Try this- - - - -rev the engine to around 2,00-2,500 RPM and hold the speed steady- - - -maybe set the fast idle cam of the carb on the second step or so, and monitor the intake manifold vacuum with a gauge attached to full vacuum (not the distributor). If the vacuum doesn't hold a steady 20 inches of mercury or more, or the vacuum drops steadily the longer the speed is held constant, look for an exhaust restriction. I've seen collapsed exhaust pipes, rodent nets in mufflers, etc. that would cause extreme back pressure and detonation anytime the engine got loaded at all. A quick check is to loosen a pipe at an exhaust manifold and do a test drive.
Jerry
