Intermittent idle problems

ChicagoBob

ChicagoBob

Senior Member
I'm trying to diagnose an intermittent carb problem. Zenith in a '30. The car runs fine most of the time, but will occasionally stall at a stop sign and then fail to idle when restarted. Carb is clean, float is properly set and idle mixture screw is open 1.5 turns.

I initially suspected a clogged idle or compensator jet, and so pulled the carb bottom, checked the jets, and the car ran fine for a while.

The next time it occurred, I just flushed the sediment bowl, then removed the carb drain plug and ran some fuel through the system for a few seconds, and the car then idled well.
What seems strange is that a fuel contaminant would affect only the idle circuit. Could I be looking in the wrong places? Would merely opening the GAV a bunch flush things out?

What do you experts think?
 
If you have not removed the welch plugs then it is time to do so. Then clean the dirt out of the passage ways with a torch tip cleaner. Plug the passages back up.
Wick
 
Might also look at Orange county model a ford club web site(los angeles area club of mafca). Under technical articles a procedure for rebuilding the carb has an alternate technique for cleaning the carb circuits. Bob
 
The "idle mixture" screw only adjusts the amount of air for the idle circuit and is very much interacts with the throttle idle setting which is dependent of ignition timing. Just to say one sets it a 1.5 turns tells me it is not tuned per procedure explained in the link.

http://www.model-a.org/idle_adjustment.html

Because of the interaction between idle setting and the air adjustment the procedure may have to be done a couple of times to work up to the sweet spot.

The GAV has very little affect at idle when the carb is tuned properly because there is little to no air movement in the venturi to act upon the GAV jet.

GW
 
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As with most if not all carburetors, one and one half turns off the seat is only the beginning setting for the idle mix screw. If the idle mix screw is turned clock wise it richens the mix, counter clockwise leans the idle mix. Opening the gav a bunch will cause a rich mix and a rough lopey idle. My thought is that the float is not set correct.
 
You mention it stalls occasionally when stopping at a stop sign. Is that not a sign of improper float height?? Mine used to do that until I "played" with float level
Paul in CT
 
"I'm trying to diagnose an intermittent carb problem. Zenith in a '30. The car runs fine most of the time, but will occasionally stall at a stop sign and then fail to idle when restarted. Carb is clean, float is properly set and idle mixture screw is open 1.5 turns."

If this only hapens on hot days I'd say you have fuel boiling problems. I had the same thing happen when the temps were mid 80's and hotter, until I quit using gas with ethanol.
 
Thanks, but I don't have a source of non-ethanol fuel. Wouldn't the problem clear up once the engine cools? Mine doesn't.

Maybe I'll try Sta-bil with each fill-up. I am going to go through the carb again and make sure the passages are clean and the float set properly.
 
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As a test, remove the idle jet, put your finger over the end next to the threads and suck on the other end. You should not be able to suck any air. If you can it is an indication that there is an air leak either along the stem where it may have split or at the junction where it is swaged together.

I have found a number of idle jets like this. In operation it is sucking air and making the idle mixture leaner. I found one in a friends carburetor yesterday that would not idle well and stalled at stops.

Tom Endy
 
I suffered with the same problem for years--I'm convinced it's vapor lock, although those are fighting words to some folks. I tried numerous carburetors, distributors, ignition systems, fuel line insulations, fuel additives, etc., but the final fix seems to have been a somewhat more modern carb adapted for the Model A, the Zenith 13922, sometimes disparaged as the "forklift carburetor". You can find previous threads on it using search, if you're interested in going that route.

2010parts009.jpg


Steve
 
I have a simular problem with a loping engine. It's annoying. It's like it's getting too much gas. It seems to straighten itself out when it gets warmed up, and then I can open the gav. I have the gav closed all the way & it still does it when it is cold. Last carb I had on did the same thing. Both were supposed to be rebuilt & bought through the parts suppliers.
 
I think my problem has been caused by the float hinge pin sliding out of position and allowing the fuel level to rise too high. I can't find any other explanation, as every passage and jet is clean and operating properly, and this problem occurs only intermittently, and is cured by removing and replacing the bottom of the carb (and checking the float pin at the same time).

I mashed one end of the pin and pressed it onto the hinge. We'll see if that solves the problem.

Thanks, everyone.
 
ChicagoBob said:
Thanks, but I don't have a source of non-ethanol fuel. Wouldn't the problem clear up once the engine cools? Mine doesn't.

Maybe I'll try Sta-bil with each fill-up. I am going to go through the carb again and make sure the passages are clean and the float set properly.
Click to expand...

Marvel Mystery Oil is worth a try.
 
Just saw your other thread where you said you found the problem. But I still stand by MMO. :)
 
Re: MARVEL'S MYSTERY OIL

broommaker said:
How much marvel's mystery oil would you use per gal of gas????
Jim
Click to expand...

It depends on the particular conditions, but usually I use about 1 oz/gal. But I don't use it all the time; just real hot weather or high altitude driving.
 
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