Model Engine Maker
Supporting => Engine Ancillaries => Topic started by: Brian Rupnow on March 11, 2014, 03:53:31 PM
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Myself and another fellow from the HMEM forum have been doing a lot of research to find a small I.C. carb with a throttle on it. Hit and miss engines do not have a throttle on them, as the speed is controlled by the governor system. There is no throttle in those carbs--they run "wide open" all the time, as anyone can attest to who has ever had the misfortune to have the governors stick on their hit and miss engines. However, not all of the model engines we build are hit and miss style. Now don't get me wrong.--There are lots of carburetor designs out there for I.C. engines, but they are HUGE compared to the size of the engine itself. We have been searching for carbs that won't look out of place on engines from 1/2" to 1" bore with a single cylinder. After looking at many different plans, we have decided that the best design out there is a design originally put together by George Britnell for use on his 4 cylinder engine. The only fly in the ointment is that George's carburetor has a relatively large bore in it at 0.260" diameter. On the small I.C. engines that I build, they depend on air flow thru the carb venturi to create enough vacuum to pull fuel up to the carb from the fuel tank. If the bore of the carb is too large, the resulting vacuum created by too large a carb bore compared to the engine displacement won't create enough venturi vacuum to suck up the fuel to the needle valve spray "nose". I have taken Georges carburetor and redesigned it to accommodate a bore of 0.156" in the venturi area. This has allowed me to make the overall carb body smaller and to make the attendant air bleed screw and idle speed screw smaller as well. George knows I am doing this, and has even offered up some helpful advise. I am going to build this carb, and use the Webster engine I built a few years ago as my "test bed". Stick with me, and I will post detail drawings as I progress. It will probably be safer to wait until the end of the build to actually download any drawings, because they may change as this thing develops.----Brian
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/FULLASSY-RUPNOWCARB_zps26e63a83.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/FULLASSY-RUPNOWCARB_zps26e63a83.jpg.html)
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This looks to be an extremely interesting journey Brian, Thanks.
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I'm not certain if this is the most difficult part or not. It certainly seems to have the most machining operations on it, so we will start with the outer body.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/RUPNOWCARBBODY_zps07d04bba.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/RUPNOWCARBBODY_zps07d04bba.jpg.html)
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I started a discussion on carburettors a while ago:
http://www.modelenginemaker.com/index.php/topic,2842.0.html
I built a carb based on Gray's design for my vertical engine but have not got around to trying it yet. (I have to run my engines outside as my workshop is in a communal cellar). I will follow this design with interest. :ThumbsUp: :ThumbsUp:
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Well, I think that covers everything I am going to do in the lathe. Started out with 3/4" square brass, mounted it in the 4 jaw, and turned the big end spigot to finished diameter but about 1/2" long. Put 3 jaw on chuck, reversed the part and held it by the round spigot to turn the other end to 5/16" and left it about 1/2" long. Threaded o.d. of small spigot and drilled 3/16" dia. hole thru all, then reversed part and put threaded spigot in 3 jaw. Drilled 0.260 dia half way thru from big end, then faced big end off to correct length and bored taper as called for on drawing. Removed part from lathe, ran a 0.2" thick nut all the way down the threaded end and ground off anything sticking out past the nut away on my stationary belt sander.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/carbbody-1001_zps3f277d5d.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/carbbody-1001_zps3f277d5d.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/carbbody-1002_zpsdd8f28cd.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/carbbody-1002_zpsdd8f28cd.jpg.html)
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Hey, I have a 23rd addition of Machinery's Handbook too. It was a good year :) Then I got a free 25th addition from the publisher, as a faculty desk copy. Actually I like the binding of #23 better. Sorry for digressing Brian.
Bill
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Bill--That edition you see on my desk was bought for me by Karl Fricke, the German head of engineering for Volkswagen Canada. He was one of the best bosses I ever had.
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Well There!!! That wasn't so bad. The biggest worstest part is finished. Its too bad that my camera picks up all the filings as well as the part I am trying to show. It certainly is a tiny little rascal.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/casrbblockfinished001_zps0a03ccb0.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/casrbblockfinished001_zps0a03ccb0.jpg.html)
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then reversed part and put threaded spigot in 3 jaw
Brian - I'd be very scared putting a threaded part into the jaws of the chuck, did you do anything to protect it?
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No, not really. There is a flat "land" about .093" wide at the tip of my chuck jaws, and it doesn't seem to damage the threads. Of course you can't do heavy turning on the part when gripped this way either. If you do mark the threads up a bit you can always run a die over them and clean them up afterwards.
