[Farmall] 1951 Cub won't start when very hot

[jimbon88 at aol.com]

Thanks everyone for your suggestions.    
For more background on my Cub, I have kept it the original 6 volt positive ground system.    It has worked fine for almost 60 years so I never converted it to 12 volts.   I also use the pertronix coil specified for that system, although I haven't ohm'd it yet.    I'll try that and see if that gives me what pertronix says I should use.   After I installed the pertronix unit, I think i've gotten the brightest spark I've ever seen on that engine and it started right up on 3 cylinders as soon as I turned it over for the first time!    I checked the timing also with my 12v timing light.   It seemed to light up fine on 6 volts.  I wonder if 6 volts delays the light?    I timed it to the No.1 cylinder at  0 degrees.   I hope that was the correct cylinder.     The manual was very clear about it.    It was a little off when I first checked it, but maybe it was that way all along?   In any case, I have been having this hot starting problem for a couple of years now whether I use a new 6v coil or the pertronix one.    The valves are the one thing I haven't checked so I'll check that next.   The engine was rebuilt about 20+ years ago so maybe they have gotten out of adjustment.    I have to go out of town for a few weeks soon so I probably won't get to that for a while.   If anyone thinks of anything else, please send it along.
Thanks again!
Jim