1959 Plymouth Belvedere (230 Flathead Six) – Runs Fine Cold, Then Gradually Loses Power and Stalls After 10–15 km

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1

Hi everyone,

I’m hoping someone with experience on old Mopars can help me track down a problem that is driving me crazy.

Vehicle

  • 1959 Plymouth Belvedere
  • Original 230 Flathead Six
  • 12V system
  • Mechanical fuel pump
  • Carter B&B 1-barrel carburetor

Symptoms

The car starts immediately and runs perfectly when cold.

After about 10–15 km (6–10 miles) of driving, it begins to:

  • lose power gradually
  • stumble and hesitate
  • feel like it’s running out of fuel
  • start jerking under load
  • eventually stall completely

It is not an instant shutdown. The power fades away slowly as if the engine is being starved of fuel.

After it stalls, I have to wait about 10 minutes.

Then it starts again and runs normally until the problem repeats.

What I have already replaced

To rule out ignition problems, I installed:

  • New Blue Streak UC12X ignition coil
  • New Standard Motor Products DR70 condenser

The result:

Absolutely no change.

The car still stalls in exactly the same way after roughly the same distance.

Ignition

Because the failure remained identical after replacing both the coil and condenser, I am beginning to think the ignition system may not be the cause.

The engine does not suddenly cut out.

Instead it slowly loses power and starts bucking before it dies.

Fuel Related?

I am now wondering if this could be:

  1. Vapor lock
  2. Weak mechanical fuel pump when hot
  3. Blocked tank vent
  4. Restricted fuel line
  5. Partially clogged tank pickup screen
  6. Something inside the carburetor

Additional Information

  • Modern fuel is being used.
  • The problem appears only after driving for a while.
  • After cooling down for around 10 minutes, the car runs normally again.
  • The symptom feels exactly like fuel starvation.
  • The problem occurs regardless of the new ignition components.

Questions

  1. Does this sound more like a fuel delivery problem than an ignition problem?
  2. Has anyone experienced similar symptoms on a Flathead Six?
  3. What would be your next diagnostic step?
  4. Could vapor lock cause symptoms this consistently after roughly the same driving distance?

Any ideas would be greatly appreciated.

Thanks!

Michael

2

Michael @nollenberger, I don’t have any insight into this particular problem - but I know someone who might. I’m copying @RICKYMOPAR here in hopes he can weigh in. Ricky, have any thoughts?

3

Michael @nollenberger , @RICKYMOPAR has this to say:

Clogged sock in tank.

I believe you can remove the sending unit without dropping the tank. They’re readily available in the U.S., e.g., Filter Sock 4 7/8" 5/16 Mopar.

Inspect it, and keep us updated. Good luck!

Dan

4

hey thanx for your fast reply… i bought already a new set. I’m going to try to install it now and take a look at the tank with a camera—since I didn’t replace that one. Is there any guidance on how to remove the filler neck so I can take the tank out?

5

There’s a lot of other discussion in this thread, but read from here on down: