Won’t start when hot...

[frijolito]

So here is the problem I’m have. Car starts fine when cold/cool. The problem is when it gets warm/hot it won’t start. The car runs great not over heating at all the temp gage is normal meaning 180° thermostat opens cools to 160°. Fuel is at the carb when I try to start it warm so no vapor lock. It turns over strong for a 6v. 
 

it’s a 1939 Plymouth 201. Any advice?

Edited May 3, 2021 by frijolito

[Merle Coggins]

It's a common problem on these old vehicles. The gas in the float bowl tends to expand as the engine heat soaks after shut down. This expansion causes gas to push out the jets into the intake and cause a flooded condition. More often than not it'll start up easier by opening the throttle slightly. My truck has the stomp starter, so my practice is to use my heal on the gas pedal to open to 1/4 - 1/2 throttle while pressing the starter pedal with my toe. Starts every time. 

 

You have also need to lower your float level slightly. This will help reduce the flooding effect. 

[Frank Elder]

Weak coil maybe, it is a major heat soak.

[ratfinkxxx]

Check for spark?

 

[sidevalvepete]

Do you have a heat shield between the fuel pump and the manifold. Without this the heat evaporates the fuel from a hot manifold and creates an air lock for when you restart it hot.

[frijolito]

1 hour ago, ratfinkxxx said:

Check for spark?

 

Yes there is spark. 

[frijolito]

58 minutes ago, sidevalvepete said:

Do you have a heat shield between the fuel pump and the manifold. Without this the heat evaporates the fuel from a hot manifold and creates an air lock for when you restart it hot.

Yes I’m have the heat shield 

[JerseyHarold]

1 hour ago, Frank Elder said:

Weak coil maybe, it is a major heat soak.

I've put a plastic bag with ice on the coil to cool it down.  If the car starts after that, I'd suspect a weak coil

[frijolito]

1 hour ago, Merle Coggins said:

It's a common problem on these old vehicles. The gas in the float bowl tends to expand as the engine heat soaks after shut down. This expansion causes gas to push out the jets into the intake and cause a flooded condition. More often than not it'll start up easier by opening the throttle slightly. My truck has the stomp starter, so my practice is to use my heal on the gas pedal to open to 1/4 - 1/2 throttle while pressing the starter pedal with my toe. Starts every time. 

 

You have also need to lower your float level slightly. This will help reduce the flooding effect. 

I’m curious why giving it more fuel would help? Thanks you

[Merle Coggins]

You're not really giving it more fuel. You're giving it more air. 

[keithb7]

Are any exhaust valve clearances too tight?  Valve stems get hot and expand. Then the valves can't seat as they tighten up.

 

It happens. I'll admit I've screwed up and caused this very phenomenon to happen myself. We've all set valves for the very first time. Some of us alone, with no mentor to guide us. Just a service manual that expects you to know the basics. Lol.

 

 

Edited May 3, 2021 by keithb7

[greg g]

Is the carb base wet with raw fuel when this occurs? If yes the the fuel expansion, percolating, float level things mentioned above are in play and if it's dry then I would look at the coil. Slowly flooring the gas pedal doesn't cause the accelerator pump to add raw gas to the situation. 

 

[Bryan G]

On my off-topic collector car I've had trouble with percolating. I floor the accelerator when I crank it, being careful to let up as it fires. I lowered the float which helped; I really need to pull it apart and lower it some more. Last summer I added a gallon or two of diesel or kerosene with every fill-up and that made a real difference. 

[1949 Wraith]

16 hours ago, Merle Coggins said:

It's a common problem on these old vehicles. The gas in the float bowl tends to expand as the engine heat soaks after shut down. This expansion causes gas to push out the jets into the intake and cause a flooded condition. More often than not it'll start up easier by opening the throttle slightly. My truck has the stomp starter, so my practice is to use my heal on the gas pedal to open to 1/4 - 1/2 throttle while pressing the starter pedal with my toe. Starts every time. 

 

You have also need to lower your float level slightly. This will help reduce the flooding effect. 

I quite often use the throttle cable on the dash, pull it out for 1/4 throttle before engaging starter.

[Kilgore47]

 

I'm not familiar with the P7 so this may not help.  On the P15's the fuel line is routed from the fuel pump up to the carburetor on the passenger side of the engine. This runs the fuel line close to the exhaust manifold and causes vapor lock when hot.  I rerouted that part of the fuel line and the problem is solved.  Starts when hot now.  The other option would be to build a heat shield for the fuel line.  I saved the original steel line and may do that one day.  