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And the next part of our saga looks like this. I decided to go with steel for this part, because I was afraid that aluminum or brass wold "gall" and seize in the brass outer body. (It has to turn freely). When I put the 0.375" hole in the brass body I used a 3/8" endmill, so my hole is a couple of thousandths oversize. I will turn this part from 1/2" dia. cold rolled steel, because round cold rolled in its stock form is about .0005" undersize and I don't want a sloppy fit. I know the drawing calls for 0.375" diameter but I will probably end up at .376 when I make the part.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/RUPNOWCARBBARREL_zpsce8ac237.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/RUPNOWCARBBARREL_zpsce8ac237.jpg.html)
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So---Now we have two parts. You may notice that the top of the barrel doesn't quite agree with the drawing. As I was about to cut it to length I had a revelation---The single bolt which holds the throttle handle on is right on the center, and the handle is sure to spin on that bolt instead of turning the barrel. What better time to make the barrel a shade longer and put a 1/4" slot on the top. Align the slot with the hole thru the barrel and I have a quick visual reference of whether the throttle is open or closed. The handle will fit down into the slot. I am going to post an updated barrel drawing as soon as I finish this post. That Canadian quarter in the picture is the same size as an American quarter.--A really nice feature of this carb (Thank you Geo Britnell) is that the #2-56 bolt that interacts with the slot in the barrel has two functions. It acts as a throttle stop, to adjust how far the throttle can close before being stopped by the end of the bolt, and the side of the same bolt holds the barrel in place so it can rotate but not fall out.---Very nifty "dual purpose" design. The hole thru the barrel is 0.156" diameter. As for the end view---If that hole in the barrel doesn't appear to be perfectly concentric with the hole in the carb body, well----That is a testament to the fact that I am much further advanced in my design capabilities than I am in my machining abilities. A sharp eyed individual will also note that my taper in the intake side of the body doesn't nicely taper to a matching diameter with the hole thru the barrel. That is because of constraints imposed by the .040 diameter hole thru the side of the carb body which is part of the idle circuit. The law that governs how a venturi in a carburetor creates a low pressure area where there is a restriction in the airflow, (to suck fuel up hill from the gas tank) doesn't explicitly state that it must be a smooth and seamless transition from the large diameter passage to the small diameter passage.--Also, the nose of the needle valve outer shell is going to stick up in the center of that 0.156" diameter, causing a further restriction in the air passage. If there is a problem, I will machine a nicely tapered insert and put it in the intake side of the carb body.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/CARB-TWOFIRSTPARTS002_zps542119a8.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/CARB-TWOFIRSTPARTS002_zps542119a8.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/CARB-TWOFIRSTPARTS003_zps107499c6.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/CARB-TWOFIRSTPARTS003_zps107499c6.jpg.html)
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We have to get some fuel into this thing, and the fuel jet is made up from two parts soldered together. No real rocket science here, but I did have to call and order $100 worth of 10-40 taps and a 10-40 die.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/RUPNOWFUELJET_zps229712cb.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/RUPNOWFUELJET_zps229712cb.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/RUPNOWFUELINLET_zps105780ab.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/RUPNOWFUELINLET_zps105780ab.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/SUBASSYRUPNOWFUELINLET_zpsc68ab107.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/SUBASSYRUPNOWFUELINLET_zpsc68ab107.jpg.html)
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The needle valve itself is of two piece construction. There are no threads on the fabricated "needle"--The threads are on the inside of the cap.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/ASSEMBLY-RUPNOWNEEDLEVALVE_zps79efbe7e.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/ASSEMBLY-RUPNOWNEEDLEVALVE_zps79efbe7e.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/RUPNOWNEEDLEVALVEPART-2_zps893a0a0a.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/RUPNOWNEEDLEVALVEPART-2_zps893a0a0a.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/RUPNOWNEEDLEVALVEPART-1_zpsc806f519.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/RUPNOWNEEDLEVALVEPART-1_zpsc806f519.jpg.html)
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I had asked George Britnell a question about the location of the air bleed hole location and throttle opening here, but now realize that George had answered me in a previous email. It just took me until this afternoon to get my head around what he was saying.
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I guess if you are going to have a throttle, you will want a handle to turn it with.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/RUPNOWTHROTTLEHANDLE_zps85480f0c.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/RUPNOWTHROTTLEHANDLE_zps85480f0c.jpg.html)
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I know, I know---I said to make the handle out of steel. But when I got to my "cupboard of all bits and pieces" the only 1/16" stock I had was brass. I don't think you can bend brass too readily without it breaking, so I heated it up with my oxy acetylene torch till it was dull red and then bent it in my vice. I'm somewhat amazed at how few pieces there are to this carb. All that's left to make is the fuel inlet and the needle and cap. I might solder up the fuel inlet tomorrow and get it ready for when my taps and dies arrive.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/THROTTLEHANDLE-2001_zps8ad128de.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/THROTTLEHANDLE-2001_zps8ad128de.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/THROTTLEHANDLE-2002_zps73fad20a.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/THROTTLEHANDLE-2002_zps73fad20a.jpg.html)
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Okay---At this stage of the game, I am beginning to realize that ---I muffed it!!! On the carburetor main body, the "thru-hole" that the air flows thru has to be the same diameter both entering and leaving the round central chamber where the throttle barrel is.--Otherwise, as I have just figured out (much to my dismay) the amount of opening or closing on one side of the barrel (for instance the inlet side) is totally different than the amount of opening or closing on the opposite side. I am going to have to reconfigure the main carburetor body--as in redesign and remake it. That's okay. I'm learning as I go along here. I think I can save the barrel and the handle.
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There wasn't a lot of redesign involved. I made the hole thru the air intake side the same diameter as the hole in the air outlet side at the point where it breaks into the 3/8" hole that the throttle barrel fits into, and moved the air bleed hole and screw around a bit to optimize their location according to advise from Geo. Britnell.---George had sent me this information earlier, but I couldn't really get my head around what he was saying until I read thru it again today while I was puzzling things out. I can re-use the barrel and handle.--Interestingly enough, this throttle goes from closed to open in 45 degrees, unlike the butterfly valve in an automobile carb that must rotate thru 90 degrees.
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This morning, before diving headlong into another piece of brass, I am doing a bit of taper experimenting. I wanted to know if I can drill a 20 degree included angle taper thru material 0.361" thick and go from a diameter of 0.315 to a diameter of 0.187". My selection of small boring bars is, well, minimal would be a good word. I didn't want to have to make up a special fluted taper cutter. I reproduced the critical geometry in aluminum--a 3/16 centered hole going .361" before breaking thru to a 3/8" diameter cross hole. I found that by drilling a couple of stepped holes first to remove as much meat as I could from the outside end of the aluminum piece without cutting into what would become the tapered hole, I was able to get my much abused boring tool thru and give me the correct taper and correct diameters. That's a big plus!!! Now I am off to my supplier to buy some more 3/4" square brass. The outer body could be made from aluminum, but I like the look of brass better.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow012/TAPERTRICKS002_zps874afc75.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow012/TAPERTRICKS002_zps874afc75.jpg.html)
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Brian, I think the added feature of the tapered (seamless) inlet and outlet is to reduce or eliminate turbulence which probably has an effect on how well it works. I've found that a simple, D style reamer works about as well as a fluted reamer for these small applications.
One thing I'm kind of figuring out with these small carburetors is that fit and finish is critically important to their operation. I think one reason that the sewing needle works so well for a needle valve is the polished finish on the needle's surface.
Chuck
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Chuck--I'm really glad you chimed in here. I know you have been fighting these carburetor wars yourself for a while and I value your input. I agree with you about the turbulence in the air stream. I think my ability to put a nice taper on the air inlet side is only going to help. It makes me a little bit crazy that I completely finished the brass piece before having the revelation about the size of holes in the outer body having to match the holes in the barrel. I wasn't that happy with the way that first body turned out anyways, as I had miss-measured something and the bores were not as "in line"as they should have been with the barrel.---Brian
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Brian, as I understand it the hole on the inlet side needs to be larger than the hole on the outlet side. If you have a simple jet and venturi followed by a butterfly throttle the mixture becomes richer as the speed increases (apparently the mass of petrol delivered is increased by the air velocity whereas the mass of air decreases due to the reduction in pressure caused by the increase in air velocity). By making the inlet side hole bigger the amount of incoming air is increased more rapidly and if correctly sized compensates for this effect.
Quite how this works in practise I am not sure, but both my commercial model aircraft type carbs have a bigger hole on the inlet side.
