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Does Size Matter?: A Talk About Airbleeds


Photo: Henry Ricardo's Engine designed for Daimler in 1917 for British Tanks which became the first mass

produced internal combustion engine in Britain. (The Tank Museum, Dorset England)

( This was in response to a post on a carb forum.I didn’t want to comment on the thread, but this post was a valuable teaching lesson, so here we go.)

Engine in question has a Thumper camshaft, 243/257@.050 duration with modest lift. These camshafts are designed for sound mostly, performance is second. I would imagine in his scenario there is idle vacuum in the 6-8 range. Carb is a 950


Lets unpack: This particular person was having issues leaning out at part throttle when applying more throttle. He wrote (and I quote) "I changed the power valve from a 5.5 to two times larger a 3.5”

Lets start with that. Power valves do not have sizes, they have opening points. Power Valve Channel Restrictions (PVCR) are the two holes under the power valve. That controls the fuel added when the PV opens.

By going from a 5.5 to a 3.5 he was delaying the opening of the power valve and subsequently delaying the addition of fuel.

As we all know, the main jets provide fuel to the main circuit. In addition the power valve adds fuel when vacuum drops. The stamped rating on a power valve is the vacuum required to hold the valve shut. To make fuel come in sooner, the opposite direction makes more sense, ie use an 8.5 PV.


Next item to discuss. Drilling throttle plates. What does it do? -Adds unmetered air into the engine. -Raises idle speed -Leans idle mixture significantly

This is often suggested when a radical camshaft is used and you cannot maintain transfer slot opening. Makes sense, add air, close plates, reduce t slot exposure. Typically you need to enlarge idle feed to compensate for this unmetered air. The exception is when the carb is already way too rich. In a camshaft as detailed, you need a lot of fuel at idle due to the weak signal provided. IE high vac pulls hard, low vac pulls soft, so you enlarge idle feed restriction.

In many extreme circumstances this procedure makes sense. In the Original posters case, it was likely unneeded. It could have still helped reduce slot exposure.


Oh, and if you are going to drill plates, you should probably remove them from the baseplate, or at the least debur them so there aren't metal chips falling in your engine.

Now the poster also drilled out the High Speed Air Bleed (HSAB) and Idle Air Bleed (IAB). We will also discuss the Idle Feed Restriction (IFR).


How does it work? Ok, your ifr provides ALL idle fuel. The fuel splits and goes to both the transfer slot and the mixture screw. All of this fuel in not pumped or leaked into the engine, it is pulled by the vacuum created through the pistons moving downward. Pressure differential really, but lets not get tied up with semantics.

The total fuel is provided by the size of the restriction. In most cases this ranges from .026-.038 which is a large difference in area. The only means of countering or shaping this fuel that is pulled is via the IAB.

This is an "air corrector" and it functions to both time and shape the fuel entering the idle circuit. Yes it mixes air into the fuel and makes an emulsion of sorts, but lets ignore that at the moment. Lets take an ifr of .033 and a iab of .070 to use as an example. In this case the fuel is pulled steadily and meets engine needs. If we were to reduce this iab to .060 we have richened the circuit, but more than that we have lengthened it. Why so? With a smaller suction hole in the circuit it takes less pressure differential to pull. If we enlarged the iab to .080 we do the opposite. Not only are we adding more air into the idle fuel, we are also lessening how hard the fuel is pulled. The old example is putting a hole in a straw.

So by the original poster drilling his idle air bleed out to some undetermined size, he has leaned the mixture, which is already lean due to him drilling four holes in the throttle plates. But he has also reduced the duration of the idle circuit. Large hole - less draw on the fuel. Opening the throttle blades reduces the vacuum/pressure differential the idle circuit sees and so cuts off the fuel being pulled significantly.

The only way to counter this is to enlarge the IFR so less pull is needed to provide fuel. This was not done.

The same scenario plays out with the boosters/main circuit and hsab. (lets ignore emulsion bleeds at this time as it's a whole nother thing) A larger hsab reduces the signal that is seen by fuel in the mainwell. It will also lean the mixture overall by adding more air. A smaller Hsab increases the signal seen by the booster as there is less of a leak in the system. This provides earlier flow. It also richens the mixture overall.

This is most easily noted at what rpm the booster begins to flow. Lets pick an arbitrary engine speed. In the exact same carb, if the main circuit comes online at 2500rpms with a .028 hsab, drilling the hsab to .035 may make it not start until 3000rpms. if we reduce it to .025 the boosters may start earlier at 2000rpm.

An often ignored factor is fuel level in the float bowls. As would seem obvious, the fuel needs to be lifted out of the metering block and through the boosters. Lowering fuel level requires more air flow to pull the fuel. Raising float level requires way less. The hsab and emulsion help by aerating the fuel so it is lighter. This lighter fuel emulsion also responds way faster to changes than solid fuel would. It is obvious when you think about it.

This is also why when looking at average HSAB in different sized carbs, it makes sense (assuming the same emulsion bleeds) 550cfm .033 600cfm .031 715cfm .029 780cfm .028 850cfm .025


Why is this? in the 550cfm carburetor, the signal seen by the booster is high due to the very small venturi, so the main fuel sees a much higher draw. Take the 850cfm carburetor, the venturi is massive in comparison. In order to get the main fuel to flow at all, it needs a very small break in the line, so the fuel itself can be pulled with ease with less pressure differential.


So, back to the original post. By drilling the throttle plates, the engine admits more unmetered air and less fuel is pulled from the idle well. By drilling the idle air bleed, the idle circuit is leaner and gets phased out sooner. By drilling the high speed air bleed, he was delaying booster draw.

By drilling a handful of holes, a large gap between circuits has been created. In a normal, well functioning carb, with steadily increasing the throttle. the engine starts rich, leans out as light throttle is applied. As speed comes up and throttle is increased, the engine richens again as booster draw begins. It will continue to lean out until the power valve opens.

Anyway, this is what goes through my head... and also why I typically avoid posting or even reading most carb forums.

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