haven't had to post for awhile but can't work this out
after about 30 mins or so the engine starts to splutter and back fire and seems to only run on two cylinders at startup it runs beautifully I have had the rocker gaps checked and set and added all new modern parts etch electric fuel pump, electronic ignition and new coil but still have this problem
someone said it night be the rear two cylinders getting overheated but I can't work how or why they are open to the air any input is great thanks

Vapor lock in the gas line?

gas cap vented ?

Yep just saw the pics.
Gotta have the tins on there particularly if you plan to run in the florida summer heat.
Hope it works out for you.

Be Big,
AMAC

I had the same problem with mine and I was getting to the point of just parking it and forgetting. Tried everything I could think of. Then one day I was fiddling with the fuel line to the carb and noticed that the Electric Fuel Pump Shut Off was hand loose in the side of the carb. I tightened it with a wrench and it has been running great since.