Carburetor vacuum leak question ...

Crazy Horse · Apr 26, 2019

I'm working on a motorcycle carb here but the question could apply to tractor, cars, or trucks with carburetors so here goes.

I'm reading an article on line that says that a vacuum leak can happen between the carburetor air intake throat and the airbox (air cleaner). For example, visualize an air cleaner that has a rubber boot tube that hooks up to the intake throat of a carburetor. If the boot has a hole or crack in it, can this result in a "vacuum leak"? It would be my understanding that a vacuum leak would have to be between the carburetor and the engine (not between the carb and the air cleaner). If that could be a vacuum leak, then how could an engine even begin to run with the air cleaner removed?

Charlie M · Apr 26, 2019

The old tractors can have a bad gasket between the carb and manifold, the manifold to the engine and rusted through holes in the intake part of the manifold that can all suck air and make the tractor run bad but it will still run unless something is way gone. I could see on a motorcycle where everything is smaller such a leak causing it to not run at all.

timcasbolt · Apr 26, 2019

(quoted from post at 19:22:25 04/26/19) I'm working on a motorcycle carb here but the question could apply to tractor, cars, or trucks with carburetors so here goes.

I'm reading an article on line that says that a vacuum leak can happen between the carburetor air intake throat and the airbox (air cleaner). For example, visualize an air cleaner that has a rubber boot tube that hooks up to the intake throat of a carburetor. If the boot has a hole or crack in it, can this result in a "vacuum leak"? It would be my understanding that a vacuum leak would have to be between the carburetor and the engine (not between the carb and the air cleaner). If that could be a vacuum leak, then how could an engine even begin to run with the air cleaner removed?

Yes, you have it right. A leak between the air filter and the carb means the engine is ingesting unfiltered air. A vacuum leak is between the carb and the engine.

Dean · Apr 26, 2019

Sounds to me that the author of the article does not know that about which he writes.

Yes, small changes in intake vacuum did/do result from removal of the air cleaner due to the restriction of the filter element.

Though technically a true statement, such small changes in intake vacuum resulted in imperceptible changes in carburetor performance prior to the advent of electronic controls in 1980 and beyond.

One with a vacuum carburetor synchronizer (once sold by JC Whitney and others) MIGHT have been able to detect removal of the air cleaner had there been provision to use the synchronizer with the air cleaner installed, but carburetors of the day, even sophisticated carburetors such as the Rochester 2GC, 4GC and Quadrajet, did not meter fuel accurately enough to compensate for the change unless the air cleaner was clogged or otherwise restricted.

Back in the day, we would remove the air cleaner/filter so that we could hear the Whaaaaaaaaa when we tromped on the throttle. Though we thought that we were now making much more HP, it made little or no difference if the air cleaner/filter was properly maintained. It didn't matter if it was Mom's 283 Powerglide or your first 389 tri-power GTO.

Dean

Ian Beale · Apr 26, 2019

Another example of aircleaner effect. A local here was carting logs to the mill with an ex-WW2 Ford "Blitz" with flat head V8. With what was judged a "fair overload" the normal.

There was one particular hill on the run where the rig would just make it if tackled flat in first gear. But only if you first stopped and took the air cleaner off.

rustyfarmall · Apr 26, 2019

Yes. A vacuum leak HAS to be between the carburetor and the engine. A leak on the air cleaner side is just that, a leak.

Janicholson · Apr 26, 2019

I disagree with my fellow YTers, though I do understand their position. The reason the leak you discuss is a vacuum leak is that the air cleaner (no matter what type) provides a slight restriction. this causes a leak as described to have a vacuum (though slight) applied to the break or hole. This does two things, it provides an entry point for abrasive dirt, and changes the effective mixture. Jim

JMOR · Apr 26, 2019

(quoted from post at 16:54:27 04/26/19) I disagree with my fellow YTers, though I do understand their position. The reason the leak you discuss is a vacuum leak is that the air cleaner (no matter what type) provides a slight restriction. this causes a leak as described to have a vacuum (though slight) applied to the break or hole. This does two things, it provides an entry point for abrasive dirt, and changes the effective mixture. Jim

rue enough, but certainly splitting hairs!

timcasbolt · Apr 26, 2019

(quoted from post at 21:35:05 04/26/19) Another example of aircleaner effect. A local here was carting logs to the mill with an ex-WW2 Ford "Blitz" with flat head V8. With what was judged a "fair overload" the normal.

