"Will.... Be sure to clean tho
"Will.... Be sure to clean those carbs thoroughly, including the fixed high speed jets that are located in the center bottom of the float chambers, and also the small brass feed tubes that lead from the float chamber to the upper body.
The check valves you speak of are actually called recirculating valves. look at the small hoses that lead to the intake manifold. Those hoses will lead you to the (6) valves. They cannot be repaired, and the passage way of fuel should lead from the inside of the valve out to the hose connection. If even a small amount of air pressure can be felt or seen flowing backwards, replace the valve.
Your s/plugs should be Champion QL77JC4 plugs, gapped at .040 . Compression should be approx 100+ psi and even on all cyls. Spark (with the s/plugs removed) should jump a 7/16" gap on all cylinders at cranking speed.
Our questions require answers. If you refuse to answer our questions, how can we answer yours?