I have a 1968 Chevelle with a 1974 350 chevy with HEI, and a 600 single feed holley carb. I ran through more spark plugs in that 350 than my father runs through fuel for his 307. In the last 5 years, I replaced dozens of spark plugs. I have to tell you first about how often this car has been run, before you trying to tell me how to troubleshoot it. The car hasn't seen the road in 10 years and is run at least once a week mostly idling. Does excessive idling with a holley carb cause spark plugs to be fouled out? When the most of the spark plugs foul out, the carb and exhaust backfires loudly. NOW heres the kicker......when I install new AC delco R44T gapped at 0.35 (I have that memorized now), The 350 purrs like a kitten and runs great. Over the past 5 years that I owned it, the engine recieved a great, but used 350hp 350cuin cam with matching lifters and push rods. I know it can't be something inside the engine due to when I change spark plugs, it runs fine. I have a feeling its in the holley carb. Does excessive idling on a rebuilt holley cause fouling of spark plugs?