51 plymouth hesitates at crusing speed

[Seanemry]

Just picked up the plymouth 2 days ago some what new to classic cars. I have alot of hesitation at higher rpms. At 45 50 it hesitates then clears up for a bit. Any suggestions? It has been converted to a 12v as well as a mallory ignition. 217 flat 6

Edited August 8, 2016 by Seanemry

[knuckleharley]

Sounds like it might be a fuel delivery problem. That's the first area I would look at if it were me,anyhow.

 

I'd start by installing a new see-though fuel filter before the fuel pump,and another one between the fuel pump and the carb. The old sediment ones with glass bowls work well for this because you can see any trash floating in them,and you can quickly dump them and clean the filter in the bowl before replacing it.

 

If you get a lot of trash in the filter/sediment bowls,you know it's time to remove the gas tank to flush it out and maybe coat it inside. 

 

You might also want to test the fuel pressure between the fuel pump and the carb to make sure you have between 4 and 6 lbs of fuel pressure.

 

Make sure you rev the engine up to highway RPM's to get accurate tests.

 

If none of this helps,then look at the ignition to see if it is breaking down at highway RPM's.

 

BTW,if the car has been sitting for a long time,you might as well go ahead and budget for a new fuel pump and water pump right now because it is going to need them before long.

[Seanemry]

Thank you for the reply. The car has sat for the year with old gas but was driven alot last summer. It is running extremely rich.

[dpollo]

Treat your car to an engine tune up by the book. Take a compression test, install new plugs, points and condenser, carb overhaul

and new in-line gas filter.  Clean and re oil the air filter. Retorque head bolts and set the valve tappets if they are noisy..

These cars are very nice runners but require routine maintenance just like any other car of their era.

[Seanemry]

I changed the plugs and rebuilt the carb. The airfilter is aftermarket but clean. I dont have a condenser because of the mallory ignition box. Cap and rotor look good. No plug wires are arcing. Im going to check my points and gap them tonight. Think it could be a coil?

[Don Coatney]

I once tried the pictured Mallory setup on my car. On the initial test run the engine just died while sitting at a stop light. I was able to re-start after a few minutes and I drove straight home and removed this stuff. Went back to the stock setup and never had another problem. Have you ever driven the car with your Mallory ignition?

 

 

[Seanemry]

I purchased the plymouth with it already installed. It ran great until i put on about 40 miles. I talked to the previous owner who told me it was put on 4 years ago when he converted to a 12v system

[Don Coatney]

Have you tested the Mallory system to insure you are getting spark?

[Seanemry]

Im not sure how too. Could you walk me through it?

[Don Coatney]

Would be basically the same procedure as shown in your shop manual. Power going to the coil? Power going to the unit etc.

[Seanemry]

This car is driving me crazy. New plugs, cap, rotor, coil. Rebuilt carb adjusted points and its still missing.

[Jeff Balazs]

Do you have the literature on this Mallory system. Could it be an extended duration type system?........if so that may also be the problem.

Getting a extended spark system timed correctly may be next to impossible on an engine like this.

Jeff

[JerseyHarold]

May be a weak valve spring that becomes more noticeable at highway speed.  Have you connected a vacuum gauge to check for unsteady readings?  If your distributor still has its advance mechanism, check to see if it moves freely or if there is a hole in the diaphragm.