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Roger--I am not exactly sure how that works either. If you take the taper on the inlet side into consideration, the intake side of my carb is much larger than the outlet side. So---And this is only assumption at this point--Air enters the large end of the tapered bore at one speed. As it gets farther down the taper it must flow faster to get the same volume of air thru an ever decreasing hole. When it gets to the center of the barrel where the nose of the needle valve sticks up into the air flow as an obstruction, the air must flow even faster to get the same volume of air that entered the carb thru the center of the carb----And according to my friend Bernouli, as the air increases in speed its pressure drops. So you have the maximum pressure drop and Venturi effect right at the needle valve nose. Now what happens on the other side of the needle valve nozzle, I'm not sure if the passage must become larger again to slow the air down a bit, or if I stay with this reduced diameter all the way into the cylinder head. I will be conducting tests with a vacuum meter t check this out as work progresses.---Brian
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Brian, it's certainly a complex subject. I hope to get outside to try my version of Graham Meek's carburettor this weekend. Chuck had a go at producing a very basic carb which worked, but in the end I don't think that he was satisfied with the low speed throttling.
In the end, I think' a lot depends on what you want. Is it 'display' engine where you want a nice low speed idle? Is it a stunt plane engine where revs and power are the goal? Is it for a tractor (one of my plans) or a locomotive (another plan) where flexibility is important?
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The carburetors on my old Honda 350 had vacuum operated diaphragms that operated a fuel metering needle valve, which I think helped control the mixture depending on speed. I found a description of how it operates:
http://www.bikerenews.com/Stories_Archives/Carburetors.html (http://www.bikerenews.com/Stories_Archives/Carburetors.html)
(http://enginuitysystems.com/pix/Microfiche-Teardown-Picture-Carburetor-Honda-2005-CB250-A-Motorcycle-800x399.png)
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Roger--My engines are small bore (3/4" to 1") x similar stroke lengths, and they lead pretty sheltered lives. The twin opposed cylinder was my sixth bar stock internal combustion engine. Three of these engines are hit and miss engines where the speed is controlled by governors and the carburetors have no throttle. I have also built an Atkinson, a Webster, and the Dual Opposed cylinder engine, and they are all running Traxxas model airplane engines because they need to have a throttle. As far as I know, there are no simple/free/small/ suitable for single cylinder engines in my bore range, throttle style carburetor plans out there floating around on the web. What Chuck has done, and what I am attempting to do, is to develop a simple small carburetor which is throttle capable, work up a complete set of plans which I will post free to download as I do with all the engines I design, and post the build as well as an engine being successfully ran by it.
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Brian, my views are much the same. The two engines I have built ( a 25mm bore 50mm stroke horizontal 'slow speed' engine and a 16mm x 16 mm 4 stroke vertical engine) have commercial model aircraft carbs. I felt that the carbs were designed for high performance two stoke engines and did not really suit my engines. I started looking into my own design and came into contact with Graham Meek who had designed a carb based on the same concepts for Seagull (iirc).
My build is illustrated in my 4 stroke vertical thread, but I was not intending to post the drawings until I have tried it.
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This is the build thread. The carb stuff is around page 6.
http://www.modelenginemaker.com/index.php/topic,2093.75.html
I have also attached my usual electronic back of an envelope sketches ::)
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Brian, did you ever try your version of my carb design on the opposed piston engine after you had worked out some of the other issues? I'd be interested in getting your views on how well my design works compared to the design you are developing in this thread...
Chuck
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So many projects, so little time...
So true!
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Chuck--I am going to try both carbs on the Webster. Before I even had a chance to give your carb a fair shake you had posted that you were not happy with it, so I put it in my tin full of treasures and went directly to the model airplane carb. Right now I'm doing the "One step forward-Two steps back" thing with my carb build. There is still 24" of snow here and daytime highs around +5C so I'm not in a real rush to get finished and work in my main garage---it's too cold.
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It's been a looooonnnng day!!! I have remade the brass outer body to the new design, and it looks good.---But---I just found out I can not re-use the barrel. This was a case of two wrongs not making a right. When I made the first brass body, I got something off center. Scratched my head for a minute and thought--"Oh well, I'll just drill the hole thru the barrel off center a bit so it will match up".---BAD MOVE!!! Now the barrel with the off center hole won't work with the new brass body. OH POOP!!! I'm going upstairs and read for the rest of the day. Fresh start tomorrow!!!
