1941 Flathead slow starting after warmup

[Jim Miller]

Every spring I clean my ground connections, use non ethanol gasoline with a marvel mystery oil chaser, and two 6 volt Optima batteries in parallel. Two Optima in parallel gives you six volts and a lot of cranking amps. Two fit nicely in the battery box.

[LaSalle5019]

In my opinion, what is happening is that a motor starts easily when cold because the piston clearances are "normal". When you drive, particularly at higher speeds, the motor gets hot and then the ignition" is turned off, the coolant is no longer circulating. In this situation the pistons as well as other parts get very hot and then expand. This expansion causes the piston clearances to decrease, creating a greater amount of friction which causes the starter motor to turn more slowly. With only 6 volts and with possible dirty grounding points it doesn't take much extra load to cause this to happen. After a while, when the motor cools down, the car will start normally. 

I find that a hot engine is much easier to hand crank than a cold engine. The thinner oil is the primary reason for easier cranking.

If you look at where the starter is located on this engine it is directly above the exhaust pipe and gets pretty warm. Electrical resistance in the armature and field coils increases with temperature so the starter gets less efficient when hot and delivers less torque for the same amperage as compared to a cold starter. That is why you may get slower cranking when hot.

If you have a decent battery, clean connections and a healthy starter you may find that hot vs cold isn't all that different in cranking speed as the starter has more than enough torque both cold, when the engine is more difficult to spin, and hot when it is a bit freer. If anything is marginal in the electrical supply or starter you will see a marked difference due to the increased resistance in the starter windings.