[Marshall V. Daut]

Possibly a bent valve that missed the builder's attention is hanging up in its guide. Or there wasn't enough clearance in a guide for the valve to slip through easily. Run a compression check on each cylinder, both when the engine is cold and warmed up. That will reveal if a valve is sticking. This is always a good starting point in order to eliminate an internal engine problem before troubleshooting the electrical and fuel systems. The best ignition and best carb in the world won't overcome internal problems.
Marshall