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Nothing picture worthy accomplished today. I spent the whole day messing with sump pump stuff. I wasn't totally impressed with the 20 degree included angle tapered hole that I had machined on the inlet side of the new brass body. So--I turned a 20 degree included angle taper on a piece of 3/8" cold rolled and used some really coarse, nasty automobile valve grinding compound on it to hand finish (can't honestly say "lap" ) the inside of the taper. It turned out quite nice actually. I don't really have any boring tools small enough to bring this off properly.
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Progress has come to a stop. The new brass outer body is finished. In the process of making the new barrel, I convinced myself the safest way to ensure concentricity was to insert it firmly into the brass outer body and drill it in place. This worked fine for the 0.156" diameter thru hole. I then rejigged the part and went to spot thru the #2-56 threaded hole for the idle stop screw so I would be able to get the slot in the correct place. I opened the drawer with my 1/16" end mill in it, and guess what!!! The Shop Pixies have taken it away. I don't want to tear down my set up, so everything will set until tomorrow when I can get over to the tooling shop and buy a couple of new end mills. I can't spot thru with a drill, because it is not on the center of the 3/8" diameter steel, so a drill would slide sideways and tear up the threads in the brass part.
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Today we are back to the point we were at when I figured out that my first design had a fatal flaw. The carb outer body is remade, and the steel center barrel is remade as well. What a pin in the butt!!! One pic shows the assembled carb setting up posing for an "overall" shot, one shows it with the throttle closed. (The closed position is adjustable) The throttle never closes completely, otherwise the engine would stop. It is only when the carb is closed to the position showed that the air bleed screw comes into play.--And remember, since the openings in both sides of the brass barrel are the same size on each side of the throttle barrel, the amount of opening on the side closest to the cylinder head will be open exactly the same amount as the opening on the air intake side. If the engine falters when the throttle is in this closed (idle) position shown, then the air bleed screw can be adjusted to smooth out the idle. Once the throttle is swung into a more open position, the idle circuit is blocked by the side of the barrel and has no effect on the carburetor.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/carbredone001_zps9f0c04b7.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/carbredone001_zps9f0c04b7.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/carbredone005_zps4508906d.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/carbredone005_zps4508906d.jpg.html)
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This is the best shot I could get of the carb with fully open throttle. What appears to be a bit of dirt right at the top of the barrel bore is actually a bit of metal displaced by the drill when I tapped the hole for the bolt holding the handle on. I will clean that up and plug it with a bit of thread sealant on the handle bolt.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/throttleopen001_zps0fe89cee.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/throttleopen001_zps0fe89cee.jpg.html)
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What you see here is a temporary disposable jig, made from aluminum. There is simply no other good way that I know of to position the two brass pieces correctly in relationship to each other, and then not have them move while silver soldering them. The jig is "temporary and disposable" because it has to be cut apart to get the soldered assembly out of the jig. The short piece of brass is for the fuel line to attach to, and has been turned to its finished size, and the hole drilled part way thru the center of it. The longer brass part is 1/4" diameter and will be turned down to finished size on each side of the fuel inlet before being threaded #10-40 on both ends. At some point while it is in the lathe chuck, the tapered end will be machined and the thru hole will be drilled. After that, the hole part way thru the short piece will be extended until it intersects with the central fuel passage.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/SOLDERINGFUELJET003_zps1256465e.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/SOLDERINGFUELJET003_zps1256465e.jpg.html)
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This is the result, immediately after silver soldering the joint. I hold the aluminum in an old drill press vice so I can position it in whatever way I have to so I can reach all areas of the soldered joint.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/SOLDERINGFUELJET006_zpsbeac4c6a.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/SOLDERINGFUELJET006_zpsbeac4c6a.jpg.html)
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And here we have the soldered part, freed from the jig and cleaned up a bit. Now I get to do some more lathe work on the main 1/4" diameter part.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/SOLDERINGFUELJET008_zps0dd7bcec.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/SOLDERINGFUELJET008_zps0dd7bcec.jpg.html)
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I've been holding my breath for the last two hours machining this fuel inlet, but I think I've got it nailed. I don't know why I hold my breath while I'm doing these really finicky things, but I bet you do too, don't you!!!
