Model Carburetor

[cfellows]

Chuck. That is interesting because I seem to have trouble with all of my carburetors being too sensitive. 1/16 turn between running and not running. With the spool floating the adjustment would be very variable.

My design is sensitive as well. But my throttle barrel fit pretty snug, so it wouldn't move around from vibration.

In retrospect, I really don't like the barrel throttle design. While simple in design, it's kind of big and clunky. My newest design uses an external throttle sleeve and it's easier to see it's position. I do think I will try some refinements so the throttle isn't so sensitive. Here's a link to my latest thread which has some pictures and a drawing.

http://www.homemodelenginemachinist.com/f31/vertical-single-4-stroke-ic-engine-21383/

A problem with small engines is that they really don't require a very big venturi and/or throttle opening. My newest design has a venturi less than 1/8" and could very well be smaller in my opinion.

Chuck

 

[gbritnell]

As Chuck said, smaller is better. The only thing that will happen if the venturi is too small is the engine will run out of air so therefore limiting the rpm. It's easier once you make the components to enlarge them as needed. There's a fine line between the engine's operating rpm range and the size of the venturi. Usually if the engine is slow running like a hit and miss you can make the venturi smaller to get a greater pressure drop at the needle. The nesxt thing is to put the finest thread you can on the needle valve. This will help reduce the sensitivity. Most of my engines that have a variable rpm range use rotating throttle barrels. I did build one of the Jerry Howell designed carbs and it works but is a little more complicated to build.
gbritnell

 

[AussieJimG]

Your thread(s) have me thinking about carby design and I have built my latest one using your offset spraybar principle.

After breaking several tiny drills (0.5mm) while trying to drill the fuel jet, I am wondering if the small fuel jet is needed at all. My thinking is: as the needle goes past the venturi hole, it partially blocks it thus doing the same job as when it blocks the fuel jet.

I can't test the theory yet as my current "engine" is still in doorstop mode even with a vapour carby on it.

Am I totally round the bend?

Jim

 

[cfellows]

Your thread(s) have me thinking about carby design and I have built my latest one using your offset spraybar principle.

After breaking several tiny drills (0.5mm) while trying to drill the fuel jet, I am wondering if the small fuel jet is needed at all. My thinking is: as the needle goes past the venturi hole, it partially blocks it thus doing the same job as when it blocks the fuel jet.

I can't test the theory yet as my current "engine" is still in doorstop mode even with a vapour carby on it.

Am I totally round the bend?

Jim

Jim, on my last carburetor, I avoided the really small hole by using 1/16" brass tubing, which has a .030" ID. I drilled a #55 hole all the way through, then came up from the bottom with a 1/16" drill. Then I soldered a short piece of the 1/16" tubing in place to serve as the needle seat.

Chuck

 

[AussieJimG]

Good thinking Chuck, much better to buy a hole than to make one. But I am still wondering about the need for it in a spraybar carby.

And still trying to coax life from the doorstop to check.

Jim

 

[cfellows]

Good thinking Chuck, much better to buy a hole than to make one. But I am still wondering about the need for it in a spraybar carby.

And still trying to coax life from the doorstop to check.

Jim

Can't really say whether just the needle closing off the aperture would work or not. I do know that with the seat in place, the adjustment is really sensitive with less than 1/8" turn making the difference. C'mon, Jim, get off the couch and go give it try!

Chuck