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Carb vent modification question
Posted by: Saxon ()
Date: June 18, 2022 08:23AM

When exiting the offramp at a medium radius turn at 15-20 mph, the fueling cuts or is flooded.

I am reading about the first step remedy which is to install vent tubes inside and above the carb.

If the fuel is sloshing up from the float bowl and into the vents and then spilling into the carb intake, why the need for tubes inside the carb?

Northern CA
66 Monza Coupe 4-spd 140



Edited 1 time(s). Last edit at 06/22/2022 04:53PM by MattNall.

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Re: Car vent modification question
Posted by: azdave ()
Date: June 18, 2022 08:32AM

Saxon Wrote:
-------------------------------------------------------
> If the fuel is sloshing up from the float bowl and into the vents and then spilling into the carb intake, why the need for tubes inside the carb?

It is flooding. In a hard turn, fuel is forced to one side of the bowl where it accidentally spills out the vents and into the engine. By installing tall tubes (vent extentions) you have raised the level the sloshing fuel has to reach before it will spill over.

Dave W. / Gilbert Arizona
65 Corsa 140/4
66 Corsa 140/4
66 Corsa 140/4 w/factory A/C
66 Corsa 455 Toro V8
65 Monza Convertible 110/4
66 Monza Convertible 140/4 A/C
65 Monza 4DR 140/PG w/factory A/C
65 Monza 4DR EJ20T/5



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Re: Car vent modification question
Posted by: Saxon ()
Date: June 18, 2022 08:56AM

Yes, I understand, but why the need for tubes INISIDE the carb. Why not just outside the top vents and directing it up into the spider air box with a long flexible tube.

Northern CA
66 Monza Coupe 4-spd 140

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Re: Car vent modification question
Posted by: Saxon ()
Date: June 18, 2022 10:05AM

After further consultation with a racer, I am going to just drill into the top vents just enough to seat in some brass tubing without falling thru into the carb.

Then, add sufficient fuel line hosing into the spider intake air cleaner assembly to take up the volume of gas that would otherwise spill into the carb throat during my Andretti hard-to-port maneuvers.


It will certainly make cleaning up burrs easier.

Northern CA
66 Monza Coupe 4-spd 140



Edited 1 time(s). Last edit at 06/18/2022 10:06AM by Saxon.

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Re: Car vent modification question
Posted by: RobertC ()
Date: June 18, 2022 10:43AM

Saxon Wrote:
-------------------------------------------------------
> When exiting the offramp at a medium radius turn at 15-20 mph, the fueling cuts or is flooded.
>
> I am reading about the first step remedy which is to install vent tubes inside and above the carb.
>
> If the fuel is sloshing up from the float bowl and into the vents and then spilling into the carb intake, why the need for tubes inside the carb?

Because that is where the fuel is coming from -

But, given the sedate speed / force that this occurs, you could try lowering the float level a little.

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Re: Car vent modification question
Posted by: Timothy Shortle ()
Date: June 18, 2022 12:02PM

Installing these tubes is not a remedy for normal driving issues. I have heard that people that like to autocross can benefit from this modification but for normal street driving there is no need. Are you trying to fix a problem with these tubes? If "yes", what is your running fuel pressure? Float level adjustments? Are floats binding in any way or heavy? What kind of needle and seats are you running? In all the years I have owned Corvairs (since 1977) I have never had a need to install tubes.

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Re: Car vent modification question
Posted by: Frank DuVal ()
Date: June 19, 2022 09:55PM

Just how tight is this “medium radius” turn and what speed are going? I have not seen these tubes on a street driven only car.

Autocross tight turns, yes.

Float level too high? Fuel pressure too high?

Frank DuVal

Fredericksburg, VA



Edited 1 time(s). Last edit at 06/19/2022 09:56PM by Frank DuVal.

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Re: Car vent modification question
Posted by: Saxon ()
Date: June 21, 2022 04:08PM

I have a rudimentary understanding of carbs but improving.

Here is what I know:

The float setting was 1 & 3/16 to 1 & 1/4 on both primaries. I set them all to 1 & 3/16. The shop manual said 1 & 1/16-1/8".