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/FUELINLETFINISHED001_zpsefdf813c.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/FUELINLETFINISHED001_zpsefdf813c.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/FUELINLETFINISHED002_zpscd31c8ce.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/FUELINLETFINISHED002_zpscd31c8ce.jpg.html)
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WOW!!! I didn't think I was going to be able to do this. That small diameter is 0.050". The smallest thing I had ever machined before was 0.093" dia. valve stems. I used a 3/16" cold rolled rod, and put the taper on the end while the rod was still at full 3/16" diameter. Then I started turning at 1000 rpm for 1/8" at a time right down to 0.050, then move ahead and machine the next 1/8" down to 0.050. I followed the same procedure to turn the 0.100" diameter. I tried to keep all machining as close to the chuck jaws as possible. I'm not even going to try to part it off. I will use the hacksaw while it is still locked in the chuck to cut it off, then use my belt sander to cut it to length.--To keep from feeding the swarf bunnies, I will wrap a length of masking tape around the part before I cut it off.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/NEEDLE002_zps195486d7.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/NEEDLE002_zps195486d7.jpg.html)
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Model aircraft engine air bleed carbs seem to open the air bleed near half open area. You might find a rich mid range if the air bleed closes too early. Finding the sweet spot for the air bleed hole is I'm sure a matter or trial and error.
The 20° included angle on the needle will be very coarse for gasoline. The reason sewing needles work well here is due to the small taper.
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So, Brian, if I understand, the 1/4" rod was solid bar when you soldered the pieces together? And the smaller piece was only partially drilled, I would guess on the outboard end?
Chuck
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That's right Chuck. I was able to use a center drill while I had the part in the chuck, and drill part way through the length before soldering. If I had drilled all the way through, it would probably have filled up with solder during the soldering operation. Then when I drilled it through in my mill as the very last operation, I didn't have to frig around finding the center of it to start the drill into.
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There we go---All finished. I cheated on the locknut---took a #10 std. pitch nut and rethreaded it. The soldering of the needle valve went very well, with no miss-haps. I have much better luck with silver solder than I do with plumbers soft solder. I coated the knurling on the needle valve with white out to keep any overflows of silver solder from sticking in the knurling. It works a treat, but I had a devil of a time getting the white-out off after the fact. I tried hot paint thinners, varsol, dishsoap, Goo-Gone, and toothpaste and hot water, all scrubbed with an old tooth brush, and I think there is still some on there. The drawings all have to be updated and changed to reflect what actually got built, but I will do that later today. Damn, it's a lot of work when you have to make the two main parts twice!!!--The $64,000 question will be now---Does it work!! As soon as the weather gets nice enough that I can work out in my main garage without freezing to death, I will be testing this carb and Chuck Fellows' carb on my old tried and true Webster, and will try and give an unbiased report and a video of the results.---Brian
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CARBFINISHED002_zpse35766be.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CARBFINISHED002_zpse35766be.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CARBFINISHED-2001_zps2a201748.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CARBFINISHED-2001_zps2a201748.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CARBFINISHED-2002_zps9c34cd8e.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CARBFINISHED-2002_zps9c34cd8e.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CARBFINISHED-2003_zps903238a4.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CARBFINISHED-2003_zps903238a4.jpg.html)
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Looks real good. I'm anxious to see how well it performs.
Chuck
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As of this evening, 18-March-2014, I have replaced all of the drawings in this build thread with the most current drawings. They are all up to date now.---Brian
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Chuck--I will be very interested too. After building both carbs, I can honestly say that the one you designed is much simpler to build. This is a picture showing the size difference between the carb I just built based on a modified George Britnell design, on the left, and one I built a few weeks ago to Chuck Fellows design on the right.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CARBSIZECOMPARISON003_zps90e3d4cf.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CARBSIZECOMPARISON003_zps90e3d4cf.jpg.html)
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This is the Webster engine I will be trying the two different carburetors on. As you can see, it already has an adapter on it to hold the Traxxas Pro 15 model airplane carb on it. (the Traxxas has a 10mm plain outer barrel diameter.)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/WEBSTER001_zps8d1be85a.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/WEBSTER001_zps8d1be85a.jpg.html)
This is the adapter I will make today so the two different carbs can be tested. The small diameter of the adapter must be Loctited into the Webster intake manifold.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CARBADAPTERTOFITWEBSTER_zps2615af5f.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CARBADAPTERTOFITWEBSTER_zps2615af5f.jpg.html)
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A very minor change in direction this morning- I realized that with the air bleed adjustment screw on the bottom of the carb as originally planned, it would be difficult to get at and adjust while the engine was running. So--I drilled and tapped the #2-56 hole completely through to the top of the carb body and plugged the bottom of it with a short #2-56 shcs. Then I made a very small notch for clearance in the handle, machined up a knurled adjustment knob, and threaded it, then Loctited it to a #2-56 screw x 3/4" long. Now I can adjust it from the top side where it is easy to reach. Also in these pics you can see the adjuster which will let me attach this carb or Chuck Fellows' carb to my Webster engine.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/airbleedmoved001_zpsa6f6fd5e.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/airbleedmoved001_zpsa6f6fd5e.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/airbleedmoved002_zpse2e2c282.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/airbleedmoved002_zpse2e2c282.jpg.html)
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Before I try and run the Webster with my carburetor or Chucks carburetor on it, I thought it would be a good idea to start it up and make sure it ran okay with the Traxxas Pro-15 model airplane carb on it. Its a good thing I tried, because the ignition points on it had gotten damaged somehow. I replaced the points (You can see the box setting in the background beside my Power Box.) The movie kind of says it all------Brian.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/th_WebsterwithTraxxascarb-march-2014_zps3de96575.jpg) (http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/WebsterwithTraxxascarb-march-2014_zps3de96575.mp4)
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That looks good and I wait with interest to see how the other carbs perform. I am just putting my vertical engine back together to see how my Graham Meek based carb works (hopefully before I go back to China next week :( )
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Brian,
Very nice work, can't wait to see the next step.
