1951 Plymouth rough idle after hot

[hep2jive]

I have a 1951 Plymouth Belvedere.  Runs great until I turn the car off. Wait 15 minutes, come back and the idle is chugging. Would this be too lean or too rich? After about 45 seconds it gets back to normal with me reving the motor. Seems like my idle speed is a high, but it lowers after driving a few miles.  It noticed it did the same when sitting at a long stop light I had to give it gas to have it smooth out. It only did that once at the light though... I'm thinking it's something carburetor related. Any ideas? Thanks!

[Doug&Deb]

Sounds like fuel percolation, what most people refer to as vapor lock. Usually it causes hard starting and stalling until under hood temperatures come down. It’s also possible that the carb needs rebuilt. When was the last time you did a tune up? Does the problem occur when the engine is cold or only after being shut off? 

[Los_Control]

What you describe sounds like vapor lock? Seems unlikely in your area at this time of year though.

 

When you shut the car off the cooling system no longer is working and the engine heats up .... modern fuel sitting in the carburetor & the fuel line from the pump to the carb will heat up & turn to vapor. .... Kinda sounds like yours is right on the verge of doing so ... it will run but not smoothly til you get the engine running and the fuel moving through the lines again.

 

Same thing at a long stop light, the engine is running producing heat, the carb is filled so the float is closed  .... the fuel just sits in the hot fuel line above the exhaust manifold and turns to vapor.

 

What others have done is add a electric fuel pump near the tank .... when the problem comes up, turn on the electric pump for a few miles until the issue goes away then shut it off.

 

The stock manual pump sucks, it is very good at sucking fuel .... but it can not suck vapor.

The electric pump pushes, it sucks at sucking .... but it will push fuel a long ways.

So the electric pump mounted near the tank, will push the cool fuel to the carb .... pushing the vapor out of the way. ..... Hope that makes sense.

 

This is only a problem with older vehicles ..... all modern vehicles run electric pumps in the tank and have a return line back to the tank, so the injectors are always getting fresh cool fuel.

 

Doug&Deb also seem to think this is the issue and I agree with them that it is possible something else is causing it .... but it sure sounds like typical vapor lock to me.

[kencombs]

I also think it is heat related but believe the problem is the hot fuel(maybe even vaporized partially) creating pressure in the pump and the line from the pump to the carb.  In effect that line is a sealed environment, check valve in the fuel pump,  inlet needle at the carb.  Pressure for the heat soak when shut off pushes the needle off its seat and since the carb bowl is full that excess fuel hits the intake.   So, on start the engine is rich until that excess vaporizes and gets burned.  

 

If I'm correct is this guess, you should be able to smell the rich burn at the tail pipe on restart, maybe even see some black smoke.  

 

A fuel pressure regulator with a return line will help as will adding a heat shield to the line and pump.

[Sniper]

I don't see the exhaust flapper being stuck mentioned.

 

[hep2jive]

Thank you everyone for responding. 

 

Now that it's getting colder outside and I drive my car daily I hope it won't be as much of an issue. I thought maybe the fan belt was too loose making the timing off at the start? But then after it runs smoothly so you think if timing was off it would always be off. 

If I stop and get gas it's fine. It's like when I stop for 15 minutes is when it has the issue. It's 40 outside but the engine is certainly hot. 

[hep2jive]

@Sniper that was removed

[Sniper]

The fan belt has nothing to do with timing.

[Los_Control]

Sings Hi Ho ... Hi Ho  off to San Angelo I go.

[hep2jive]

All of a sudden it started doing this after I adjusted the fan belt. I've had this car for 10 years it just started this vapor locking. Car idles quietly and smoothly. Had some issue when cruising at 40 mph it would miss but tightened the coil to distributor wire and it stopped that. But this vapor locking is weird.

[hep2jive]

@Los_Control I'd be vapored out there! LOL!!!

[Sam Buchanan]

I don't know what kind of choke your carb has but if it is automatic it may be closing the choke when it shouldn't. If the choke isn't manual make sure it is functioning and adjusted correctly.

[hep2jive]

When the engine is warm I see the flap is open on the top of the carb. When I look down into the carb it's closed. Are there two flaps in the carter ball and ball? 

[Los_Control]

16 minutes ago, hep2jive said:

this vapor locking is weird.

I will only suggest that the quality of the fuel has been going down year after year.

10 years ago the fuel was better and not a issue to most, then a couple years go by and gubment regulations change the fuel, then a couple more years go by and the regulations change again .... just saying this is getting worse & worse as time goes on. We notice it slowly.

 

Modern cars do not have a problem with the lower quality fuel .... so they just keep lowering the bar until they do have a issue.

 

It still can be another issue altogether though.

