1951 Plymouth rough idle after hot

[FarmerJon]

My p15 had a very similar problem. When it sat for 10+ minutes after driving it would be very hard to start. When this happened the carb base wpuld be wet with gas.  I added an under carb heat shield from a '55, but it made little difference.

Finally I lowered my float, and the problem seems to have gone away completely. It also seems to have leaned out the fuel mix a little. 

I only have a mechanical fuel pump, heat riser gutted. 

 

Your car is beautiful,  I am glad to hear you are using it as a daily!

[hep2jive]

Okay thanks everyone.  I'll try lowering the float a tidbit then. I should bend the float tab closer to the needle then. Is that correct? I hate messing with the float!

[hep2jive]

So I lowered the float. Didn't do anything. When the car is warmed up it started missing while driving... slightly but still missing. From take off at a stop or cruising at 40 mph... I'm starting to think maybe I need a valve job. I had the motor rebuilt last year and the rebuilder said after 50 hours of driving to do a hot valve job...I never did and 12k miles later here I am. It's still doing what I've stated (idles rough for a minute after the hot engine has been resting for 10 minutes plus) but drove fine until the entire engine was warm and started hesitation on starting from stop and when cruising it stumbled. Definitely something with heat it doesn't like. 

[Kilgore47]

Adjusting the valves while the engine is hot is probably the most accurate way to do it.  But I was too chicken to do that.  Burned hands and face while working that close to the hot exhaust manifold wasn't something I wanted to try.  I removed the manifolds and adjusted the valves on a cold engine.  That's a little extreme but I was replacing the intake manifold and it was a good time to adjust the valves with everything out of the way.  The valves can be adjusted with the manifolds on and if I ever have to do it again that's the way I'll do it. On a cold engine.  Just make sure you use the cold clearance settings for your engine. 

[Dave72dt]

You're going to have to determine if you have a fuel problem or an ignition problem.  You should be able to duplicate your problem sitting in your driveway.  Hook up a timing light and when the problem occurs check for consistent flashing.  Compare it to the engine when it's cold.

[greg g]

I didn't go through all the posts.  Did you ever pop the hood after dark?  If you see a light show that might give you some direction.  Before I put my Tractor Supply wires on, I had sparks arcing between wires, down the sides of spark plugs and within the loom tubes, and down the outside of the distributor cap.

[FarmerJon]

I adjusted my valves hot. With standard lenght wrenches and a split exhaust manifold, burnt hands are a real possibility.  I bought a pair of long "tappet" wrenches and 12" feeler gages for next time. 

Adjust the valves, but then i think you will still need to do some diagnostic work.

 

I would hook up a vacuum gauge, and note the readings at good idle, rough idle, cruise, and average acceleration. Check that your vacuum advance canister isnt leaking air. Check your points are clean, and that the dwell is in spec. Test your condenser, if possible. Check that the advance plate/points plat can move correctly and that the connector wires are not shorting out. Check for leaky sparkplug wires. Check your base ignition timing. Check if the carb is wet when it starts acting up. Check it for vacuum leaks. 

 

Once everything is in spec, you can start adjusting things, a little at a time, noting any changes in the vacuum readings as you go. 

It may not be one thing wrong, but a cumulative effect of several minor things adding up.

 

[Bob Ritter]

Great looking car, love the color combination...

[hep2jive]

I wonder if the car needs a valve job what are the symptoms? They don't clatter.

[DonaldSmith]

Baed on my one year of mechanical knowledge repeated sixty times, I would opine: 

 

Valve job needed: 

 

Low compression in one or more cylinders.

 

Vacuum gauge needle swinging widely in sync with engine speed.  

 

Magical suction test at the exhaust pipe, with dollar bill alternately blown out and sucked back.  

 

 

Edited November 29, 2023 by DonaldSmith
typo, out. space

[DonaldSmith]

Fess up. hep2jive, is that really your picture?  Recent?  or 60 years ago?

[Kilgore47]

If those spark plug wires have been on the car for the 10 years you have owned it then it's probably time to replace them.  

[hep2jive]

13 minutes ago, DonaldSmith said:

Fess up. hep2jive, is that really your picture?  Recent?  or 60 years ago?

That was January of 2023!

