'55 runs rough, backfires and stalls out at highway speeds

[Pete986]

I never even thought about the carb.
I'm going to check the air filter now, too.

I don't ever remember this happening in 2nd.

Thanks for your input!

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[Grahamp]

If the issue does not happen in 2nd gear at an equivalent rpm, then it is unlikely to be fuel or air related . Does the problem occur quickly at 60mph or does it take awhile? If it takes awhile, you may have to stay in second gear for an equivalent amount of time to see whether the problem repeats in which case it is definitely an rpm issue and sounds like a fuel/mixture problem. If the motor quits fairly quickly, you might have an ignition coil problem, probably a good idea to replace the coil and put in a new set of points and a new condenser just to be sure it is not an ignition problem - not expensive to replace

 

[Pete986]

Just got back.

I took the air filter off, it looked pretty cruddy. How do I replace it? It seems to be 1 unit.

I drove it with the whole air assembly off, the car seemed a little smoother at slower speeds.
But again, the problems occurred at 60MPH. It's pretty immediate. It starts running rough, then it back fires. It takes a little while longer for the car to actually stall, and I let off the gas to avoid it.

This definitely does not happen it 2nd. I kept it there and let the RPMS get up there. No issue

 

[Grahamp]

Very mysterious! Let me think about it, I will get back to you

 

[Grahamp]

I conclude that you should replace the ignition coil. The fact that you cannot duplicate the problem in second gear means that the problem only occurs when the engine is under greater load, in other words it is taking more mechanical effort to drive the car in top gear than in second. It is possible that the spark is too weak to create enough power to overcome the greater load and the engine stalls. Ignition coils break down as they age and cannot keep up with the engines demand for high voltage, high frequency sparks at each plug. I have had coils exhibit this type of behavior. Might be a simple and inexpensive fix. Assume you have checked the vacuum advance on the distributor, as well as the timing ?

 

[Pete986]

OK.

I will pick up some parts tomorrow

 

[Grahamp]

Has the carburetor been rebuilt?

 

[biddle]

I had posted a few weeks ago about this issue.

Please don't double post on the forum. Find your original post and reply to it. I've merged the topics together.

 

[Pete986]

I truly have no idea.

I had found that people stopped responding on that post

 

[biddle]

I truly have no idea.

I had found that people stopped responding on that post

That happens, but you should not create a new post for the same topic. Thanks.