[kencombs]

22 hours ago, Merle Coggins said:

It's a common problem on these old vehicles. The gas in the float bowl tends to expand as the engine heat soaks after shut down. This expansion causes gas to push out the jets into the intake and cause a flooded condition. More often than not it'll start up easier by opening the throttle slightly. My truck has the stomp starter, so my practice is to use my heal on the gas pedal to open to 1/4 - 1/2 throttle while pressing the starter pedal with my toe. Starts every time. 

 

You have also need to lower your float level slightly. This will help reduce the flooding effect. 

Early 50s Buicks had a builtin solution from the factory.  The starter switch was under the accelerator pedal!  If you needed to clear a flooded condition or set the auto choke on a cold start, either way it served its purpose.

[chrysler1941]

16 hours ago, Bryan G said:

On my off-topic collector car I've had trouble with percolating. I floor the accelerator when I crank it, being careful to let up as it fires. I lowered the float which helped; I really need to pull it apart and lower it some more. Last summer I added a gallon or two of diesel or kerosene with every fill-up and that made a real difference. 

Exactly.

Kerosene or electric fuel pump.

Lower the octane or force fill bowl preventing evaporation.

[Sniper]

Kerosene in a gas engine.  Interesting

 

 

[Los_Control]

I imagine it will work. From past experience, I had a 1969 fastback mustang with a 351 windsor, my buddy had a 1970 with a Boss 302.

We made a weekend fishing trip to the mountains in his car. We kinda got lost a few times driving around to find the lake we were looking for. Old logging roads on state land & poor maps.

We found the lake & had a great weekend. Lots of good sized trout and crawdads.

 

On Sunday heading home we were very low on fuel. We came across a state road grader parked on side of the gravel road and we siphoned some diesel from it.

The Boss 302 was not happy with it, did smoke a little and not the pep normally had ... the diesel got us home. I compare kerosene to a low grade diesel. Both have a oil in them that would lubricate the valves .... is wrong in a gasoline engine in sooo many ways, not really going to destroy anything. IMHO

[kencombs]

14 minutes ago, Los_Control said:

I imagine it will work. From past experience, I had a 1969 fastback mustang with a 351 windsor, my buddy had a 1970 with a Boss 302.

We made a weekend fishing trip to the mountains in his car. We kinda got lost a few times driving around to find the lake we were looking for. Old logging roads on state land & poor maps.

We found the lake & had a great weekend. Lots of good sized trout and crawdads.

 

On Sunday heading home we were very low on fuel. We came across a state road grader parked on side of the gravel road and we siphoned some diesel from it.

The Boss 302 was not happy with it, did smoke a little and not the pep normally had ... the diesel got us home. I compare kerosene to a low grade diesel. Both have a oil in them that would lubricate the valves .... is wrong in a gasoline engine in sooo many ways, not really going to destroy anything. IMHO

Depends on the mix.  Too much diesel and it won't run.  Buddy of mine in HS 'borrowed' some diesel from a construction site in his 49 Plymouth.  He got about a mile or so before it started smoking badly then died.  I towed him home the next day and he spent the day draining, flushing and refilling w/gas.

[Los_Control]

I should have added, we were guessing about 2 gallons gas in tank & guessed about 2 gallons diesel added. Either way we had about 40 miles to get home, 2 gallons gas was not going to get us there. So we were looking at a 50/50 mix or less .... Who knows we think is 2 gallons in the tank & may have been 4 ... we just added enough to get us where we were going.

When your gas gauge is sitting on E and you trying to remember when exactly it hit E, after a 3 day camping trip ....

[JBNeal]

additional information - flathead hot-starting remedies

[tom'sB2B]

On 5/3/2021 at 12:57 PM, Merle Coggins said:

It's a common problem on these old vehicles. The gas in the float bowl tends to expand as the engine heat soaks after shut down. This expansion causes gas to push out the jets into the intake and cause a flooded condition. More often than not it'll start up easier by opening the throttle slightly. My truck has the stomp starter, so my practice is to use my heal on the gas pedal to open to 1/4 - 1/2 throttle while pressing the starter pedal with my toe. Starts every time. 

 

You have also need to lower your float level slightly. This will help reduce the flooding effect. 

Had the same problem. Dropped the float level. Problem solved.

[Rosco]

I also had a problem with my ‘50 Plymouth not starting when it was hot.  I then had an unrelated problem with a wire shorting out inside the distributor, causing the car to stall while driving.  My mechanic replaced the wire, points and condenser and adjusted the carburetor and timing.  So far, since then the car has started right up every time, hot or cold.  Just my two cents worth.