1951 Plymouth rough idle after hot

[JerseyHarold]

Just a WAG.....Could the exhaust manifold heat control valve be flopping around and intermittently restricting air flow through the engine?

[hep2jive]

18 minutes ago, JerseyHarold said:

Just a WAG.....Could the exhaust manifold heat control valve be flopping around and intermittently restricting air flow through the engine?

Hey there! The flap was removed.

[desoto1939]

Have you pulled off the distributor cap and rotor? Check to see if the breaker plate that holds the points and condenser is not loose. There should be some movement but no slop. A friend of mine has a 39 Desoto just like me and he was getting popping and stalling similar to what you are having. We took it to my old time mechanic and he pulled the cap and rotor and showed us that the breaker plate assembly was loose and needed to be replaced. Came back to my garage pulled the dizzy replaced with a NOS breaker plate and this solved the pbm.  If the plate has never been replaced suggest that you get a NOS one.

 

Rich Hartung

Desoto1939@aol.com 

[hep2jive]

2 hours ago, desoto1939 said:

Have you pulled off the distributor cap and rotor? Check to see if the breaker plate that holds the points and condenser is not loose. There should be some movement but no slop. A friend of mine has a 39 Desoto just like me and he was getting popping and stalling similar to what you are having. We took it to my old time mechanic and he pulled the cap and rotor and showed us that the breaker plate assembly was loose and needed to be replaced. Came back to my garage pulled the dizzy replaced with a NOS breaker plate and this solved the pbm.  If the plate has never been replaced suggest that you get a NOS one.

 

Rich Hartung

Desoto1939@aol.com 

Thanks I'll keep it mind. 

[Kilgore47]

Distributor cap - that's another thing to check.  When I got the P15 one of the things I found was that the plug wires were not pushed all the way down into the holes on the cap.  This caused the plug wires to arch from the cap to the wires and eventually burned the cap in those places.  Point is that it could be one thing or a combination of things that's giving you the problem you have.  Most likely a combination of things.  

 

The PO said they had a hard time getting the car to start and couldn't get it to stay running.  After going through all the systems I couldn't believe they could get it to start.  Just keep fixing all the things that have been discussed here and one day you will start it for a test run and realize that all is well again.

[Sniper]

40 minutes ago, Kilgore47 said:

When I got the P15 one of the things I found was that the plug wires were not pushed all the way down into the holes on the cap.

 

I ran into an issue where my plug wires popped off on me.  If you have good sealing boots, and you should, they can pop off when the air entrapped in the boot heats up and expands.  What you need to do is pinch the boot as you install it so that any excess air can vent out as you slide the boot over the plug or cap tower.  This should prevent this issue.

[hep2jive]

I had my trusty mechanic do the valve adjustment. It needed it. A valve was stuck and they needed the hot adjustment. Before the adjustment they made no noise but now when warming up I hear a slight clatter... better the clatter than too tight I hear people say. Tomorrow I'll do a test drive and see how it runs! 

[Saskwatch]

What a Beautiful car ! ??

[oldodge41]

I hope it runs as good for you as it looks. That is a pretty car.

[hep2jive]

56 minutes ago, oldodge41 said:

I hope it runs as good for you as it looks. That is a pretty car.

Thanks! If it doesn't I'M bricking it into the LAKE!

[keithb7]

My understanding is when cold, a slight exhaust valve tick is fine. When exhaust valves heat up they expand and the valve lash tightens. Then the tick goes away. A little wider lash means better exhaust cooling. Especially important at longer distance hi-way speed trips.  If lash it too tight, when the valves heat up, some may not be closing completely, leading to an engine miss. Most noticeable at idle . Eventually leading to a burnt valve and a dead cylinder.

 

The valve lash very well could have been your initial problem. It sounds like you have found a decent mechanic who understands these old flatheads. Not an easy thing to find!

Edited December 2, 2023 by keithb7

[hep2jive]

3 hours ago, keithb7 said:

My understanding is when cold, a slight exhaust valve tick is fine. When exhaust valves heat up they expand and the valve lash tightens. Then the tick goes away. A little wider lash means better exhaust cooling. Especially important at longer distance hi-way speed trips.  If lash it too tight, when the valves heat up, some may not be closing completely, leading to an engine miss. Most noticeable at idle . Eventually leading to a burnt valve and a dead cylinder.

 

The valve lash very well could have been your initial problem. It sounds like you have found a decent mechanic who understands these old flatheads. Not an easy thing to find!

Thank-you!  I love your videos! It seems to have been a valve

My mechanic said one was stuck and they needed adjusting.  Before I'd be cruising and after the car was warmed up it get rough and kinda slightly miss and jerk. Did it randomly. Musta been that stuck valve.

[hep2jive]

Thanks everyone! I'll update tomorrow after I drive some more see if it randomly misses but I think we're good for now lol. It still will vapor lock a little (After 15 minutes of hot engine being OFF and i turn it back on) but I think that's the winter gas. The car always turns right on and a few revs of the gas pedal gets it running smoothly again. If I get gas or or stop for 5 minutes it fires right up. Never had this issue until winter gas here in Chicago but oh well. 

[Los_Control]

Glad you got it ..... seems like there is always a little tinkering that needs to be done with these old cars.

 

I noticed with my old truck that is not driving on the road yet ..... after working on it for a few years preparing it for the road.

 

I noticed that when I first start it up the other day,  it is not running on all 6 cylinders ...  Seems after it warms up it is running on all 6.

It could be a bad plug or a wire .... Not worried about it. I just know it wants some attention. And it is telling me it wants some love & care.

 

They have ways of talking to you.

