Model I.C. Carb with Throttle

[Brian Rupnow]

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

[Heffalump]

This looks to be an extremely interesting journey Brian, Thanks.

[Brian Rupnow]

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.

[Roger B]

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. 

[Brian Rupnow]

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.

[b.lindsey]

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

[Brian Rupnow]

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.

[Brian Rupnow]

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.

[Heffalump]

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?

[Brian Rupnow]

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.

[Brian Rupnow]

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.

[Brian Rupnow]

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.

[Brian Rupnow]

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.

[Brian Rupnow]

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.

[Brian Rupnow]

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.