Another Hot Starting Question

[packick]

Here's a question that pertains to an early 50's car (Buick, Packard, etc.) with a 6V system.  When the car is cold it starts normally.  But when the car is hot, like pulling into a gas station to get gas, the car just cranks and cranks and cranks and not start.  And because of the 6V system, the cranking is very slow.  After it cools down a bit it will start.  

 

So here is my question.  With an accelerator pedal starting system, when pushing down on the accelerator pedal to restart a hot car, is the system pushing more gas into the carburetor thus flooding the engine, or does the initial pushing of the accelerator pedal NOT push anymore gas into the carburetor?  The suspicion is that the engine is getting flooded and is not vapor locked.

 

Your thoughts?  Thanks for the help.

[nat]

     With a foot to the floor starter, a cold engine with the choke on will provide the ungodly blast of fuel that all old engines seem to love.

     If and when an engine is flooded, flooring the accelerator opens the throttle to such an extent that, (when the choke is open), cranking speed lets in lots of air and little or no fuel.  I don't know for sure but I have long thought that is the reasoning behind the accelerator activated starter.  

     Is your manifold heat valve working properly?

     I suspect that when you shut it off hot, some fuel overflows from the carburetor.

     Maybe... Get it hot, remove the air filter, drive it around the block, open the hood, pull the coil wire, open the throttle and use a match or flashlight to look down the carburetor for fuel running in.

     

[1937 Buick 66C]

I do encounter hot starting problems on all of my old cars (Buick, Chrysler, Packard). 

 

Hot-start problems are generally caused by one of two conditions, vapor lock or flooding caused by percolation.  Let's look at each of these issues independently:

 

Vapor Lock:  Vapor Lock occurs when fuel boils in the fuel system, replacing liquid gasoline in the fuel line with fuel vapor.  The engine is fuel starved and will not start.

• Modern fuels, particularly fuels with ethanol, have a much lower vapor pressure (they boil at a lower temperature than pure gasoline), therefore are more likely to cause vapor lock.  Use ethanol-free fuel whenever available.  Treating the fuel by adding Diesel, Marvel Myster Oil or Stabil fuel additive will reduce, but not eliminate, vapor lock.
• Mechanical fuel pumps are intended to move liquid, not vapor.  When fuel has boiled in the fuel line is usually take place in the engine compartment and cannot be overcome by the mechanical fuel pump. 
• Fuel no longer reaches the carburetor, and the engine will not start due to fuel starvation.
• Using an electric fuel pump, placed between the fuel tank and the mechanical fuel pump, will push cooler fuel from the tank into the fuel system, displacing the vaporized fuel with liquid fuel which will eventually allow the car to start.  To hot start a car with an electric fuel pump, the fuel pump needs to run until the fuel system is pressurized.  This is similar to running the electric fuel pump to prime the engine when the car has not been used for an extended period.

 

Percolation (hot flooding):  Percolation occurs when residual fuel in the carburetor bowl becomes heat-soaked and boils out of the carburetor into the intake manifole when the hot engine is stopped.  This causes the engine to flood, and difficult to restart.

• You have percolation taking place if you observe any raw fuel leaking from the carburetor shortly after shutdown when heat from the engine has reached the carburetor.  Watch around the throttle shaft a few minutes after shutdown for evidence that raw fuel is boiling through the carburetor.
• In order to hot start a flooded car, you want to crank the engine while holding the throttle open.  When you press on the throttle you trigger one pump fuel from the accelerator pump in the carburetor, however this is not sufficient to flood the engine unless you repeatedly "pump" the throttle.  On cars like Buick and Packard, where the throttle is used to engage the starter, you may need to hold the throttle open for a long enough period to deplete your battery charge.  In these cases, you may want to sit in the car, engine off, and hold the throttle open for several minutes to allow the surplus fuel to evaporate without cranking the engine.
• You need to crank the engine long enough to allow surplus fuel to be evaporated from the carburetor, intake manifold and cylinders in order to hot start the car.
• Do NOT pump the throttle when attempting to clear a flooded engine, as each stroke of the throttle will inject additional fuel into the intake.  Hold the throttle open, and crank the engine. 
• Percolation can be reduced by putting an insulating block between the carburetor and intake manifold to reduce the amount of head being conducted from the engine into the carburetor.  A spacer made from a block of phenolic material can be of great value here.

