stumbles after 5 minutes

[70spider]

Hi everyone. I have developed a bit of an issue. The car starts fine and runs great for about 5 to 10 minutes then stumbles, and at time dies, when accelerating. I can restart it and limp back. Is this a fuel or ignition problem?

[18Fiatsandcounting]

A few things to check:

- Vacuum building up in the gas tank. Loosen the gas cap slightly and see if the problem is still there.
- Clogged fuel filter, debris in the gas tank, failing fuel pump, etc.
- Another (remote) possibility is that the fuel inlet in the tank is getting plugged up.
- Weak coil? Bad spark plug wires? Bad plugs, or the plug gap is too large? On the early cars, I set the plug gap to 0.025" or so.

When it stops running, pull the spark plugs and take a look. They should be a light tan or grey, but not black or oily. If the engine is running super rich, you could be slowly fouling the plugs although that usually happens at idle.

-Bryan

[70spider]

Vacuum building up in the gas tank. Loosen the gas cap slightly and see if the problem is still there.

ran it for a few minutes this time before problem occurred, loosened cap no change.

Weak coil? Bad spark plug wires? Bad plugs, or the plug gap is too large? On the early cars, I set the plug gap to 0.025" or so.

New coil, New wires and plugs were clean, a little rich but not to bad. the gap is @0.030 because I am running a pertronix kit.

Clogged fuel filter, debris in the gas tank, failing fuel pump, etc

New filters, new tank and new carter electric fuel pump (P4600HP)
It acts like a fuel starvation problem, i will just have to figure it out. I might go back to the block mount fuel pump.
Thanks for the help especially the vacuum idea I thought for sure that was going to be it.

[18Fiatsandcounting]

I agree it sounds like a fuel starvation issue, although I'm not ruling out an ignition problem caused by heat, such as a bad Pertronix unit.

Here's what I would do, although make sure you do this in a safe manner if you try it. Drive on a deserted road and wait for it to start stumbling. As soon as it does, immediately cut the engine off and coast to a stop. Remove the float bowl cover, at least enough to peer into the float bowl with a flashlight. See if you have at least an inch or so of gas in the bowl. Don't forget to bring the necessary tools along. And a cellphone in case you need to call someone...

A safer option would be to temporarily install a fuel pressure gauge in the car, with some long tubing running to and from the fuel line and carb. See if your fuel pressure starts dropping when the problem happens. That would eliminate an ignition problem.

A remote possibility is that the electrical connections to the fuel pump are cutting in and out. But I would think that would be a problem all the time, not just after the car has been driven 5 or 10 minutes.

-Bryan

[70spider]

Problem solved. The issue was the electric fuel pump. There was no way of mounting it below the level of the tank per instructions so I mounted it in the same location as the newer Spiders. This caused the problem, when the fuel level was below half a tank it would stumble. So i went back to the mechanical pump and drove it for 30 minutes with no problems.

[18Fiatsandcounting]

Glad to hear that you fixed the issue. I have not heard of that issue with electric fuel pumps, but thanks for letting us know.

-Bryan

[70spider]

I can only guess it is because the pump is a Carter rotary vain style and are a "push" type designed to be gravity feed.