By the way, your drawings are the best I've seen here, and that is from someone who draws for a living. All others should look at them closely.
Mosey
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Thanks Mosey. I started out as an apprentice draughtsman in 1965 and I've been doing it in some capacity ever since.---Brian
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Fantastic. I love to watch Brian's videos, I'm not sure maybe its the fact that he has one of those voices that you could drift off to sleep listening to. Like a Canadian David Attenborough.
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So--Here we go with the first run of what I'm going to call the Rupnow/Britnell carb. The engine runs. The throttle works. The needle valve is VERY touchy, and I'm going to have to put a spring around the outer body to bear against the needle adjusting screw because it rapidly screws itself in or out depending on the engine vibration. Right now I can't see that the air bleed screw makes much difference, but I haven't ran it long enough to make that judgement quite yet. I was afraid this morning when I assembled things for the final time that it might leak fuel at the bottom of the carb, because the of the needle valve going in from the bottom, but that doesn't seem to be the case. You will notice in the video that I have a small piece of cardboard setting directly below the carb to show up any fuel drips.--However, it doesn't appear to be dripping any. I have a few more things to do today, and then later today or possibly tomorrow I will put Chuck Fellows carb on and test it.---Brian
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/th_WEBSTERWITHGEOBRITNELLCARB_zps9f772547.jpg) (http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/WEBSTERWITHGEOBRITNELLCARB_zps9f772547.mp4)
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As promised, a video of the Chuck Fellows carburetor running on the Webster, and a bit of a surprise.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/th_WEBSTERWITHCHUCKFELLOWSCARB_zps581ad5b0.jpg) (http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/WEBSTERWITHCHUCKFELLOWSCARB_zps581ad5b0.mp4)
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Sure looks good to me Brian and Chuck.
Gail in NM
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:Love: Fantastic!!!
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AWESOME!! Congrats Brian & Chuck! You two need to team up & create a Rupnow-Fellows or Fellows-Rupnow design engine.
John
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If I have done this correctly, this link should get you all of the drawings as .pdf files.---Brian
http://www.mediafire.com/download/cjgmg9kj2vcgy70/RUPNOW-BRITNELL_CARB.zip
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Great test, Brian! Needless to say, I'm pretty tickled with the results. While watching your progress on the Britnell carb, I've also been doing further testing with my original design, although I haven't posted anything.
I learned a couple of things.
First, the rotational position of the needle valve assembly is very touchy. The fuel aperture into the air passage should point directly at or slightly in front of (away from the engine) the throttle screw. This makes a considerable difference in the way the carb performs.
Second, it turns out the problem I was having wasn't the fault of the carburetor, it was the ignition I was using. As you recall, I was testing the carburetor on my vertical single and I hadn't really shaken all the bugs out of the engine itself. So, with a proper working ignition installed, the results I'm getting are much better. I built another carburetor for the vertical single, sticking to my original design, but I made the venturi 5/32" instead of the original 1/8". I also used a 10-32 throttle screw instead of the 8-32. This carburetor works quite well and I'm happy with it.
I'm still using my first carburetor with the 1/8" venturi on my plumbing parts engine and it runs flawlessly. It will idle down to a very slow speed and never miss a lick.
So, in summary, I was a bit quick on the trigger to abandon my original design and your test seems to confirm my later results. I think both your carb and mine will satisfy most needs for slow running model engines.
