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The effects of an air leak into a float bowl

General engine tech -- Drag Racing to Circle Track

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Chargermal
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The effects of an air leak into a float bowl

Post by Chargermal »

Recently my AFs went mad at idle.I assumed a blocked air bleed, but discovered that one of the Edelbrock top plate screws had stripped the thread, so the top plate was not sealing.

A tap and cap screw fixed the problem.

But...me being me..I’d love to know how a top plate leak can effect my idle mixtures etc?

Can someone draw a picture of what went on?

TIA
Geoff2
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Re: The effects of an air leak into a float bowl

Post by Geoff2 »

Assuming we are talking about Edel AFB/AVS/QJ carb. No air leak as described will not affect idle, as carbs already have float bowl vents to the atmosphere.
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Re: The effects of an air leak into a float bowl

Post by mag2555 »

The fuel bowls must be referenced to atmospheric pressure as it's this pressure differential that allow the bowls to feed the carbs fuel circuits.

It's this effect that's makes gas coming out of a gas can pass out very slow until you open the vent hatch on the can.
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Re: The effects of an air leak into a float bowl

Post by Circlotron »

For most carbs I've seen, the fuel bowls are referenced to the carb side of the air cleaner. That way, any air cleaner restriction won't have much of an effect on the a/f ratio. If the air cleaner was restrictive, a leak into the float chamber would upset the pressure balance achieved by the bowl vent to the carb inlet and cause the mixture to go somewhat richer because the leak stops the reduced air pressure at the carb inlet going through the vent tube to the float chamber "pulling back" on the fuel headed toward the idle circuit.
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Re: The effects of an air leak into a float bowl

Post by 408swinger »

I have had an AVS2 600 and 800 with top plates that seemed to rock a bit while installing . I wondered if that would be an issue but figured they would seal once torqued doen .
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Re: The effects of an air leak into a float bowl

Post by Walter R. Malik »

Air is supposed to freely enter the float bowl with an NA engine.
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Re: The effects of an air leak into a float bowl

Post by Circlotron »

Everybody else has the opposite opinion to me...
Let me state it a different way. You drive through a dust storm and the air cleaner becomes super clogged up. With the engine idling there is now a partial vacuum where the air enters the carb, courtesy of the clogged air cleaner. This partial vacuum would normally cause extra fuel to be sucked out of the idle circuit and into the airstream, causing things to go way rich. However, if we add a path from the carb air entry (below the air cleaner) to the float bowl this partial vacuum will also act on fuel that has not yet entered the idle circuit, so the partial vacuum that is trying to suck additional fuel into the airstream is exactly countered by itself on the opposite end of the idle circuit. Its like a tug-of-war, if we add one man on the left end then an equal man on the right end then nothing changes, and that’s how we want it. The only time a float bowl could be exposed to the atmosphere without ill effect is if there was zero restriction to the inlet air stream. If there is an air leak into the float bowl the vent tube can’t do its job of countering the effect of inlet restriction causing richening of the the mixture.
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Re: The effects of an air leak into a float bowl

Post by Schurkey »

Chargermal wrote: Mon Jul 12, 2021 4:19 am Recently my AFs went mad at idle.I assumed a blocked air bleed, but discovered that one of the Edelbrock top plate screws had stripped the thread, so the top plate was not sealing.

A tap and cap screw fixed the problem.

But...me being me..I’d love to know how a top plate leak can effect my idle mixtures etc?

Can someone draw a picture of what went on?

TIA
The top plate no longer sealed a fuel or air-bleed passage?. It's not that the float bowl itself was affected, it was the passage that was affected. Air leaking into an otherwise calibrated fuel or air passage would tend to lean the idle mix.
Chargermal
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Re: The effects of an air leak into a float bowl

Post by Chargermal »

I think the ‘tug of war’ scenario is probably what was happening. Either way, the new cap screw and tapped thread appears to have resolved the issue.

Thanks all for your views.
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Re: The effects of an air leak into a float bowl

Post by BOOT »

Circlotron wrote: Mon Jul 12, 2021 6:40 am For most carbs I've seen, the fuel bowls are referenced to the carb side of the air cleaner. That way, any air cleaner restriction won't have much of an effect on the a/f ratio. If the air cleaner was restrictive, a leak into the float chamber would upset the pressure balance achieved by the bowl vent to the carb inlet and cause the mixture to go somewhat richer because the leak stops the reduced air pressure at the carb inlet going through the vent tube to the float chamber "pulling back" on the fuel headed toward the idle circuit.
I think the reason that the bowl vent is inside the air cleaner, is to reduce emissions while not running.
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