This, was a common issue back in the day.... Low vacuum doesn't keep the metering rods down, the plugs get loaded up & soon the engine barely runs... Run the engine a little higher RPMs & the vacuum signal improves pulling the metering rods down & the plugs recover...
Pull the aircleaner & with the engine idling watch the metering rod yoke, it should stay fully down, if it's bouncing either increase the vacuum signal or reduce the spring tension on the yoke...
