Building small carburetor with throttle

[Brian Rupnow]

This post is not an original design by me, but rather an "imperialized" copy of a design originated by Malcolm Stride, which was originally designed in metric. I built one of these carburetors during the past year, and was impressed by how well it works.  I am building a second one to try on my side valve engine. I really didn't do a lot of "rounding off" in the conversion from metric to Imperial, but suffice it to say that the inside dimension of the throat is .195" (4.95 mm), the thread on the outside of the carburetor body is 5/16"-18, and the throttle  barrel  is .394"---a direct rounding off of 10 mm. The tapered bore in the throat was put in there with my very recently made tapered D bit, and has a 16 degree included angle. This carburetor seems to be excellent on single cylinder engines in the 3/4" to 1" cylinder bore range.

[Brian Rupnow]

On any carburetor with a square or rectangular body, the choice is always--Do I start with square stock and hold it in the four jaw chuck to turn round ends on it, or do I start with round stock and turn it first, then mill the square/rectangular sides. I had some 1" diameter aluminum round stock, and I am really more comfortable with the 3 jaw chuck, so my first step was to turn the diameter down to 0.835" (which is the distance across the corners of the square body), then turn the end which will receive the tapered bore down to .394". I left everything attached to the bit of 1" diameter parent stock so I would have something to hold onto with the chuck, and roughed out the diameter on the other end but left it well oversize. With this type of carburetor, it is best to drill the through hole with the throttle barrel in place, which ensures that the holes will line up properly on final assembly. Of course, I don't have the throttle barrel machined yet, so I drilled through to what would be the approximate center of the throttle barrel at .195" (4.95 mm), but left the other half of the carburetor undrilled. The hole I created allowed me to ream the tapered hole with my newly created D bit in the tailstock chuck. I then transferred the part over to the chuck on my rotary table and milled away the sides which left the square body. I then drilled and reamed the .394" diameter hole through for the throttle barrel, and drilled and tapped the holes for the bolts which hold the top and bottom plates in place, and also drilled and tapped the throttle stop screw hole, the air bleed screw hole, and drilled the air bleed hole. The attached picture shows the result up to this point, with the "part in progress" held in the chuck on my rotary table, which is mounted on the bed of my milling machine.

[Brian Rupnow]

Of course, as often happens, I got ahead of myself, and after squaring up the carb body, I realized that I should have arranged to machine the square top and bottom covers in the same set-up, but it was too late. Thankfully, the piece of parent stock was long enough to flip "end for end", put it back into the lathe, and turn the diameters which appear on the top and bottom covers. I took two pictures of this operation, and both of them tuned out blurry---and of course I don't know that until I download my camera to the computer after I have moved on and went to the next step. --A hint--the portions with layout dye on them are the large diameters that will get squared off in the milling machine.

[Brian Rupnow]

At any rate--Here we are, back in the rotary table, with the round portions squared off. Now it was simply a matter of moving things back to the lathe, drilling, reaming, threading, and parting off----

[Brian Rupnow]

And "Hey Presto"--Here we have 3 almost finished parts. No finish sanding has been done at this point, the clearance holes for my #4-40 capscrews are not yet drilled in the top and bottom covers, and the hole through the air horn of the carburetor is still only drilled through to the center. You will see that I did turn the other end of the carburetor down to .312 so I could thread it 5/16"-18, and parted it off from the parent stock.

[cfellows]

Hey Brian, always interested in carburetor designs.  Did that design come from Malcom's book, online, or ...?  Just curious how much detail you're comfortable with providing.  Does the carb have an air bleed for idling?  How about the needle valve, anything special about that?  Or should I just sit back, watch the show and save questions for the end?   

