I have a 84 F150 351W. I have had a problem for a while with a hard start when engine is hot. The starter turns but struggles. If I let the engine cool down a bit - it starts right up. I have replaced the starter, the solenoid, replaced both the positive and negative battery cables (with 1ga cables) and cleaned the engine block ground.

Any suggestions.....?

 

Try running a ground strap from one of the starter mounting bolts to the frame, it allows more current to run thru the starter and also pulls heat away.

 

I had an old 78 ford that would do that. It turned out to be the coil. In the summer time it was really bad. When it would warm up, the starter would drag so bad, it would barely turn over. After replacing the starter and solenoid, which didn't help, my Dad said to replace the coil. Didn't have any more problems after that.

 

check your timing