[Sam Buchanan]

34 minutes ago, Sam Buchanan said:

I don't know what kind of choke your carb has but if it is automatic it may be closing the choke when it shouldn't. If the choke isn't manual make sure it is functioning and adjusted correctly.

 

15 minutes ago, hep2jive said:

When the engine is warm I see the flap is open on the top of the carb. When I look down into the carb it's closed. Are there two flaps in the carter ball and ball? 

 

Must not be a choke sticking shut if the choke plate is open, the bottom plate is the throttle, it should be closed at idle. Oh well, it was worth checking.

 

Beautiful car!

Edited November 21, 2023 by Sam Buchanan

[hep2jive]

@Sam Buchanan thank you!  Okay that makes sense. 

 

Thanks for the compliment!

[keithb7]

Maybe watch this video through. It may add something to ponder while you are troubleshooting your carb.

 

 

Edited November 21, 2023 by keithb7

[hep2jive]

@keithb7 hey thanks! Love your videos!!!

[hep2jive]

2 hours ago, keithb7 said:

Maybe watch this video through. It may add something to ponder while you are troubleshooting your carb.

 

 

I guess it's just vapor lock. Maybe ill get an electric fuel pump like you have. I hope we have a cold winter! Lol!

[Sam Buchanan]

I don’t think it’s vapor lock. Has the idle mixture been set properly? Sounds like it may be rich, loading up then having to clear.

Edited November 21, 2023 by Sam Buchanan

[hep2jive]

36 minutes ago, Sam Buchanan said:

I don’t think it’s vapor lock. Has the idle mixture been set properly? Sounds like it may be rich, loading up then having to clear.

Hmm yeah it only does it when hot. Driving it's fine, but sitting at that long light it stumbled a second but I gave it gas and smoothed out again. Plugs don't look black but maybe it is too rich. It drives and idles very smoothly but if I turn it off for 15-20 minutes it turns right back on but chugs and if I give it gas it smooths out. Takes 25 seconds to get it smooth again and after that it's fine. Yet if I turn the car off for under 3 or 5 minutes, say to fill up the tank, it's fine and smooth.

[soth122003]

I know some states have a summer and winter fuel blend. Maybe the change over (if Illinois does it) had something to do with it. Also the 10-15 minutes it might sit allows the hot engine to really warm up the fuel lines and the carb. You might try lowering the float in the carb just a bit to prevent excess fuel from being forced into the manifold. 

 

As far as the long light issue, I got nothing. It might be perking your fuel and a lowering of the float might help, but I'm really not sure on that. A carb rebuild and lowing the float about a 1/16" should definitely help.

 

Joe Lee

[hep2jive]

42 minutes ago, soth122003 said:

I know some states have a summer and winter fuel blend. Maybe the change over (if Illinois does it) had something to do with it. Also the 10-15 minutes it might sit allows the hot engine to really warm up the fuel lines and the carb. You might try lowering the float in the carb just a bit to prevent excess fuel from being forced into the manifold. 

 

As far as the long light issue, I got nothing. It might be perking your fuel and a lowering of the float might help, but I'm really not sure on that. A carb rebuild and lowing the float about a 1/16" should definitely help.

 

Joe Lee

Thanks yeah I HATE messing with the float ? buy I might have to. It gappened all of a sudden...illinois does have winter gas maybe that has something to do with it.  What if I leaned the carb out I wonder if that would help??

[Kilgore47]

Your problem does sound fuel related.  But when I got the P15 it did the same thing.  Sitting at a light it would start running rough.  Blip the throttle and it would run smooth for a few seconds and then go back to running rough.  I went through the fuel and ignition systems and it still did the same thing.  The engine had about ten thousand miles on it per the PO.  The valves needed to be adjusted because I could hear them under the hood clacking away like an old sewing machine.  Finally got around to adjusting the valves and the rough idle went away.  If your valves are talking to you then that's another path you could consider.

[Sam Buchanan]

8 hours ago, hep2jive said:

Hmm yeah it only does it when hot. Driving it's fine, but sitting at that long light it stumbled a second but I gave it gas and smoothed out again. Plugs don't look black but maybe it is too rich. It drives and idles very smoothly but if I turn it off for 15-20 minutes it turns right back on but chugs and if I give it gas it smooths out. Takes 25 seconds to get it smooth again and after that it's fine. Yet if I turn the car off for under 3 or 5 minutes, say to fill up the tank, it's fine and smooth.

 

Your description of hot starts is typical of what I see with my '48 P15 but once it starts it idles smoothly and it will idle for long periods of time with no difficulty. It does seem to be fuel percolation and the suggestion to try lowering the float is something you could try. 

 

However.....I have a full-time electric fuel pump and that may be preventing problems you could be having with the old suction pump. I've never experienced what I would consider vapor lock and it gets HOT here in north Alabama.

Edited November 21, 2023 by Sam Buchanan