The Stovebolt Site Forums The Shop Area The Electrical Bay Truck won't start when hot (1947 GMC)

I thought I had a vapor lock issue, which I may still have?
I installed a 6V electric fuel pump to prime and hopefully pump when hot to help start.
That seems to be working correctly and does pump gas to the carb when it is hot (it used to not fill the line till cool down).
But now, even with the fuel pump, the truck does not start when hot. I need to let it cool down, then it will start.
I'm wondering if it is something electrical?
I have a new coil with an internal resistor.
Can I install an external resistor?
when looking for these, I have not seen a 6 volt resistor - yet.
Do I need a 6 volt or are they interchangeable?

Thanks for any info or assistance!

Assuming the rest of the electrical system is 6v, your coil should not have a resistor, internal or external.

Welcome to the 'bolt. Please introduce yourself in the welcome forum.

Someone will come along soon with recommendations for your problem.

Any time somebody thinks he has a vapor lock situation, he doesn't. It is almost always something else, often not fuel related.

Vapor lock isn't as common as people think.

Thanks Bill,
Yes, the whole system is 6 volt positive ground.
I was wondering about that after reading some comments on coils etc.
Do you think the resister would cut the voltage down and reduce the spark for starting - especially when hot?

Thanks Otto,
I'm pretty sure, at least part of the problem is/was a vapor lock.
When the truck was hot, there was no fuel in the line to the carb as well as an inline filter in the area.
And even with prolonged cranking, the gas would not get to the carb.
The elec fuel pump has seemed to correct this but still problems starting.
Thats why I am wondering if there is some additional problem perhaps electrical.

Was the mechanical pump a relatively new one prior to replacing it with the electric pump? New ones are known to be absolute crap. I, had a new pump fail on me and also thought it was vapor lock. Nope. Rebuilt original pump cured it right up.

Otto,
Actually, I probably had the same problem on the mechanical pumps as you.
I did purchase a new one(s) with most not working at all.
Had one work for a bit but seemed to be pumping air so put my original one back in and that seems to work best.

I should say the Elect fuel pump is an axillary pump that I use just to prime the line after not running for a while and to try and overcome any vapor lock issues,
I can turn it off and just use the mechanical pump for driving.
The electrical pump has a check valve bypass for the mechanical pump.
I did this after reading some recommendations from others on the pump.

Sorry for any confusion.

OK, this sounds familiar now. This was a year or so ago, right?

Yes.

See this post again.

Fuel pump not working is not vapor lock
Vapor lock is when the components are so hot the fuel
Is boiling into a vapor inside the lines and components
Even with empty line and filter the carburetor will have enough gas in the float bowl to start the vehicle

But anyway
Not starting hot sounds a lot like a problem I had
It was the coil going bad
The internal insulation breaks down and become heat sensitive so you get too weak of a spark hot while the starter is dragging the battery voltage low
I could roll start the truck fine
But no amount of cranking hot would work
New coil fixed the issue instantly
-s

Check the voltage at the coil + terminal during cranking. If it's less than 4.5V from a 6V battery, find and fix the high resistance between the battery and the coil. Look for loose, dirty primary wire connections, and/or dirty contacts in the ignition switch. Never,- - - - -ever, run a resistor on a 6V coil- - - -internal or external.
Jerry

Jerry even though an engine will crank over, if it's not getting any spark at the coil could it be a problem with the ignition switch?

Yes- - - -particularly with the foot starter. Those trucks have a completely different circuit for cranking and ignition. The pedal start works even with the ignition turned off. Dirty or burned contacts in the ignition switch, particularly when hot, can cause a crank- - - -no start condition.
Jerry

Jerry thanks for the reply. So even in a later model truck the ignition switch when engaged could crank the engine over but not provide a circuit to the coil to generate spark?

I have a new coil in it but just purchased another one with no internal resister.
I’ll try to let you know if it helps.

No.

On later model trucks an ignition switch with a spring loaded "start" position was used in combination with a solenoid on the GM starter motor. The "start" position caused the solenoid to "pull in", pushing the starter drive ("Bendix") into the teeth on the flywheel and at the same time switching on battery power to the starter motor.

On typical 12volt electrical systems a set of contacts inside the solenoid was also activated, providing full battery voltage to the ignition coil + terminal. This caused a hotter spark while cranking the engine to start. When the key was returned to the "run" position, the solenoid disengaged, the contacts providing battery voltage opened and lower coil voltage was supplied through a resistor while running.