[Booger]

2 tone? wheel skirts? Locked that thing up at night bro. That thing is primo

Desoto 39 provided you that chart. Ignition parts are very specific. (And dont use Champion spark plugs)

[Sam Buchanan]

3 hours ago, hep2jive said:

Thank-you!  I love your videos! It seems to have been a valve

My mechanic said one was stuck and they needed adjusting.  Before I'd be cruising and after the car was warmed up it get rough and kinda slightly miss and jerk. Did it randomly. Musta been that stuck valve.

 

3 hours ago, hep2jive said:

Thanks everyone! I'll update tomorrow after I drive some more see if it randomly misses but I think we're good for now lol. It still will vapor lock a little (After 15 minutes of hot engine being OFF and i turn it back on) but I think that's the winter gas. The car always turns right on and a few revs of the gas pedal gets it running smoothly again. If I get gas or or stop for 5 minutes it fires right up. Never had this issue until winter gas here in Chicago but oh well. 

 

Hope your test drives prove the drivability problem has been addressed. However......for sake of education we need to return to the matter of semantics.

 

What your mechanic remedied with the valve adjustment (not valve job!) was a tight valve, not a stuck valve. A stuck valve is just that, stuck open which prevents compression from occurring in that cylinder. It is a mechanical defect that usually requires some engine disassembly to resolve.

 

What your mechanic did was restore valve lash via the adjustment procedure so the valve could close completely. It wasn't "stuck" open but was still operating normally with the exception of not having enough clearance between valve and lifter to completely seal the cylinder.

 

Now.....vapor lock. What you are describing with your hot starts isn't vapor lock but fuel percolation. Vapor lock is when fuel boils in the fuel line so a "bubble" of vapor develops which prevents the fuel pump from developing sufficient suction to pull and push fuel to the carb.

 

Fuel percolation occurs when fuel boils in the carb bowl while a hot engine isn't running and overflows the bowl sending fuel down into the intake manifold. This causes a fuel puddle to form in the manifold which in turn creates a rich mixture when the restart is attempted. Once the engine is running the rich mixture clears and the engine again idles smoothly.

 

Hope this helps your understanding of what you are observing and will allow you to converse in a more accurate manner when describing engine issues with your mechanic.

 

Enjoy your ride!

 

[hep2jive]

15 minutes ago, Sam Buchanan said:

 

 

Hope your test drives prove the drivability problem has been addressed. However......for sake of education we need to return to the matter of semantics.

 

What your mechanic remedied with the valve adjustment (not valve job!) was a tight valve, not a stuck valve. A stuck valve is just that, stuck open which prevents compression from occurring in that cylinder. It is a mechanical defect that usually requires some engine disassembly to resolve.

 

What your mechanic did was restore valve lash via the adjustment procedure so the valve could close completely. It wasn't "stuck" open but was still operating normally with the exception of not having enough clearance between valve and lifter to completely seal the cylinder.

 

Now.....vapor lock. What you are describing with your hot starts isn't vapor lock but fuel percolation. Vapor lock is when fuel boils in the fuel line so a "bubble" of vapor develops which prevents the fuel pump from developing sufficient suction to pull and push fuel to the carb.

 

Fuel percolation occurs when fuel boils in the carb bowl while a hot engine isn't running and overflows the bowl sending fuel down into the intake manifold. This causes a fuel puddle to form in the manifold which in turn creates a rich mixture when the restart is attempted. Once the engine is running the rich mixture clears and the engine again idles smoothly.

 

Hope this helps your understanding of what you are observing and will allow you to converse in a more accurate manner when describing engine issues with your mechanic.

 

Enjoy your ride!

 

Thank you very much! Yes valve ADJUSTMENT not a job. It must be percolating of gas then, not really vapor LOCK. Thanks for the clarification makes sense then! 

 

My mechanic knows I'm nuts I just say I think a tire is weird and it's a headlight switch. ?  

[kencombs]

12 minutes ago, Sam Buchanan said:

 

 

Hope your test drives prove the drivability problem has been addressed. However......for sake of education we need to return to the matter of semantics.

 

What your mechanic remedied with the valve adjustment (not valve job!) was a tight valve, not a stuck valve. A stuck valve is just that, stuck open which prevents compression from occurring in that cylinder. It is a mechanical defect that usually requires some engine disassembly to resolve.

 

What your mechanic did was restore valve lash via the adjustment procedure so the valve could close completely. It wasn't "stuck" open but was still operating normally with the exception of not having enough clearance between valve and lifter to completely seal the cylinder.

 

Now.....vapor lock. What you are describing with your hot starts isn't vapor lock but fuel percolation. Vapor lock is when fuel boils in the fuel line so a "bubble" of vapor develops which prevents the fuel pump from developing sufficient suction to pull and push fuel to the carb.

 

Fuel percolation occurs when fuel boils in the carb bowl while a hot engine isn't running and overflows the bowl sending fuel down into the intake manifold. This causes a fuel puddle to form in the manifold which in turn creates a rich mixture when the restart is attempted. Once the engine is running the rich mixture clears and the engine again idles smoothly.

 

Hope this helps your understanding of what you are observing and will allow you to converse in a more accurate manner when describing engine issues with your mechanic.

 

Enjoy your ride!

 

Excellent tutorial in the basics of troubleshooting lingo.   I might add that IMO, real vapor lock is really rare as it requires the pump itself to have vapor in the pump body.  Only if the check valves  and /or diaphragm are surrounded by vapor  will they fail to work.  As long as the pump is full of liquid it can push vapor that might be between it and the carb on through the needle/seat area.