The Ford Barn

ssg57 · 03-31-2023, 12:38 PM

I have a 31 Model A PU I purchased recently.
Being new to the A's I am slowly learning them.

My problem is the engine surges at idle and low speed while driving.

I've have replaced the Modern Plate condenser and points (gap is .020), new plugs (gap .035) Timing is set with the Nu-rex tool as directed. The Marvel Schebler carb is clean and the flange is straight, not warped. I'm having to run the GAV between 1/2 and 3/4 open. I've sprayed starting fluid around the intake with no change. The compression in all cylinders are 62psi. Good filtered fuel flow from the tank to carb. Float level is good. Checking the play in the distributor shaft and bushing I feel like it is ok.

It was running good until a week ago when it died and there was no fire to the distributor. Which I traced to the bottom plate was being shorted out. Once I solved that issue I got the truck running, but now I'm have the surging issue.

Thanks Scott in Texas

Gene F · 03-31-2023, 01:04 PM

Ignition tends to be good, or failed with no inbetween. The fact that you open the GAV and it helps leads you towards the fuel system. This time of year, have you tried a bit of drygas?

AL in NY · 03-31-2023, 06:13 PM

Surging is usually caused by a lean fuel mixture. Try adjusting the idle air screw on the top of the carburetor.

mcgarrett · 03-31-2023, 08:56 PM

Turn the idle mixture screw at the very top of the carb slightly counter-clockwise to enrichen the mixture and take note if this adjustment eliminates the surging. Sometimes it takes adjusting both the gas adjusting valve needle and the idle mixture screw together before you find the sweet spot that the carb likes to idle at.

If these adjustments don't seem to make any appreciable change in performance, you may need to disassemble the carb and clean all the passageways with spray carb cleaner and compressed air. It's possible that there may be some foreign material blocking the proper flow of air and/or fuel.

SAJ · 04-01-2023, 04:27 PM

On a Zenith one must screw the mixture adjustment inwards(closing the air path) to enrichen, not counter clockwise . It is an air control valve, not a fuel valve.
SAJ in NZ

ABento · 04-01-2023, 09:00 PM

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Pull the carb apart and check the jets, most likely have a piece of crud in one.
I believe the idle adjustment does adjust the air fuel mixture, counter clock wise does emmit more fuel in the idle circut.

03-31-2023, 12:38 PM	#1
ssg57 Junior Member Join Date: May 2012 Posts: 4	31 Pickup Surging at Idle and Low Speed I have a 31 Model A PU I purchased recently. Being new to the A's I am slowly learning them. My problem is the engine surges at idle and low speed while driving. I've have replaced the Modern Plate condenser and points (gap is .020), new plugs (gap .035) Timing is set with the Nu-rex tool as directed. The Marvel Schebler carb is clean and the flange is straight, not warped. I'm having to run the GAV between 1/2 and 3/4 open. I've sprayed starting fluid around the intake with no change. The compression in all cylinders are 62psi. Good filtered fuel flow from the tank to carb. Float level is good. Checking the play in the distributor shaft and bushing I feel like it is ok. It was running good until a week ago when it died and there was no fire to the distributor. Which I traced to the bottom plate was being shorted out. Once I solved that issue I got the truck running, but now I'm have the surging issue. Thanks Scott in Texas

03-31-2023, 01:04 PM	#2
Gene F Senior Member Join Date: Mar 2012 Location: Ohio Posts: 1,973	Re: 31 Pickup Surging at Idle and Low Speed Ignition tends to be good, or failed with no inbetween. The fact that you open the GAV and it helps leads you towards the fuel system. This time of year, have you tried a bit of drygas?

03-31-2023, 06:13 PM	#3
AL in NY Senior Member Join Date: May 2010 Location: Southern Upstate New York Posts: 1,160	Re: 31 Pickup Surging at Idle and Low Speed Surging is usually caused by a lean fuel mixture. Try adjusting the idle air screw on the top of the carburetor. __________________ AL in NY

03-31-2023, 08:56 PM	#4
mcgarrett Senior Member Join Date: Dec 2013 Location: Waxahachie, Texas Posts: 949	Re: 31 Pickup Surging at Idle and Low Speed Turn the idle mixture screw at the very top of the carb slightly counter-clockwise to enrichen the mixture and take note if this adjustment eliminates the surging. Sometimes it takes adjusting both the gas adjusting valve needle and the idle mixture screw together before you find the sweet spot that the carb likes to idle at. If these adjustments don't seem to make any appreciable change in performance, you may need to disassemble the carb and clean all the passageways with spray carb cleaner and compressed air. It's possible that there may be some foreign material blocking the proper flow of air and/or fuel.

04-01-2023, 04:27 PM	#5
SAJ Senior Member Join Date: Jul 2010 Location: Auckland, New Zealand Posts: 515	Re: 31 Pickup Surging at Idle and Low Speed On a Zenith one must screw the mixture adjustment inwards(closing the air path) to enrichen, not counter clockwise . It is an air control valve, not a fuel valve. SAJ in NZ

04-01-2023, 09:00 PM	#6
ABento Senior Member Join Date: Dec 2010 Location: San Luis Obispo Posts: 237	Re: 31 Pickup Surging at Idle and Low Speed Sponsored Links (Register now to hide all advertisements) Pull the carb apart and check the jets, most likely have a piece of crud in one. I believe the idle adjustment does adjust the air fuel mixture, counter clock wise does emmit more fuel in the idle circut.