I have been chasing this problem for months now. Was reluctant to post it here for fear that it is something really simple that I am overlooking, but thought I would describe it anyway since we are all friends here.
My 56 Customline dual quad motor has developed a strange, inconsistent stumble when it reaches operating temperature or above but only under a load. Idles fine and revs up fine in neutral. Dizzy has a Pertronix ignition, and I have recently changed the cap and rotor. 8mm Silicone wires, new coil also. Did long periods of winter storage in Maine, so I even ran a couple of bottles of carb cleaner through the tank. Timing is good, so now I'm thinking that maybe it's a vapor lock issue or a worn distributor shaft bushing? It's been VERY hot here...over 100 degrees, but the motor usually runs at 180 in this weather with shrouded 2,500 CFM puller fan. So, any comments or ideas would be welcome.
Thanks,
Rono

  Ron Lane,  Meridian, ID