The float angle is symmetrical but not fully parallel to the cover. About 10-15 degrees tilt upward away from air horn. I read this could help with lower fuel level.

I set the float drop to the factory spec.

I don't know what kinda needle and seat brands, but what Clark sells in their kit.

There was no binding in the float system. The floats passed the warm water float test.

The carbs spend a few hours in the dunk. All passages checked and blown.

What I don't know:

I don't know what the fuel pressure is. Have never measured such a thing before.

At hard left turns, Am I getting fuel starvation or fuel flowing up thru air vents.

Here is the repeatable scenario. I am exiting the freeway off ramp and braking hard in a straight line from about 50 mph. I estimate my turn in entry speed for a 90 degree left hand lighted intersection is about 25-15 mph (at least that what it feels like). A few feet past the apex, I apply throttle, but there is no response from the engine. When the car has exited the radius and is straight lined, the car responds to an increase throttle pedal.

Wish I could dispatch little green dwarfs in the bowls for observations. For now, the topside tubes have been installed and will get tested in a few weeks.

Northern CA
66 Monza Coupe 4-spd 140



Edited 6 time(s). Last edit at 06/21/2022 04:15PM by Saxon.

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Re: Car vent modification question
Posted by: dryenko ()
Date: June 22, 2022 02:31PM

Floats may actually be set too LOW

Bob C aka Dryenko
Dobson, NC 27017

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Re: Carb vent modification question
Posted by: Seth Emerson ()
Date: June 22, 2022 05:34PM

Floats set too low causes starvation on one side or the other, depending on the direction of the turn, as the fuel flows away from the jet. Just raising the floats up can cause the fuel to dump in. Adding the tubes can extend the range of float settings without letting the fuel "fall" in.

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Re: Carb vent modification question
Posted by: corventure Dave ()
Date: June 23, 2022 09:52AM

Check your float springs also. I have resolved a lot of carburetor issues by adjusting the float springs so the floats hang just about weightless.
I also resolved the hard turn cutout issue by keeping the floats full. Proper adjustment and I found that the ball type float valves Clarks sells flow more fuel than needle valves and solved cornering cut out issues. However these float valves need to be installed "properly" to function correctly.

Corventure Dave

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Re: Carb vent modification question
Posted by: isucorvair ()
Date: June 23, 2022 12:15PM

If you are losing fuel on a 15-20mph turn, You need to resolve a different issue, first, as others are suggesting.

I can take highway cloverleaf turns at 65mph on stock carbs, and not starve.

Now, tight turns on an autocross with quicksteering and wide sticky tires...that car needs vent tubes (And relocated jets!)


Eric P.
DeWitt, IA

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Re: Carb vent modification question
Posted by: RobertC ()
Date: June 26, 2022 12:41AM

isucorvair Wrote:
-------------------------------------------------------
>
> I can take highway cloverleaf turns at 65mph on stock carbs, and not starve.
>
> Eric P.
> DeWitt, IA

Depends on the age / design of the cloverleafs - they changed over the years.

You cannot do that (65 mph) on the Pasadena (CA) freeway built in the early 50's.



Edited 1 time(s). Last edit at 06/26/2022 12:43AM by RobertC.

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Re: Carb vent modification question
Posted by: JimBrandberg ()
Date: June 26, 2022 06:23AM

Sometimes I like to run the engine so the fuel level in the bowls stabilizes, then shut it off and loosen a gas line to drain into a paper towel to relieve the residual pressure. Then remove the carburetors to the bench and remove the tops to see where the fuel level in the bowl is at.
Although fuel level in the bowl rises and falls somewhat, it's nice to have some idea where it's at. I usually like the level to be a little under the "ramp" on the main jet side of the bowl. I'd sure like to hear if you have a different opinion.
Some of the needle and seats on the market may be a little large. I like the ones that are about .086, some are more like .102. There's argument about whether this makes any difference. Most of the time these days if I'm waging a carburetor battle it's because of fuel dripping from the venturi cluster when the engine is running.
I only bring all this up because it seems you need to figure out if you have a problem with too much gas or not enough.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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