Carburetors again

[Gordon]

On the last couple of engines I have been using a throttled air bleed carburetor which is roughly like ones designed by George Britnell and Chuck Fellows. I have not seen much difference in running when using the air bleed adjustment. On the last engine I was having the usual initial start/run problems so I actually plugged the air bleed hole and could not tell any difference.

Am I doing something wrong or does it work on some configurations and not on others?

Gordon

 

[Brian Rupnow]

Gordon--I don't see where the air bleed makes a lot of difference. I have built carburetors to George Britnels design, Malcolm Strides design, and to my own design,and although it may have a very small effect on idle speed, it doesn't make much difference.--Brian

 

[Gordon]

Brian: That was my experience. I am just finishing up my latest project, a Snow engine and I decided to clear up some of the clutter in my CAD drawing files. I frequently make partial drawings as I am building an engine to check fit etc so I have quite a few bits and pieces. I found that I have about eight different carburetor drawings, most of which are again partial drawings. I decided I should try to either complete them or at least consolidate them. I found that I had at least five different partial drawings or layouts for the Snow so I consolidated them into one sheet and deleted the individual drawings.

House cleaning time before starting the next project.

Next project will probably be an Atkinson Differential. I have to decide whether to make the castings or make it from bar stock. The alternative probably should be to try to sell some of the completed engines.

 

[stevehuckss396]

The air bleed is supposed to lean out the mixture at idle only. A good running mixture will typically be too rich at idle. The air bleed should not and is designed in a way not to affect the way it will run off idle.

 

[gbritnell]

Gentlemen,
I know this has been covered before so I'll try to be concise without getting too wordy.

First of all an engine will run rich but not lean. How long it runs rich depends on how rich it is.

Venturi: a reduction in diameter in the middle of a tube which causes the flowing air to increase in velocity and therefore creates a pressure differential between the large diameter and small diameter.
A carburetor is fed fuel through some type of metering orifice. It can be fixed or adjustable. The amount of fuel is determined by the flow of air through the venturi. In a carb like the type used on hit and miss engines or constant throttle engines once the fuel is metered correctly no further adjustments are needed.

Choke: a restriction to the flow of air through the carb. When in the closed position it creates a great vacuum condition which pulls more fuel through the orifice or jet.

A throttling carburetor like the type used on most variable speed engines has a venturi area which meters fuel through a jet placed in the middle of of the venturi area, or the area of lowest pressure (highest vacuum)
The throttling carburetor also has a butterfly or slide valve (motorcycle) which controls the amount of air passing through the carb. When this butterfly is in the almost closed position (low speed or idle) the air flow through the venturi is minimal so there is not enough vacuum to pull fuel from the main jet. To get fuel into the carb while in this condition a second set of ports are drilled where the edge of the butterfly meets the carb bore. At this restricted area another venturi is formed. As the butterfly starts to open the position of the venturi changes because the edge of the butterfly is in a different position relative to the wall of the carb so additional porting is added to still pull fuel at this low speed condition. In an automotive carb there is a slot which is known as the transition port. Once the butterfly is open wide enough so as to not provide any more venturi action then the main venturi takes over.

We'll disregard accelerator pumps and power valves as they aren't pertinent to our modeling carbs.

The purpose of an air bleed carb is to meter the fuel as close to the ideal mixture ratio throughout the entire throttling range. The air bleed carb controls the air flow through the carb by means of a rotating barrel positioned across the throttle bore. Generally this barrel has a smaller diameter through hole which creates the desired venturi affect at wide open throttle settings. This type of carb has a needle valve which controls the fuel flow through a small tube which projects into the center of the air stream. When the throttle barrel is rotated towards the closed position it now starts to act like a choke plate. It does create a venturi affect as the edge of the through hole rotates toward the bore of the carb but there is no porting at this point so it basically becomes a restriction to the air flow. This now choked condition starts to pull more fuel through the jet (see choke description above) In doing so the engine starts to run rich from the increased fuel flow. To compensate for the added fuel an air bleed hole is drilled into the throttle barrel of the carb at a point where it starts to be uncovered when the barrel is about 1/3 open.
By introducing this supplemental air flow the vacuum signal is decreased so extra fuel isn't pulled through the jet. The air bleed port has it's own adjusting screw which is no more than the tip of the screw passing by the air bleed port.

So you built a carb for your throttling engine and it doesn't have an air bleed port. You say it runs fine like that. As with all these carbs I have found that everything is a compromise. If your engine sits there and putts at low speed you can adjust the needle accordingly. When you go to throttle it up and it stumbles you adjust the needle a little to the rich side. Now it will rev up but when you bring the speed back down it has to go rich so you make an adjustment that suits both high speed and low speed but basically shaded toward the high speed setting.

On all of my engines I have air bleed type carbs. With the carb adjusted to where the engine runs the best (high and low speeds) I do this. With the engine warmed up and running at slow or idle speeds I put the tip of my finger over the air bleed port. Almost instantly the engine starts to falter due to the overly rich condition.

Hopefully this will clarify the purpose and use of the air bleed type carb.
gbritnell

 

[Gordon]

Thanks George. That makes sense. As long as I am content to have a good running engine at either high speed or low speed I do not need the bleed port. If I want both high and low speed the bleed port come into play at the low speed end and is pretty much ignored at high speed.