Effectiveness of "air bleed" screws on i.c. carburetors

[Brian Rupnow]

I have built and ran small i.c. carburetors originally designed by George Britnell and by Malcolm Stride. Both of these carburetors included "air bleed" screws in their design. Recently, I seen a post by George Britnell saying that he no longer used the bleed screws in his carburetor design. I myself have found that they don't seem to have much effect on how well an engine runs. They give a very small bit of influence on how the engine idles, but I question whether or not any improvement they may add will offset the bother of using them at all. I am soliciting opinions from other knowledgeable folks out there who have built and ran these small carburetors with "air bleed adjustment screws" on them.---Brian rupnow

[Jasonb]

Probably more use on engines that will actually get use rather than just bench run.

The mixture will be set for when they are running a or near max revs which may cause them to run poorly at idle so the air bleed is used to give a steady idle but still have the correct mixture higher up the rev range.

Our bench run engines tend to have the main needle set to give a steady tick over and they only rev higher when the throttle is given the odd blip and thats when they have little or no load.

[tangler]

The few simple carbs I've made for aero engines have been of the air bleed type.  I made a carb for my Wyvern without the air bleed and I think it runs far too rich when throttled back so I intend to add an air bleed (when the round tuits are more readily available).

Rod

[Brian Rupnow]

Jason--Thank you for your response. You're right, none of the engines I run are designed for high speed like an aero engine. I much prefer a slow, steady "tick over" and some mid range rpm rather than a "full throttle wide open" run. On the Malcolm Stride type carburetors, (as used on the Bobcat and Jaguar) an air bleed adjustment screw is something that can always be added "after the fact" if it is really necessary.

[Allen Smithee]

The airbleed only has any effect at all over the initial 10% of throttle opening (if that). At larger throttle openings the main intake is so much larger than the [tiny] bleed hole that its effect is negligible, and at anything more than (typically) 20% opening it's actually blocked off by the throttle barrel.

But having said that they can be VERY effective at correcting the mixture at idle in a simple carb. The OS10FSR had an airbleed carb which could be adjusted to give a really good idle and pick-up on a wide range of props, and the airbleed carb on the Oliver Tiger Major RC was stunningly effective, with adjustments as small as 1/2 turn on the screw giving noticeable results and the best throttle response I've ever seen in a diesel.

As most of your running it at idle speeds I'd suggest that you would be best advised to continue having adjustable airbleeds.

YMMV,

AS

[Graham Meek]

For my part I have found the air bleed that I have added during the evolution of my carburettor design was of most benefit when opening the throttle, ie flexibility.

Before the inclusion of the air bleed into the design there was always a flat spot as the engine revs started to rise. As pointed out already for bench running engines at a steady rpm, the advantage would not be noticed. However if the engine duty is such that it is expected to operate over a wide rev range then my advice would be to fit one.

My best regards
Gray,

[Brian Rupnow]

Thank you, gentlemen!!

[stevehuckss396]

I have built and ran small i.c. carburetors originally designed by George Britnell and by Malcolm Stride. Both of these carburetors included "air bleed" screws in their design. Recently, I seen a post by George Britnell saying that he no longer used the bleed screws in his carburetor design. I myself have found that they don't seem to have much effect on how well an engine runs. They give a very small bit of influence on how the engine idles, but I question whether or not any improvement they may add will offset the bother of using them at all. I am soliciting opinions from other knowledgeable folks out there who have built and ran these small carburetors with "air bleed adjustment screws" on them.---Brian rupnow

This is my own opinion but I think the air bleeds we use don't make sense to me. When using the carb off idle the air bleed is blocked, makes sense. At the idle position the air bleed is exposed to the top of the barrel hole and bleeds air through the barrel the same as if it were off idle sucking fuel with it. I believe the air bleed should short circuit the barrel and feed air straight to the bottom of the carb so no fuel is brought in with the air. Then I believe there will be a leaner mixture and a more noticeable difference in how they respond to the bleed. If the bleed air is fed into the top of the barrel the mixture won't change. I plan to modify the V8 carb to test my theory but time has not permitted.

[gbritnell]

