1939 Ford Flathead starting problems

A

Airrace

Junior Member
The motor starts up fine when it's cold but when it's hot it takes a while to start up or I have to let it cool down. The motor runs perfectly. I have checked for any vacuum leaks and found none. The voltage to the distributor seems to be low after it is hot.

Any ideas or thoughts on what to do next?

Thanks in advance for your time!
 
My 39 ford had the same problem until recently. You are about to hear everything from vapor lock, condenser, coil, carburetor, coil, points, ignition switch, wires, etc. And that is what makes owning one of these old fords fun, finding the problems.

It took me years and several engines later to find out it was my coil resistors. It will cost you nothing to check. When the car will not start, check the voltage at the coil. If it is low, I think that is your problem. Solution upgrade to a new style resistor.

I did it backwards and replaced everything including building a new motor which did the same thing. As per murphys law, the last thing I checked was the problem. Your problem could be anyone of the above. But I would start with this simple test and have fun.
 
I'm with Lawson on this one..I have seen people run a temporary wire from the battery to the coil,just long enough to crank the car..if it starts, time to have your coil rebuilt ,or replaced.
 
If you test the voltage at the coil (engine not running and points have to be closed with ign turned ON.) when all is COLD you should get 6 volts. When engine and ignition resistor are hot the voltage at coil should read approx 4.5 volts. This is how the resistor works. Resistance when cold is lower and allows higher current flow(amps) and voltage which gives the coil a bigger spark while cranking and starting. Once all is warmed up resistance is higher and allows lower current flow (amps) and voltage. The original flathead coil is designed to run all day on 4.5 volts. This also promotes a long distributor contacts life by lowering the coil current draw (amps). Make sure all connections at resistor, ignition switch, ammeter terminals, starter solenoid on firewall are clean and tight. You need to be very fussy here. Maybee your problem is fuel related when hot ? Check all these out. Regards, Kevin.
 
Just another tip. Old ignition switches are prone to wear on their internal contacts. To test this is simple by jumping a short wire accross the switch terminals and then see if starting improves. Remove switch an dismantle and check if the terminal plate has a circular groove leading around to one brass contact, OR moving contact is worn or broken or lost tension. New terminal plates and moving contacts are available. This is a very common fault on old ford V8s and is often overlooked. Regards, Kevin.
 
koates said:
If you test the voltage at the coil (engine not running and points have to be closed with ign turned ON.) when all is COLD you should get 6 volts. When engine and ignition resistor are hot the voltage at coil should read approx 4.5 volts. This is how the resistor works. Resistance when cold is lower and allows higher current flow(amps) and voltage which gives the coil a bigger spark while cranking and starting. Once all is warmed up resistance is higher and allows lower current flow (amps) and voltage. The original flathead coil is designed to run all day on 4.5 volts. This also promotes a long distributor contacts life by lowering the coil current draw (amps). Make sure all connections at resistor, ignition switch, ammeter terminals, starter solenoid on firewall are clean and tight. You need to be very fussy here. Maybee your problem is fuel related when hot ? Check all these out. Regards, Kevin.
Click to expand...

Koates,

My findings have been with the old style resistor (new-several) the voltage at the coil cold was 2.98 v. After it warmed up, it was closer to 2.5 v. I used a new style resistor of the same resistance and I get a voltage of just above 4 volts all the time. My car will not start with 2.5v. I do not suggest direct wire of coil, but a "new style resistor" in place of the old. Does that follow your theory.
 
Firstly the new style resistor (repro. ? ) may not have the same electrical characteristics as the original ones which were ford quality parts. The resistance wire used to wind it might vary from the original. Many reproduction parts just dont have the same quality. I suggest you re-read my first posts and do some checks and voltage readings at those connections. If the resistor does not have a 6 volt supply to it it wont ever work correctly. To prove a point you could run a tempory wire from the battery negative post (presuming car is pos. ground.) and connect direct to resistor. Then do cold and hot voltage tests at the coil terminal with points closed dont forget . What coil do you have fitted ? An original or new old stock or rewind or a reproduction. Primary windings on coils can vary and so current draw and voltage also varies a little depending on this. And as we well know coils can be faulty. See how you go, take your time, Be a detective and track it down by a process of elimination. Regards Kevin.
 
Kevin, Thanks for the info. My 39 Mercury has this exact hot starting problem. Spark, when the engine is cold will knock your socks off but after running the engine for 30 minutes or so there is hardly a tingle at the plug. So far, I've only changed out the condenser. Nil fix. I will check the voltage at the coil and will try the jumper wire from the battery to coil when the engine will not start. I'll let everyone know the results. Thanks everyone for all of the troubleshooting info.. The adventure continues.
 
Joe KCMO said:
My 39 ford had the same problem until recently. You are about to hear everything from vapor lock, condenser, coil, carburetor, coil, points, ignition switch, wires, etc. And that is what makes owning one of these old fords fun, finding the problems.

It took me years and several engines later to find out it was my coil resistors. It will cost you nothing to check. When the car will not start, check the voltage at the coil. If it is low, I think that is your problem. Solution upgrade to a new style resistor.

I did it backwards and replaced everything including building a new motor which did the same thing. As per murphys law, the last thing I checked was the problem. Your problem could be anyone of the above. But I would start with this simple test and have fun.
Click to expand...


I bought a motor from a guy, that he took out of a 35 pickup. he thought the valve timing was off. the motor runs great it has about 10, thousand miles on it. it was just a bad resister!
 
What I have found for hot starts is a spring loaded toggle switch, that when held gives the coil all the volts available. This is what later solenoids did during the start cycle.I mounted it on the bottom of the dash, under the starter button, so it doesn't show. Two post switch, one wire on each side of the resistor, so when it is pushed there is no resistor.
Works good, cheep fix, Chuck S.
 
Where do you get a new style resistor?Or,shall I say the best resistor.I have a 59AB with the "V" distributor cap in a 40 Ford.
 
Have coil rebuilt by Skip Haney in Florida. $ 87.....worth the money. Had a spare rebuilt too just to have on hand. No probs in 2 years now. He'll also tell you what condenser to buy at NAPA.

I had the same thing and once the coil was in.....starts hot or cold right away !

Mike
 
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