Chuck

[Roger B]

+1 for what Chuck said 

[Brian Rupnow]

Chuck--That design came from the Bobcat and Jaguar plans. It does have an airbleed for idling. The needle valve isn't really "special"---but it's arrangement is different. This is one of those situations where I have "redesigned" the original carburetor, but I haven't "redesigned" it very darn much, except for fasteners, imperial dimensions,  and some imperial reamer sizes. I have a complete set of plans I would post, but I don't want to get hung by the moderators.--I will post them if a moderator okays it, otherwise I probably won't.--Brian

[Brian Rupnow]

Good news--the holes I drilled through the carburetor end caps seem to have ended up in the right place. Everything goes together!!! the first picture shows the end cap which the throttle barrel extends through to attach to the throttle handle. Two of the screws on opposing corners hold the end-cap to the main body. The third bolt you see there is the air bleed control screw. Normally I would say this view shows the top of the carburetor, but there is a catch---it isn't really the top. I will explain as we get farther into this build.

[Brian Rupnow]

This view of the opposite end cap shows a number of things. The rusty #10-32 capscrew is threaded into the hole which the "spray bar assembly" threads into. The single #4-40 capscrew thru the blank side of the carb body is the throttle stop screw which adjusts the idle speed of the engine. The small plain hole in the same side is the air bleed hole. The threaded end of the carburetor body has been countersunk in preperation for drilling the hole which will go all the way through to the center hasn't been drilled yet.

[cfellows]

No worries on the plans, Brian.  I can see enough from your pictures to understand how things go together and I am very much in agreement with protecting the original designer's intellectual investment.  Does the spray bar extend up into the center of the throttle barrel venturi?

Chuck

[Brian Rupnow]

I will tackle the throttle barrel next. Lots of things going on here. The large (.394") diameter fits into the 0.394" diameter reamed hole through the carburetor body. The smaller diameter fits thru the 5/32" diameter reamed hole in the carb end cap for the throttle handle to attach to. The notch in the bottom is where the throttle stop screw bears against to keep the throttle cracked to whatever idle RPM you want the engine to have. The length of the major diameter has to be a "good" fit between the inside of the two bolt on caps.--Snug, but still loose enough to rotate. The 0.195" (4.95 mm)" hole thru the center is drilled after the throttle barrel is installed in the carburetor with both end caps bolted in place. This is the point at which I will finish the hole through the air horn (main body) of the carburetor at the same time all in one set up. The small hole in the bottom of the throttle barrel is where the nose of the "spray bar assembly" pokes up through, and it is a clearance hole, because although the throttle barrel rotates to control the speed of the engine, the nose of the spray bar assembly doesn't. That is kind of a hard concept to get your head around, but it works very well, and does make sense after you have though about it for a while.

[Jasonb]

Brian, was there any particular reason you have used a 5/32" (4mm) hole through the carb as I would have thought 3/16" would be a lot closer to the original 5mm?

J

[Brian Rupnow]

Jason--My apologies--The hole through the center of the carburetor is .195" (4.95 mm). I had to go back and edit my previous posts where I said the hole was 5/32" diameter. I didn't realize until I made the throttle barrel about an hour ago that I had posted the wrong size. I actually did drill a 5/32" hole thru the throttle barrel and the other side of the carburetor body, and then I couldn't figure out why there was a step at the bottom of the taper on the intake side where the throttle barrel started. I had another look at the drawings and seen that the hole was actually dimensioned as .195". (4.95 mm). My bad--I redrilled the hole to what the drawings said and all is well now.----Brian

[Brian Rupnow]

So--here is the tricky part I alluded to earlier, when I said that what would normally be the top of this carburetor wasn't really the top. On all of my engines, I set the gas tank up so that the top of the gas tank is about 1/4" below the center of the carburetor to prevent gravity flooding and/or draining the tank. However, due to the way the needle valve on this carburetor screws in from what would normally be the bottom, the fuel will leak down past the 10-40 threads on the needle valve and drain the gas tank when the engine is left unattended. I solved that problem by turning the carburetor 90 degrees so that the centerline of the needle valve sets in line with the center of the main carburetor air passage. That solves the leaking problem, and the carburetor doesn't care. It will work well in any rotational aspect.