[hep2jive]

5 minutes ago, Kilgore47 said:

If those spark plug wires have been on the car for the 10 years you have owned it then it's probably time to replace them.  

Hmmm I put them on about 6 years ago. I've put thousands of miles on them. Maybe that's it

[Sam Buchanan]

7 hours ago, hep2jive said:

I wonder if the car needs a valve job what are the symptoms? They don't clatter.

 

Let's work on semantics a bit.

 

A "valve job" involves removing the head and valve train, either replacing or grinding the valves, grinding or lapping the seats and putting the engine back together. This is usually needed only after many thousands of miles of service since a previous valve job or engine rebuild.

 

A "valve adjustment" is a maintenance procedure that doesn't require any disassembly other than removing the valve covers on the side of the engine. This procedure is usually done every few thousand miles along with other periodic maintenance and tune-up work.

 

You stated earlier that your engine was rebuilt last year and your mechanic recommended checking valve adjustment after 50 hours of running. However, it has been 12K miles since rebuild (in one year!) and the valves haven't been touched. Time for a valve adjustment! Run that procedure and check back with us, there is a good chance drivability will improve.

Edited November 29, 2023 by Sam Buchanan

[hep2jive]

See the thing is idle cold or warm is great, driving the first 25 minutes is fine. It starts to miss at any speed sporadically after 25 minutes of driving. But then will get me home. Sometimes from a stop sign accelerating sometimes it won't. While cruising at 35 mph just sailing along then stumble!

[Kilgore47]

I think this has been mentioned.  In the dark, start the car - open the hood and look for sparks coming from the plug wires.  And adjusting the valves should be done.  12 thousand miles on a new engine - it's time.

[hep2jive]

3 minutes ago, Sam Buchanan said:

 

Let's work on semantics a bit.

 

A "valve job" involves removing the head and valve train, either replacing or grinding the valves, grinding or lapping the seats and putting the engine back together. This is usually needed only after many thousands of miles of service since a previous valve job or engine rebuild.

 

A "valve adjustment" is a maintenance procedure that doesn't require any disassembly other than removing the valve covers on the side of the engine. This procedure is usually done every few thousand miles along with other periodic maintenance and tune-up work.

 

You stated earlier that your engine was rebuilt last year and your mechanic recommended checking valve adjustment after 50 hours of running. However, it has been 12K miles since rebuild (in one year!) and the valves haven't been touched. Time for a valve adjustment! Run that procedure and check back with us, there is a good chance drivability will improve.

Okay thanks valve adjustment then! Never thought about the wording, thank you! 

[Los_Control]

Kind of a long thread so unsure if anyone has mentioned the coil? As it warms up a bad coil will get very hot to the touch and get sporadic ignition going in and out.

[Kilgore47]

2 minutes ago, hep2jive said:

See the thing is idle cold or warm is great, driving the first 25 minutes is fine. It starts to miss at any speed sporadically after 25 minutes of driving. But then will get me home. Sometimes from a stop sign accelerating sometimes it won't. While cruising at 35 mph just sailing along then stumble!

That sounds like valve adjustment.  The P15 did the same thing.  After adjusting the valves it was fine.

[hep2jive]

1 minute ago, Los_Control said:

Kind of a long thread so unsure if anyone has mentioned the coil? As it warms up a bad coil will get very hot to the touch and get sporadic ignition going in and out.

Hey thanks its a new coil I put on this year so it has about 8k miles since I installed it. I'll feel it when I heat up the car. Thank you!

[Los_Control]

It is my experience they get hot ..... possible it could still be bad and not get hot? I dunno.

If you still have your old one lying around you might try hooking it up and see if things change.

 

While your coil is new, with the quality of some parts today it would not surprise me to see it go bad  

[hep2jive]

1 minute ago, Los_Control said:

It is my experience they get hot ..... possible it could still be bad and not get hot? I dunno.

If you still have your old one lying around you might try hooking it up and see if things change.

 

While your coil is new, with the quality of some parts today it would not surprise me to see it go bad  

Very true! Okay I'll get another one see what happens!!

[hep2jive]

Just now, hep2jive said:

Very true! Okay I'll get another one see what happens!!

Whatever the issue is it's definitely heat related 

[Sam Buchanan]

Nearly every engine problem we deal with is heat related....the only problem my P15 engine has presented when cold is a puddle of oil....... ?