Why does engine run best with GAV almost closed

[Go doc]

My engine runs best with GAV nearly closed. It will start that way also. The carb is totally ok and has been rebuilt professionally.

[Tom Endy]

The GAV controls the amount of fuel that reaches the cap jet in addition to what it receives from the comp jet. The comp jet is much smaller that the cap jet. If the GAV is closed the only fuel reaching the cap jet is through the comp jet. When you open the GAV it ports fuel around the comp jet and the cap jet receives much more fuel.

For everything to work correctly each jet in the carburetor should be flow tested to be sure they are within the deign spec. Most reproduction jets are wide of the mark.

My car runs best with the GAV open about a half a turn. More than that and it will run rich. As long as you can control how your car runs with the GAV it is probably ok. Your comp jet may be on the large size.

Tom Endy

[Bruce_MO]

I really recommend using flow-tested jets. I installed a set from Renners Corner and it made a noticeable difference in pep and smooth performance.

[Jwilli]

One of mine runs best at about 1/4 open. The 30 runs best almost closed. I am no expert, but my understanding is to adjust to where it runs best.

[BILL WILLIAMSON]

There's no one MAGIC setting for the GAV. Carbs VARY, WEATHER, ALTITUDE, HUMIDITY varies & on & on.
With a warm engine, run @ a steady 25 MPH in SECOND GEAR, adjust GAV for best running & LEAVE IT ALONE!
AND, if your GAV affects your idle, YOU'RE IDLING TOO FAST.
I CAN'T believe all the confusion with things like setting the dad blamed timing & where should I set the GAV? Some seem to WANT to COMPLICATE a SIMPLE car. Would a MIDOL help me?
Bill W.

[Mitch//pa]

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Make sure your proff reblt carb has a proper set of flow tested jets and that your float level is set just right.. They make all the difference...

[Ron in Quincy]

If idle and comp. jets are correctly sized you should be able to close the GAV and the engine run OK.

Running with the GAV open will reduce your MPG during normal driving.

I believe the Owners Manual tells you to close the GAV for normal driving.

Ron

[Tom Endy]

I would be very careful about driving at cruising speed with the GAV closed. This causes the engine to be running lean and increases exhaust gas temperatures (EGT) that could do damage to the engine.

Several of us have constructed an engine running stand that is equipped with a number of sensors, including an EGT sensor. We noticed that when the GAV was closed the EGT would increase by quite a bit. The carburetor on the test stand has re-sized jets installed.

It is also a good idea to open the GAV more and retard the spark slightly when climbing a long steep grade.

Tom Endy

[Larry Seemann]

You don't say which carb you have. If it happens to be a "B" carb they are designed to run with the GAV closed after warm-up.

[ocpd44]

So, I gues the next question is, how do you know it's running right. I notice a difference from a full turn to only a quarter turn open but not much (if any) between a quarter turn open and fully closed.

Regards,

[steve s]

I think your situation is a cause for pride, not concern. It means that you've got a nice tight intake system that is not leaking any air in that needs to be balanced by extra fuel enrichment (i.e., opening the GAV). The most common problem with old carbs is a worn throttle shaft: Jillions of vibrations jamming against the throttle linkage takes a toll, and provides a leakage path for more air to leak in, causing the car to run too lean. The problem is, this is a project--there's no easy fix, so in my experience many rebuilders just clean it up, clear the passages that can be reached, change the jets and gaskets, paint it up and hope for the best.

Steve

[Chuck Sea/Tac]

What do you think it should be running at? And why?

[Mike V. Florida]

Welcome to the site Go doc. Do you have the Model A owners manual?

This is from an owners manual (bolding is mine),

The section on starting

3. If the engine is cold, turn the carburetor adjusting
rod one full turn to the left to give it a richer mixture
for starting. This rod serves both as a choke for
starting and as an enriching adjustment. Next pull back
the rod, at the same time pressing down on the starter button
with your foot. The instant the engine starts, withdraw your
foot from the starter button and release the choke rod, next
advance the spark lever about half way down the quadrant.
When the engine warms up, turn the adjusting rod back to the
right. Never drive continuously with adjusting rod more than
¼ turn open.

