Unanswered Carb Adjustment Question

[braol]

So, things are going great with the new-to-me '31 Tudor. It starts, runs , and drives quite well and we have gone out in it almost every other day..to the store, local tavern, restaurants, etc... But here's what I have noticed:


Car starts fine on one full turn of the GAV. Advance the spark and the engine runs faster and smooths out. Wait for it to warm up for a bit and then when I close the GAV valve engine RPM slowly climbs until I'm at about only 1/4 turn open at idle. SO far so good...this mimics the instructions in the owner's manual. The issue is while driving. I find that while driving I only get good power with about 3 to 4 full turns of the GAV valve...any less and I get low power and even the occasional backfire under full throttle. Please note that I have sprayed WD-40 around the intake manifold and carb to see if I might have an air leak of some sort...to no avail.


I read that any more than one full turn of the GAV should make no difference, but that is not the case in my situation. I can start, run, and drive all day long with 3-4 full turns of the GAV valve but I get higher RPM (the "sweet spot") at idle with the GAV valve at the recommended 1/2-to-1/4 turn open. What gives?


My instinct tells me this: With the throttle closed the idle system is performing as it should. With the throttle open/accelerating I am lean...so there must be a "main jet" that is perhaps clogged? What puzzles me is that the GAV valve is supposed to have no effect beyond 1 full turn (or so) opened...yet is DOES obviously affect the mixture with the throttle opened up. Please help.

[31pu]

I've always left my GAV 3/4 open and leave it there just advance the spark all the way and go. Never had any problems with power.

[Kurt in NJ]

When my car did that I found brown crust in the tip of the main jet-- cleaned out then normal---- happened again--- determined it was fuel evaporating on the tip of jet, turned off fuel as backing into garage, run till carb empty, no more problems except once I forgot to turn off ignition-- dead battery

[braol]

Originally Posted by 31pu

I've always left my GAV 3/4 open and leave it there just advance the spark all the way and go. Never had any problems with power.

3/4 doesn't do it for driving. I can start the car just fine with 3/4 to 1 full turn...it's the driving around part when I need the 3 to 4 full turns for power.

[31pu]

look at your spark advance rod where it goes into dist. make sure its all the against the opening. It could be that your steering column has turned a little and its not advancing all the way.

[DHZIEMAN]

Sponsored Links (Register now to hide all advertisements)

I suspect something in the carb! Idle is fine and the GAV, on mine is set at one turn out and left alone. Having the GAV that far open is a tip of something else that is being masked by turning the GAV 4 turns open. Gotta be a fuel issue in the Main jet path. Gotta be a main jet flow problem being masked by the opening of the GAV! If you can, get a carb kit for your carb and clean it all and get into the main jet hole feeds with a small wire and really clean it and let it soak. Use High air pressure to blow everything out and then clean it again!

[Big hammer]

Time for a good carburetor cleaning! Zenith carbs have hidden passages that must be cleaned. Tom Endy has great info on the Zenith, follow his instructions and you’ll have a great running car. Santa Anna’s model A club has all of his instructions! I think?

[braol]

Looking at a flow diagram of the zenith I think either the Compensation Jet or the Cap Jet needs to be cleaned.

[Bob Bidonde]

Fuel Supply:
o Needs to be adequate for high speed running;

o Assure the gas tank vent in the cap is working;

o Assure that there is no add-on gasoline filter obstructing / reducing flow;
o Assure that the gas line is not over-inserted into the carburetor. The bead should be 1/8 back from the end of the gas line;
o Make sure the carburetor's float bowl is vented as shown in service bulletin;
o Make sure the gasket between upper & lower carburetor castings fits correctly;

o Float setting needs to allow maximum gasoline into the float bowl so that with engine OFF, gasoline levels are just below the exits of the cap and main jets;
o Gasoline passages in low carburetor casting need to unobstructed by debris;

o Cap & main jets need to have factory correct hole sizes to assure correct flow rates.

Low Speed Circuit:

•As Throttle opens past idle

•Air rushing thru Venturi sucks gasoline from the Cap Jet
•Gasoline flows from the Float Bowl
•Thru the Compensator & past the Idle Jet Well

•Into the Cap Jet

•Additional gasoline comes from Gas Adjusting Valve

High Speed Circuit

•As throttle opens low speed to wide open
•More air flows thru the Venturi
•Gasoline continues to fill the Float Bowl
•Venturi sucks gasoline from the Float Bowl directly to the Main Jet
•Gasoline flows continue from the Cap Jet & the Gas Adjusting Valve

[rosenkranswa]

Yep, clean the carb. Had exactly the same thing happen on three of my cars. Bad gas slowly gumming up the works.

Pull the carb and split in half. Clean the bowl and squirt carb cleaner through all of the jets until they flow freely, don't need to remove the jets to do that except maybe the Grosse jet. Reassemble and remount. Should be good as new.

I've started running the carb dry before shutting down if its going to set more than overnight.

[braol]

thanks for all the good ideas. I'll be poking paperclips through all my carb holes and jets this weekend. It should just be a simple cleaning since I don't have any big issues. I can start the car COLD with the hand crank and my spark advance checks out OK so it just must be some debris in the slow and/or high speed circuits (Main/Cap Jets).


I guess the only real confusion I have is why turning the GAV more than one turn makes a difference? Everything I've read says that turning it beyond this amount should make no difference...but it does in my case. The only thing I can think of is that the Compensator Jet is blocked and that the Cap Jet is getting fuel purely from the GAV. That would be why at least the Idle circuit seems unaffected. (I haven't driven fast enough to test the Main Jet...working on my wobble first...)