'39 engine stumbles only under acceleration

[Bob Good]

I own a wonderful '39 Deluxe Tudor restored in 2016 by previous owner. I live in a very rural area of S/E Georgia with no available help. The car drives and runs great with one exception which is getting worse. If I am gentle going through the gears all is well. If try to accelerate quickly the engine stumbles and I have to let off on the throttle. This happens not based on rpm but torque. I can race the engine under no load, and it will not stumble. Visually, all the ignition system looks new. The engine has a '42 Crab distributor, the coil mounted on the fender well. It has a Holley 94 2bbl carburetor and a Isky 3/4 cam. Love the dual exhaust tone. This is my first post, and I am truly grateful for the opportunity to ask for your help.

[Mart]

Hello Bob, welcome to the forum.
Do you have another rotor arm and distributor cap you could try? from your (very good) description it sounds like a problem I had with either cap or rotor. The spark was finding an alternative path to ground when the combustion pressure is raised (wide throttle).
Mart.

[deuce_roadster]

Welcome Bob,
You didn't mention if your car had been switched to 12 volts. If it has, and an extra hot coil had been chosen the issue Mart described would be aggravated by it. If your coil is on the inner fender panel, the high voltage coil wire is necessarily long and under acceleration might be moving over and shorting on something also. The answer can be found and sometimes it takes a while to find it. Without the car running, if you look into the carb and you move the linkage as if you were flooring the car do you see a healthy squirt of fuel? When it stumbles, try pulling the choke out a little and see if it smooths out. If so, the problem is likely in the carb. Others here will have good trouble shooting ideas. If you were my neighbor, I would put the distributor on my Sun machine and make sure everything is as it should be. Good luck and let us know what you find.

[Flathead Fever]

Like Deuce Roadster mentions above. If it immediately falls on its face when you step on the pedal and then recovers the accelerator pump might not be squirting. With then engine off, look down the carb and see if it squirts.

When the carburetor switches between the different fuel circuits there is a momentary loss of fuel. The accelerator pump is there to give it an extra squirt while its switching between circuits. Without that squirt it momentarily runs out of gas.

[aussie merc]

some things that may help you as stated by the other knowable gentlemen check that the accelerator pump [air cleaner removed engine off look down carby throat and slam throttle open ] should see it squirt fuel 2 change spark plugs and make sure the gap is correct for your engine 3 fresh fuel always helps

[oldbugger]

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Check your spark plugs for even burnt light brown color, I had this with a bad plug

[Fairlane514]

May also check the power valve. Any excessive black smoke at idle?

[Fairlane514]

The constant tension band clamps are one of the better designs. They can expand and contract with heat and maintain a seal. They come on newer cars, but work very well.

[SoCalCoupe]

These are all good answers. I've been chasing down a stumble in my 1941 and it's turning out to be the sum of a lot of things. Every fix makes it a little better. Look at these:
1. accelerator linkage. if you tweak the accelerator pedal just a little, do you see throttle move on the carb? I didn't. I tightened up the linkage bracket that was bolted to the firewall, helped a lot.
2. Also the fitting from the accelerator rod to the carb. The internal spring on 'pull' fitting was shot. I'd tweak the accelerator and the linkage rod would move but not the lever on the carb. Bought a new fitting from Speedway motors, that helped a lot.
3. Do you have the original vacuum wipers. I do. When I discovered that they don't work I thought 'vacuum leak.' I disconnected the tubing from the manifold and put a rubber cap on it from Autozone. That helped.
4. moving on to timing. I backed out the vacuum brake on the distributor half a turn. That helped. Some people on this forum say back it out all the way.
5. My next adjustment is the basic timing at idle. I'm going to advance it slightly and see what happens.