# Carburetor



## cfellows (May 18, 2012)

Carburetion continues to be the bug-a-boo of my internal combustion engines. So, I've built a very simple mixer based on dimensions from Phil Duclos's engines and added a throttle control of my own. This allows me to adjust the air mixture and the throttle separately. Initial results on my plumbing parts engine are really good. I can idle the engine way down and yet it still runs steadily with a strong thump each time it fires. Here are some pictures. I'm also working on some drawings that I'll post when finished.

















The secret to carburetion seems to be the needle in the needle valve. Using a commercial sewing needle of .050" diameter works much better than anything I can make myself.

Chuck


----------



## dgjessing (May 18, 2012)

Nice - looking forward to seeing the drawings.


----------



## Paulsv (May 18, 2012)

It looks simple yet elegant. I'd like to see the drawing. I have a half a dozen SU carburetor needles and jets from my Triumph TR-3 days that I might be able to adapt for something like this.


----------



## compspecial (May 18, 2012)

Brilliant idea, Chuck, sort of like a constant-depression type?
               Stew.


----------



## cfellows (May 18, 2012)

compspecial  said:
			
		

> Brilliant idea, Chuck, sort of like a constant-depression type?
> Stew.



Thanks, Stew. Not sure what a constant-depression type is...

I've attached the drawing.

Chuck


----------



## cfellows (May 18, 2012)

OK, previous drawing had a mistake. The drill size for the inlet side of the jet should be #60. I've attached the corrected drawing. Sorry for any inconvenience.

Chuck


----------



## Kaleb (May 19, 2012)

I don't see any dimension given for the engine side of the carb body. You have stated that it's drilled 5/32", but you forgot to include the OD.


----------



## Deanofid (May 19, 2012)

Carbs give me trouble too, Chuck. Once I get past a simple jet and venturi or inlet type, things start to go south. I've tried butterfly types to control air flow, and they only want to work at a single setting, much the same as a straight inlet with a 90 deg jet in line. Then the engine speed is the same old rich/lean situation with seemingly only one truly good setting.
Thanks for the drawing. Something to keep in mind and try next time I try another one!



			
				Kaleb  said:
			
		

> I don't see any dimension given for the engine side of the carb body. You have stated that it's drilled 5/32", but you forgot to include the OD.



I would imagine it would be whatever is required for your particular engine.


----------



## cfellows (May 20, 2012)

Kaleb  said:
			
		

> I don't see any dimension given for the engine side of the carb body. You have stated that it's drilled 5/32", but you forgot to include the OD.



As stated by Dean, it depends on how you are connecting it to your engine. In my case, the OD is 1/4" threaded with 1/4 x 40 model pipe thread.

Thx...
Chuck


----------



## cfellows (May 20, 2012)

I've found another simple carburetor design that I want to try. It's based on a mixer with a throttle that Jerry Howell designed for his Powerhouse engine which is similar in size and layout to the Webster engine. It's a pretty elegant and clever arrangement so I'm going to design my own carb using the same concept and give it a try. Will post a progress report when I get going.

Chuck


----------



## compspecial (May 20, 2012)

Hi again chuck and thanks for the drawing! I meant can you rotate the cross-drilled hex bolt to vary the air supply?
                 Stew


----------



## Mosey (May 20, 2012)

The carburetors on model airplane engines (OS Max) seem to be very simple, and I know they work extremely well. They have a simple rotating cylindrical barrel with a cutaway in it. Air is modulated remotely with a needle. And they are cheap. Comments?


----------



## cfellows (May 20, 2012)

compspecial  said:
			
		

> Hi again chuck and thanks for the drawing! I meant can you rotate the cross-drilled hex bolt to vary the air supply?
> Stew



Yep, rotating the hex bolt closes (or opens) the radial holes which decreases or increases the air supply.



			
				Mosey  said:
			
		

> The carburetors on model airplane engines (OS Max) seem to be very simple, and I know they work extremely well. They have a simple rotating cylindrical barrel with a cutaway in it. Air is modulated remotely with a needle. And they are cheap. Comments?



I'm familiar with rotating barrel types and have made several of them with varying degrees of success. I did make a pretty successful one for my open column twin engine. The secret to its success seems to be the use of a commercial sewing needle instead of trying to grind the end of a screw to a taper. Prior to my home made version, I had a commercial model airplane carburetor like you describe mounted on this same engine. It was kind of sporadic and I couldn't get it to idle very low. I think it was probably too large a venturi... these low RPM, four stroke model engines seem to prefer a very small venturi, as little as 1/8".

Chuck


----------



## cfellows (May 20, 2012)

So here's the start on my next carburetor. I'm starting with a 3/4" x 3/4" x 2" chunk of aluminum. I'll whittle away all the material I don't need once I get it near finished.

This first picture shows the cross hole that will hold the spray bar / needle valve assembly.






And here I've drilled the inlet passage through the main body. It's 3/16" diameter on the outboard end and 5/32" on the engine side.






Here you can see what the spray bar / needle valve assembly will look like as seen through the main air passage. As stated, the jet hole will extend straight down from the spray bar.






The thottle bar will extend up into the air passage from the bottom.






Here's another shot with a better view of the throttling bar partially closing off the venturi. I'm holding the throttle between my thumb and second finger so you can't see it extending out the bottom of the carburetor body.






Chuck


----------

