Q: I am running a GMC inline six cylinder in my 52 Chevy. I recently had a misfire on 1990 Chevrolet Corvette

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
I took it in and discovered the valves had heavy carbon deposits. Some of the valves were sticking in the valve guides. we increased the diameter of the valve guides and reworked the valve seats. I have been advised to use a 20-50 Valvoline oil which contains zinc additive. I am also using an octane booster in the fuel.
(1) Answer
(2) Comments
rec. fuel system clean. ethanol treatment to 1 tank premium. ethanol blend in fuel causes build in fuel system, combustion chambers, exhaust system and causes increased wear to spark plugs. this is the carbon you are seeing. octane booster won't do it alone. result will take drive time so be patient.
rec. oil stabilizer treatment on next oil change. this will recondition seals and cylinders(improve compression). virtually all high mileage oils will suffice as long as correct grade. big box stores always have deals on these items(save money on product).
i am guessing u already did the tune up. if you haven't,rec. premium type spark plugs(e3) as replacement. inspect ignition cables and distribution for damage or wear. replace as needed. rec. upgrade from stock to performance grade. more conductivity, better spark. accel's are good and more cost effective than OEM replacements.
sounds like a fun project. very cool truck.