Hi Brian,
You are correct in one sense in that I don't use the air bleed screws any longer. I guess whenever I stated that I should have been more clear. I still use the air bleed port but just not the screw. When I make a carb now I start with what I think the air bleed port should be (diameter) and run the engine. If it gets too rich when at lower or idle speeds I enlarge the port. If the engine stalls at idle I make a bushing and reduce the port size.
Having played with and adjusted many airplane engines the air bleed port does in fact change the response of the engine. Here's a test. Start the engine, adjust the needle for a higher rpm range, idle the engine, if it idles like you think it should now stick your finger over the air bleed port and you will find that it will definitely change the running of the engine.
The principle is this. Air flowing through a restriction (venturi) increases in velocity. The same amount of air that entered the carb now has to flow through a smaller area. The greater the velocity the greater the pressure differential or vacuum. On an automobile carb the main feed nozzles are located at this point for high rpm running and also on our air bleed carbs. Also on a full size carb the throttle plates are wide open. Now we go to idle. With the throttle plates closed the vacuum signal is now located between the edge of the throttle plate and bore of the carb. There is no longer a signal at the main venturi because the air isn't flowing fast enough. Between the edge of the throttle plate and the bore wall of the carb is the idle port. At idle fuel is drawn through this port. As the throttle opens and the vacuum signal changes position (it follows the edge of the throttle plate) there are secondary idle ports above the original port. Once the throttle plates are open far enough the vacuum signal switches back to the main venturi and in some cases a booster venturi.
So with this in mind on our carbs as the throttle barrel closes (much like the throttle plates) and the crescent shaped opening becomes smaller the vacuum signal intensifies. More vacuum means more fuel is being drawn. To compensate the air bleed port is uncovered. I design mine so that this occurs at about 1/4 throttle opening.
Is it a perfect transition, only if you're very lucky but it does make a difference.
With a single cylinder engine, hit and miss type,  you might not notice much change. On a model airplane engine you're talking idle around 1000 rpm and high speed, depending on the engine size, anywhere from 14 to 30 thousand rpm. Our engines never come close to this so they don't see that great of a transition. 
gbritnell

[Allen Smithee]

This is my own opinion but I think the air bleeds we use don't make sense to me. When using the carb off idle the air bleed is blocked, makes sense. At the idle position the air bleed is exposed to the top of the barrel hole and bleeds air through the barrel the same as if it were off idle sucking fuel with it. I believe the air bleed should short circuit the barrel and feed air straight to the bottom of the carb so no fuel is brought in with the air. Then I believe there will be a leaner mixture and a more noticeable difference in how they respond to the bleed. If the bleed air is fed into the top of the barrel the mixture won't change. I plan to modify the V8 carb to test my theory but time has not permitted.

This may appear to be the case, but it's about the physical configuration of the carb barrel and the airbleed hole. In all the examples I've had the airbleed hole is actually *below* the venturi constriction, as per the attached (exaggerated) sketch.

AS

[stevehuckss396]

With a venturi hole of .093 to .125 I find it hard to believe that the air slips under and does not take fuel with it. I guess the only way to find out is to bud an example and see what happens.

[Allen Smithee]

In a standard barrel throttle when the barrel is nearly closed the engine essentially has to suck through the spraybar hole such that the primary suction mechanism is simply the primary pumping effect of the scavenging rather than the venturi-produced pressure drop around the spraybar. The venturi effect (which meters the fuel draw with the throttle open) produces a pressure drop of perhaps 0.5-1psi, where the primary pumping  effect (as you feel on your thumb if you turn the engine over with a choking thumb) can produce ~5psi of suction. All the airbleed does is provide additional air to correct the mixture - this airflow goes nowhere near the spraybar, but even if it did it wouldn't really change the pressure around it by any significant amount.

You can actually *see* the difference between venturi pumping and scavanging-pumping by simply draining the fuel from the feed pipe and then putting a starter on the engine. Observe the fuel travelling up the pipe, which it may do (but slowly) under the influence of the venturi suction. Then choke the intake with a finger and what the fuel zoom up the pipe like a rat up a drainpipe as the full scavenging suction is applied...

PDR

[Jasonb]

Its not so much that it draws air in with out fuel, more a case of reducing the vacuum so less fuel is drawn in.

Think of it a bit like choking the carb, this reduces the amount of air that it can suck in and makes the engine run rich. Shutting the throttle will have a similar effect. The airbleed allows more air to enter the engien and as it is below the jet it will not affect the flow through teh venturi and over the needle.

[gbritnell]

You stated it better than I did Allen. On my engines I always have the top of the fuel tank just below the level of the needle valve. When I fill the tank the fuel can be visibly seen in the fuel line but it will not fill completely.
Upon cranking the engine over the fuel will fill the line and prime the carb. Once the line is full it will continue to use all the fuel until the tank is empty.
Steve, when you get the chance please try out your idea because you might have a better idea. I personally have spent countless hours making and modifying carbs to try and find the perfect one. My thoughts when moving to the air bleed style carb was that an actual engineer working for a model engine company had the resources and testing equipment to design a carb so I would just go with one of them and modify it a little.
gbritnell

[CHP]

For my part I have found the air bleed that I have added during the evolution of my carburettor design was of most benefit when opening the throttle, ie flexibility.

Before the inclusion of the air bleed into the design there was always a flat spot as the engine revs started to rise. As pointed out already for bench running engines at a steady rpm, the advantage would not be noticed. However if the engine duty is such that it is expected to operate over a wide rev range then my advice would be to fit one.

My best regards
Gray,

This is the answer. this is the reason why 2 stroke engine builders are using a variable venturi and a various jet setting " the angle or shape of the needle it self are trying to give you the best mixture money can buy. Can this be applied to a 4 stroke Yes and they where Stromberg carb with having fuel mixture following the air demand changing the oil thickness of the oil