Author Topic: Building small carburetor with throttle  (Read 12408 times)

Offline Brian Rupnow

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Re: Building small carburetor with throttle
« Reply #30 on: December 16, 2014, 09:11:24 PM »
One last view thru the bore. You can see the nose of the spray bar, sticking up thru the hole in the center of the passage. The fact that it is sticking up there in the airflow is enough to make the air speed up when it is being pulled past it, and that creates the "venturi" effect which will pull fuel up from the tank 1/2" to 1". The amount of fuel available is controlled by the needle valve, with the needle "screwed into" the other end of that 1/16" diameter tube. That is how you set the mixture to "richer" or "leaner" to best suit your engine. (This is not intended to control the speed of your engine.) The amount of air rushing thru the passage is controlled by turning the throttle barrel and impeding the air flow. That is what creates the "throttle effect" and lets your engine run faster or slower.--Of course there is a direct relationship between the two factors. The more you open the throttle, the more air will rush through the carburetor, and consequently create more "venturi" effect, thus pulling more fuel thru the spray bar. When you close the throttle, less air passes though, less venturi effect is created, and consequently less fuel is pulled from the spray bar.

Offline Brian Rupnow

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Re: Building small carburetor with throttle
« Reply #31 on: December 16, 2014, 09:19:08 PM »
In the picture directly above, you will see a .045" thick brass washer just above the hex shape of the soldered spray bar assembly, next to the aluminum carburetor bottom plate. Why is it there, when it doesn't appear in any of the solid models. Well, my friends, that is the "bit" that buys me some tolerance on all the pieces that screw together and have to meet fairly tight "end gap" tolerances. If the spray bar screws too far into the carburetor bottom plate, then it will press against the underside of the throttle barrel and prevent it from being turned. During final assembly, I did a bit of measuring and determined that a washer that thick would let me tighten everything "snug" and still have a freely rotating throttle barrel. Probably if I built ten more identical carburetors, that washer would end up being a different thickness on every one of them.----Brian

Offline Brian Rupnow

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Re: Building small carburetor with throttle
« Reply #32 on: December 17, 2014, 04:31:34 PM »
I knew when I built the carburetor that it wasn't going to just be a straight bolt-on to my side valve engine, since being mounted sideways it takes up more room than the Chuck Fellows carb that I had built for this engine originally. I didn't want to build a whole new intake manifold, so I made up a 45 degree adapter to use with the original intake manifold. The 45 degree offset gives me ample room beside the "muffler".

Offline Brian Rupnow

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Re: Building small carburetor with throttle
« Reply #33 on: December 17, 2014, 05:14:33 PM »
The engine runs really well with this carburetor on it. The lowest sustainable rpm is 700 rpm, and the top rpm had increased to 3000 rpm. I have played with the ignition timing a bit, and with a lot of advance the engine will rev up close to 4000 rpm (which is very scary).  However, with the ignition advanced that far, it doesn't idle down particularly well. I backed the ignition timing off and the engine sets there and idles very well.
<a href="https://www.youtube.com/watch?v=zl_v7HHpArI" target="_blank">http://www.youtube.com/watch?v=zl_v7HHpArI</a>
« Last Edit: December 17, 2014, 05:32:31 PM by Brian Rupnow »

Offline Brian Rupnow

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Re: Building small carburetor with throttle
« Reply #34 on: December 19, 2014, 08:58:41 PM »
I played a bit more with the needle valve setting on the new carburetor and was able to get the engine idle down to 670 rpm, but it simply won't go any slower than that.
<a href="https://www.youtube.com/watch?v=CQ0vm7rpf4U" target="_blank">http://www.youtube.com/watch?v=CQ0vm7rpf4U</a>

 

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