So the ignition switch could be bad even if the engine cranks but no voltage to the coil?

The starter circuit and the ignition circuit are separate. You can step on the starter button with the ignition switch turned off and the engine will crank over but will not fire.

The key (ignition) switch only supplies current to the coil. If the coil is good, you get current to the points. If the points (and condenser) are good, with fuel it should start.

I had a similar priblem it was a bad ground .try when it is hot to park on a hill and see if it starts mine started when rolled but not with the starter when hot.

If ignition is ok:
Check the power piston if you have a Rochester B carburator. If it´s stuck it will not start, too. Had this problem on 2 of my 3 trucks I owned.....

Sven,
How do you check that?
I did put in a new carb kit but there wasn't much to it.
I'm guessing you are referring to the main piston in the carb?
I did in stall a new seal and pin on that so they should be functioning.

I put on a new non-resister coil and re-attached the ground to the motor (which was off when I got it).
I have not been able to confirm but suspect it still has a hot start problem.
Other than the above, I have not been able to check on the voltage questions yet.

Thanks All!

Make sure the carburetor fuel bowl is full. With the engine off, look down the throat of the carburetor with the choke fully open. Manually goose the throttle. You should see a healthy squirt of gasoline from the accelerator pump. If you don't see that squirt, your accelerator pump is shot.

What’s different about the new pumps assuming a new rebuild kit isn’t crap too?

Early pump housings are cast aluminum, top and bottom, and are assembled with screws. Later pumps have a stamped steel top, and are crimped together. Those are difficult, if not impossible to rebuild.
Jerry

Contact Carbking, Jon H on this site. He'll set you straight with the proper rebuild parts, if needed.

Does your pump have a glass cover on top?

Before we sweat what type of fuel pump this is, I’d strongly suggest trying the test Otto suggested to verify there is/isn't fuel in the bowl.

Its a little confusing. If you have any resistor in the ignition system. Not good. (BTW, resistors are not voltage specific). You have two fuel pumps working, not good. Have you checked the flow when hot and won't start?????? This answer is very important. This test is done by observing the flow out of the outlet of carb to see pulsing good squirts into a clear bottle. Otto's test to see gas being squirted down throat is good also.

When electrical wires and ignition components get "hot" they get high resistance and less current/volts to the points/plugs. This is due to a lot of bad connection places and corrosion. Weak gets more weak. A weak battery gets weak when hot. Heat caused by Bad timing. Exhaust manifold leaks. Lack of motor cooling....So this would be your "electrical or heat" cause. Weak spark.

The concern about switches, coil, carb, pump must take into consideration that the truck does start. Sooooooooo......
I don't believe "hot" or electrical has anything to do with it. You had the same problem with mechanical pump as now...so my guess is Gas Tank venting. You are pulling a vacuum in the tank and fuel eventually stops and it seems like hot but it's just the time it takes to pull a vacuum lock. The electrical pump was steady and stronger for a while. The truck starts after sitting and bleeding off the vacuum as tank debris clears/floats.... or a blockage of some kind bleeds off vacuum. It is not "cooling down". Take off the gas cap and drive it. See what happens. If it works, drill a 1/16 diameter hole near center of cap.


If flow test is bad, then there will be a list of things. Mostly not related to "hot" or "electrical"
1. Debris at tank pickup.
2. Fuel line collapse or block in line or valve.
3. Fuel filter
4. Carb components
5. Tank vent
6. Bad pump

If flow test is good, report before continuing.


The flow test/carb pump test and cap removal are easy and fast to get a clue. During "won't start" it's good to see some spark at a plug.

Can you advise "won't start"?
Motor does crank but no fire at all?
No sputter?
NO backfire? Thru carb or exhaust.
Choke no help?
No gas smell?
Gas tank has at least 1/2 gas in it for sure.
Squirting a shot of gas down carb from a spray bottle gets no fire? Do this if bad pump test or observing dry throttle pump (Otto)

Don't take this the wrong way, it is just part of the diagnosis. I realize you are trying things. However the mention of resistor, two pumps, check valve, switch to turn off E-pump, new carb kit, new coil.... means it is possible you bypassed the cause and have introduced, off and on, some other causes which now all need a rethink/eliminated.. This is what everyone is sorting thru now.