The section on carburetors

Adjustment of Carburetor
The method of regulating the carburetor for ordinary driving
.conditions is to tum the carburetor adjusting rod to the right
until the needle just seats, then turn the rod back approximately
one-fourth of a turn.

[BILL WILLIAMSON]

Minervas' Zenith was SUPERB, in ordinary temps, she would start with NO CHOKING, plugs burned a light TAN & ran fine with the GAV CLOSED!
I only used 1/4 turn of the GAV, or had to choke her, when the temps got down to 28 to 32 degrees. I did run a 160 thermostat, which really helped. Fortunately, I DON'T have a COLD garage.
Bill W.

[Kurt in NJ]

Originally Posted by Mike V. Florida

Welcome to the site Go doc. Do you have the Model A owners manual?

This is from an owners manual (bolding is mine),

The section on starting

3. If the engine is cold, turn the carburetor adjusting
rod one full turn to the left to give it a richer mixture
for starting. This rod serves both as a choke for
starting and as an enriching adjustment. Next pull back
the rod, at the same time pressing down on the starter button
with your foot. The instant the engine starts, withdraw your
foot from the starter button and release the choke rod, next
advance the spark lever about half way down the quadrant.
When the engine warms up, turn the adjusting rod back to the
right. Never drive continuously with adjusting rod more than
¼ turn open.

The section on carburetors

Adjustment of Carburetor
The method of regulating the carburetor for ordinary driving
.conditions is to turn the carburetor adjusting rod to the right
until the needle just seats, then turn the rod back approximately
one-fourth of a turn.

If you read further in the owners instruction book--regulating gasoline mixture--"for economical driving, reduce the quanity of gasoline in the mixture by turning the adjusting rod to the right as far as possible without affecting the operation of the engine"
the next paragraph---"On long trips some drivers make a practice of driving with the adjusting rod turned all the way off"


I have run my car under load with an emission machine hooked to the tailpipe, both with regular gas and ethonal gas, with regular gas my carb runs a little rich (4% CO gav off), with ethonal gas--gav off is too lean at 0.05% co, my jets are a good compromise for both fuels, , I checked the mixture up to 3200 rpm, baisically I wanted to know the best positions for my GAV, up to about 40 the car will run fine closed/1/4 turn, above 60 it wants almost 1/2 turn to have CO in the 2-4% range that the car runs good and gets good fuel mileage

[40 Deluxe]

So in other words, Go doc, "Since it's not broke, don't try to fix it"! This is a better idea than "Fix it until it's broke"!

[bobpo1]

Last summer I had been having backfiring problems a lot. It was nerve racking. I had the carb rebuilt and now I open the GAV 1/8 to 14 turn open. This has eliminated the backfiring. However if I close the GAV and drive my '30 for a distance, it will backfire when I turn the engine off.

[40 Deluxe]

Originally Posted by bobpo1

Last summer I had been having backfiring problems a lot. It was nerve racking. I had the carb rebuilt and now I open the GAV 1/8 to 14 turn open. This has eliminated the backfiring. However if I close the GAV and drive my '30 for a distance, it will backfire when I turn the engine off.

C'mon! Where's your inner teenager?? We used to throw the spark lever up and close the GAV when going downhill. A high school friend proudly boasted that his "popped 40 times" going down a hill!

[BILL WILLIAMSON]

Originally Posted by 40 Deluxe

C'mon! Where's your inner teenager?? We used to throw the spark lever up and close the GAV when going downhill. A high school friend proudly boasted that his "popped 40 times" going down a hill!

At about 30 MPH, Chief would turn the key OFF & hold the throttle WIDE OPEN for a ways, then turn the key back ON & "KABOOOOW"---Momma would get FURIOUS!!!
Bill W.