There was one particular hill on the run where the rig would just make it if tackled flat in first gear. But only if you first stopped and took the air cleaner off.

I've seen that situation before, but it was because the air cleaner housing was preventing the throttle plates from opening completely.

Dean · Apr 26, 2019

Bingo.

Splitting hairs, as indicated in my earlier post.

Dean

Crazy Horse · Apr 26, 2019

Thanks boys ..... so basically, the writer was wrong, maybe splitting hairs as someone said but still wrong. Made no sense to me because I have run many vehicles, tractors, and bikes without an air cleaner (for a short time) and there was no difference in engine performance or at least I couldn't detect any.

coshoo · Apr 26, 2019

Dean, for most of us, your last sentence needs to be modified slightly. "Mom's 283 Powerglide or your '49 Chev 216." I'm still waiting for my first 389 Tri-power GTO.

rustred · Apr 26, 2019

the vacuum is below the throttle plates when they are almost closed. the vacuum is at the highest when the engine is idling and the throttle plates are darn near closed. when you stomp on the gas pedal the vacuum goes to near zero. what that guy is saying is I would call it a reduction in air pressure kinda like the venturi effect in a carburetor and draw air in from out side the housing. I don't think it is classed as an actual vacuum above the throttle plates.

David G · Apr 26, 2019

I suppose both sides are technically vacuum as it takes pressure differential to move air, but I always thought of vacuum downstream of throttle plate.

Hobo,NC · Apr 26, 2019

Its gonna depend on what carb your scooter is run'N...

Mikuni's never liked to run open... BTDT A false air leak would affect them... A different air filter would affect them I have seen then run the fuel bowl dry on hard acceleration from just installing a different air filter on them... You had to re-jet it to satisfy it...

Without knowing what set up and the article I am not gunna call the author a dumb chit till I read it...

Many a scotter carb is mounted to the intake are cylinder head with a rubber insulator YES a leak there will affect it... How bout a link to whut you have read...

T.R.K. · Apr 26, 2019

Well, technically, the writer was NOT wrong.

Most of your major run issues will result from air leaks anywhere after the throttle plate. That said, to ensure compliance with EPA emissions requirements modern equipment must not allow any air to bypass the air filter or factory designed intake equipment.

I work in the small engine field. Even the same make and model engine will have different jets in the carburetor depending on whether that engine has the regular or heavy duty canister type air filter installed on it.

Since with the right equipment you should be able to measure a vacuum anywhere beyond the air filter, the writer was not wrong.

And yes, I know that a clogged air filter will change the fuel mix, but its supposed to be serviced (replaced) before it gets to that point.

MarkB_MI · Apr 27, 2019

Most motorcycle carbs are "constant velocity", meaning there's a vacuum-actuated slide that acts as second throttle so the mixture doesn't go lean when you open the throttle. I wonder if the writer was referring to this system.

showcrop · Apr 27, 2019

I agree that the author is probably correct because he would have been referring to the slide type constant velocity carb like the Amals that all the english bikes had and the extremely popular Mikunis that so any jap bikes had and probably still do. I am restoring a '67 Datsun roadster that has SU type carbs that are the slide type.

Dean · Apr 27, 2019

I never had one either, Coshoo, but I drove a few.

Friend had (still has) a 1965 Royal Bobcat GTO with tri-power 421 and 4.33 gear.

Dean

dr sportster · Apr 27, 2019

Between the carb and filter is not a vacuum leak. Between the mixing chamber and the engine is a vacuum leak which will be diagnosed by erratic idle.

Carburetor vacuum leak question ...

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