Check the ignition timing when cold versus hot and see if it retards more when hot if it does try advancing the timing another two or three degrees, make sure to check the total timing after that. Don't run anymore than 36 to 38 on pump gas at or above 3500 RPM . I weld up the advance slots in the distributor to limit the mechaical advance so I can increase the inital timing at idle, I like and use 14 to 18 initial and 34 to 36 total No vacume advnce on my toys IHTHs