Thanks for all the work you put in on this, particularly in documenting your build and in running the comparative tests!
Chuck
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Chuck--It's been fun. In the overall scheme of things, you and I seem to approach these things in similar fashions. I hope that between us we have provided the small i.c. engine community an alternate to model airplane carbs. Many thanks to you for your input, and many thanks to George Britnell as well. I also fall into the trap of "My engine isn't running right. How do I figure out if it is carburation or ignition." That was the main reason I went to electronic ignition on my Rupnow hit and miss engine. Inconsistent ignition that I simply couldn't track down.
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Excellent conclusion to a journey.
I read in a 1900's book on carburettors that a blunter angle on the needle results in a more linear increase in passage size. Given the overall size (tiny) of the components, a finer thread to control the needleis not in the cards, perhaps a slower taper will do the needful?
Gerrit
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My carburetor WAS made with a finer thread. 40 threads per inch. I'm not really sure if that makes any significant difference or not. Other carbs I have built used a 32 threads per inch pitch on the needle.
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Hi Brian
I acknowledged that, which is why I suggested using a slower taper, it will give finer control over the increase/decrease in the open area of the orifice.
What I am quoting is from http://books.google.com/books?id=0dY3AAAAMAAJ, page 73. with a straight taper "the increase in the area of an orifice is not proportional to the movement of the pin". E.g. IIRC Weber DCNF idle adjustment needles are a quite shallow taper, resulting in finer control over idle mixture (I had 8 to adjust on my car :-( )
Gerrit
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Alright Gerritv--I know what you are talking about, and I am sure that is why many carburetor builders use sewing needles when they build carburetors, because of the much more gradual taper. The carb with the 20 degree included angle taper IS very sensitive.
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FYI, I have uploaded an updated version of my carburetor drawings to the plans and drawings section. The latest design eliminates the air bleed and increases the venturi diameter to 5/32" and the throttle screw size to 10-32. I've also increased the OD of the needle jet assembly to 5/32", making it less likely to break during assembly. Finally, I included a simpler way to make the through holes in the jet tube.
Based on further testing that I've done, I'm very pleased with his carburetor design and plan to use it for all future engines.
Chuck
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Okay--Small things amuse small minds. (Nah, that isn't really true). We've got 6" of new snow here and its still snowing, so I decided to play indoors today. I wanted a throttle handle on Chucks carburetor, so today I built one. Now if you immediately ask "Why not just drill a cross hole through the head of the #8 shcs that acts as a throttle?", there is method in my madness. We don't really know what rotational position the head of the screw will be in at the desired low rpm. I wanted to put a stop pin in for the throttle lever to stop against, so I had to be able to adjust the position of the lever in relationship to the position of the #8 capscrew.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/th_CHUCKSCARBWITHTHROTTLEHANDLE_zps163fc842.jpg) (http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CHUCKSCARBWITHTHROTTLEHANDLE_zps163fc842.mp4)
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If you look close, you can see that I just had room to drill one corner of the carb body for a 1/16" diameter "stop pin" that I Loctited into place.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/ASSEMBLEDFELLOWSCARBURETOR-VERSION-2-THROTTLEHANDLE_zps89f99cf1.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/ASSEMBLEDFELLOWSCARBURETOR-VERSION-2-THROTTLEHANDLE_zps89f99cf1.jpg.html)
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And here are the sordid details of how I made it----
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/ASSY-CHUCKTHROTTLEHANDLE_zps258c73b8.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/ASSY-CHUCKTHROTTLEHANDLE_zps258c73b8.jpg.html)
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/CHUCKTHROTTLEHANDLE_zps72e6c35f.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/CHUCKTHROTTLEHANDLE_zps72e6c35f.jpg.html)
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This is a view of the carb from the far side, which shows the "stop pin" for the throttle arm. I just had enough room to mount it in the corner of the carbs rectangular body. The throttle arm is in the "fully open" position in this picture, but when it swings up against the pin and stops, that is the idle position.
(http://i307.photobucket.com/albums/nn294/BrianRupnow/BrianRupnow013/STOPPINVIEW--CHUCKSCARB002_zps7d6bc66e.jpg) (http://s307.photobucket.com/user/BrianRupnow/media/BrianRupnow013/STOPPINVIEW--CHUCKSCARB002_zps7d6bc66e.jpg.html)
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Classy industrial design....
Gerrit
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Nice, Brian. As it turns out you really don't need more than about a third of a turn to go from idle to scary fast.
Chuck