Problems at higher RPMs

[Guest Shoe1932]

Hello all.

Have a 39 Plymouth P-8 and she starts and drives well. Unfortunately at high RPMs in each gear it acts like it is starving for gas. In 3rd gear it starts at about 45-48 MPH. I thought it might be the flexible gas line which runs from the metal gas line to the carburator. I have seen where those lines can get clogged in the past. It was not clogged and it has not made a difference. The carburator was rebuilt a few years ago and should be good. What are some other ideas or possibilities? I am thinking about putting some carb cleaner down the carb when it is running in case a jet is having some issues. All ideas are welcome. Thanks in advance for your help

[broker-len]

better to put it in the tank ,,maybe the jets are gunked up

[F&J]

I fixed a 47 Ply for a local guy with same issues. He was taking the car to the same old-time shop for a few years of ownership, and it never ran right.

I pulled the hand choke just a bit, either sitting still while holding the rpm to about 1800-2000, or driving at a steady road speed. That made all the difference.

So i took the "the NORS factory rebuilt carb" apart and there was nothing wrong as far as dirt or plugged main jet, so I started gradually reaming the main jet. Took two tries to get it perfect.

I later read in an old manual, that Mopar had optional jets for "economy drivers", which are very lean. I also think methanol requires a bit more jet IMO

[hchris]

Hello all.

Have a 39 Plymouth P-8 and she starts and drives well. Unfortunately at high RPMs in each gear it acts like it is starving for gas. In 3rd gear it starts at about 45-48 MPH. I thought it might be the flexible gas line which runs from the metal gas line to the carburator. I have seen where those lines can get clogged in the past. It was not clogged and it has not made a difference. The carburator was rebuilt a few years ago and should be good. What are some other ideas or possibilities? I am thinking about putting some carb cleaner down the carb when it is running in case a jet is having some issues. All ideas are welcome. Thanks in advance for your help

A couple of other things to consider, (I am assuming you have the original Carter BB carb installed) float level required is 5/64" if this is set low then the carb will starve for fuel at higher speeds.

There is also a high speed enriching device known as stepup piston and rod, the way this mechansism works is a vacuum operated piston holds down a metering rod in a high speed jet at lower rpms, as the throttle opens vacuum falls off and a spring under the piston lifts the retainer plate and rod out of the jet to deliver a greater (enriched) fuel flow, obviously if its not working correctly then the higher rpm range suffers.

If you lift the top off the carb you will see a screw holding a retainer plate at the edge of the float bowl, it holds down the spring loaded vacuum piston; at the other end the retainer plate has a thin wire metering rod which locates inside the high speed metering jet at the bottom of the float bowl.

With finger pressure on top of the retainer plate compress the spring loaded plunger, then release the spring tension and note that the the whole mechanism rises smoothly, any hesitation here means that during normal operation you will not get the correct mixture at high speed; it is very easy to kink the metering rod during reassembly.

[StillOutThere]

It could be ignition related, particularly the points. Replacement points today come from a variety of sources, some of very poor quality. One of my cars had the same problem and just two weeks ago a new set of points totally resolved it.

[Guest jaj63207]

On my '52 Plymouth, about 45 years ago had the same problem. Turned out that the rubber fuel line was clear when I looked at it but when vacuum was applied, the line sucked closed. There was enough fuel in the float bowl to run a few seconds before fuel starvation occured.

Good luck,

Jimmy