 

You can have both conditions at once!  My 1948 Packard demonstrates both problems concurrently.  The fuel percolates into the intake manifold, flooding the engine.  I can crank the car with the throttle open to clear the intake manifold and get the car started, but shortly thereafter it will stall due to vaporized fuel (vapor lock) in the metal fuel line in the engine compartment not reaching the carburetor.   I am finding that the optimal solution is to not shut the car down unless I can leave it off for long enough to allow the fuel system to cool before attempting to restart.  Opening the hood on the side where the carburetor lives improves the cooling.

 

 

 

Edited August 13, 2023 by 1937 Buick 66C
Correction of percolation section (see edit history)

[EmTee]

Also check that the dwell and timing are set to specified values.  Over advanced timing can cause slow cranking when hot.  Also make sure the vacuum and centrifugal advance mechanisms are working as designed and are not binding or sticking.  This can also cause an over advanced situation if the breaker plate doesn't reliably return to its zero-advance position.  Lastly, check that the battery connections are clean and tight.  Heat can exacerbate a borderline condition.

[MCHinson]

Modern fuels are much more volatile than fuels from when these cars were new. In my experience, when my 1937 Buicks are shut down hot, the heat from the engine will cause the modern fuel to boil out of the carburetor bowl. It is then going to take a bit of time with the engine turning over for the mechanical fuel pump to refill the carburetor bowl. While I like my cars as close to original as possible, I have added an auxiliary electric fuel pump near the fuel tank on both of them. The electric fuel pump is controlled by a toggle switch that is mounted within reach of the driver's seat. It is also on the ignition circuit so it will be off when the car is shut down, even if you leave the toggle switch on accidentally. A short activation of the electric fuel pump will refill the carburetor bowl so the car will start as soon as you attempt to start it. This technique also solves a similar problem when the car has been sitting in the garage for several days and the fuel has evaporated out of the carburetor bowl. It is also a perfect solution for anytime that the car is exhibiting vapor lock. The electric fuel pump will stop vapor lock in a few seconds. 

[nat]

10 minutes ago, EmTee said:

Over advanced timing can cause slow cranking

       Worn starter bushings will use up the battery and crank at about 1/2 speed.

[packick]

Thanks for all the information.  From what I can deduce from it, I would lay money on the fact that the gasoline today is not the same as it was in the old days is the root cause of hard starting when the engine is hot in today's world.  I don't ever remember my father having any trouble starting his late 40s and early 50s Buicks when we traveled on vacation.  After traveling many, many miles, he would pull into a gas station, turn off the engine, fill up, and the car would immediately start.  From what I have read, even the ethanol-free gas we get today is different from the olden days leaded gas.  Maybe the removal of lead in the gas had some effect?

 

Anyway, thanks for the responses.

[zeke01]

Don’t forget to check the coil. They sometimes break down when hot but work okay when cool. Zeke

[nat]

52 minutes ago, packick said:

After traveling many, many miles, he would pull into a gas station, turn off the engine, fill up, and the car would immediately start.

    Do you remember it cranking as slow as your's does now?

[Century Eight]

Do you guys have a preferred octane rating to buy?  I have never experienced vapor lock in my ‘50 Super with the 273 engine, (owned 47 years) but in the last year I have all of a sudden experienced vapor lock in my ‘40 Century with the big 320 engine 3 times in the last year after 13 years ownership.  I usually blend 87 and 89 octane, because i think the old cars never were treated to 91 or higher octane.  Todays gas is approaching  actual E10.

 

I did install an electric fuel pump just ahead of the gas tank to push gas forward.  BIG difference!

[MCHinson]

I avoid ethanol containing fuels when possible (which is most of the time). I use the lowest octane fuel available, typically 87 or 89. Higher octane is not really beneficial, but avoiding ethanol is. 

[packick]

Nat, I was too young to remember the cranking speed.  But I don't remember sitting for long periods of time because the car wouldn't start.

 

By the way, I always burn ethanol-free gas in my old cars.  Been burned too many times with ethanol.

 

 

[Skidplate]

Another point of interest that I found out... same hot start problem. All the same symptoms. I went back and readjusted my valves after the engine was really really warmed up. And I don't mean over heated, just warmed up... like an engine is when it's been run for a hour or two. Warmed up like when you stop the engine and the exhaust system pops a few times while in the driveway. Get it THAT warmed up and then adjust the valves. You'll find that your valves aren't as well adjusted as you thought and it'll start better when hot after that.

[EmTee]

Sounds like the valves were a little too tight before.  My '38 has a couple of valve ticks at idle when I pull out of the garage and get out to shut the overhead door.  Upon return, it is dead quiet when I get out to open the garage.

[Marty Roth]

From my old racing, sports car, and hillclimb days, I recall that leaving the valve adjustment just a bit more clearance tends to accomplish at least two things:

Better low end torque

Less chance of burned valves