Author Topic: Model Carburetor Design Query  (Read 2689 times)

Offline cfellows

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Model Carburetor Design Query
« on: January 14, 2014, 05:57:42 PM »
Below are drawings of my latest carburetor design showing the air bleed in 3 different positions:

I'm assuming there would be little difference between 1 and 2 where, in both cases, the air bleed enters the carburetor above the fuel metering needle valve. However, I'm wondering about the 3rd case where the air bleed enters the fuel stream before the fuel metering needle valve.

Anybody have any thoughts about any of these three positions?

So many projects, so little time...

Offline gary hart

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Re: Model Carburetor Design Query
« Reply #1 on: January 14, 2014, 06:23:10 PM »
Chuck,   did an adjustable air bleed into the fuel line before the spray bar on a float type carburetor and it did improve running conditions.    I have just recently modified the carburetor to run on propane as the float was just not quite enough lift to prevent sticking open or shut.

Offline Roger B

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Re: Model Carburetor Design Query
« Reply #2 on: January 14, 2014, 06:39:56 PM »
Hi Chuck, I think that theoretically there is a big difference between all three due to the different densities and viscosities of liquid fuel (gas, Benzin, Petrol, etc) and air. However I also know that I tend to get too deep into the theory. 3, I suspect would struggle to suck fuel up, and 1 will have more influence than 2. I am quite happy to be proved wrong on all counts  :D
Best regards


Offline Graham Meek

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Re: Model Carburetor Design Query
« Reply #3 on: January 14, 2014, 06:58:02 PM »
Hi Chuck,
Version 3 would as Roger has already said be more likely draw air instead of fuel. Version 2 is Edgar T Westbury's design for his Seagull, Seal etc engines, it works but the size of the hole in reality needs to be quite a bit smaller than as he originally designed. When I did my Carburettor experiments with the Seagull engine I found the size of the orifice required was 25% of the original hole used by ETW. This worked out at about 0.008" (0.2 mm) diameter.

Version 1 would I imagine tend to improve the fuel mixing, a lot depends on the offset but with a lot of these ideas the only way really is to try them back to back.

My best regards

Offline gbritnell

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Re: Model Carburetor Design Query
« Reply #4 on: January 14, 2014, 08:34:41 PM »
Hi Chuck,
I'll try not to make this too long winded. In automotive carbs there are what they call air bleed holes. These are in fact ported into the fuel supply channels but for a slightly different purpose, to prevent the fuel from siphoning through the passages once there is no vacuum signal. They do add a small bit of air to the fuel flow. Motorcycle carbs have and air passage which might be called an emulsifier. It is an air cavity ported into the fuel passage which in fact helps break up the fuel droplets. When you consider the size of our carburetor and the amount of fuel they flow I don't know how a person could accurately calculate what size air bleed would be needed.
Now the air bleed (airplane type) carbs that a lot of us use is a little different animal. When the throttle barrel is wide open the main jet needle is adjusted for optimum performance. When the throttle barrel starts to close the venturi effect starts to increase because of the reduction in the air passage. This in turn creates a greater vacuum and starts to pull more fuel. When the barrel is at the idle position the vacuum signal is at it's greatest and the engine is running (if at all) way too rich. To overcome this condition the air bleed hole is ported into the throttle barrel hole (which is now almost 90 degrees to the carb bore. By doing this the vacuum signal is reduced and therefore the fuel flow. The air bleed port is adjusted by a screw that's threaded across this passage. When the engine is idling the air bleed screw will lean out the mixture to where it's about 15:1 (We hope). When the engine is idling if the air bleed port is wide open and the engine is still rich then the port size needs to be increased. If the throttle is returned to the idle position and the engine stalls (too lean) then the air bleed screw needs to be closed up a little.
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