[decooney]

Ken,
I may be off but it sorta sounds like an ignition/firing or plug problem to me. The first thing I'd be doing is reading the plugs on the left bank and comparing to the right and looking for a fouled plug, etc. My guess is it did not run this way in the beginning when you first got the motor together (first few hours) and I'm wondering if one or more of your plugs is so fouled out now that its going bad or is bad. Plug read time. Check the color of those insulators one plug at a time and mark which plug came from which cylinder one by one. Take it out, running full blast and shut off the key close to your house as possible. Coast to a stop, and pull those plugs one by one. Look at the color of those insulators.

Are those the original plugs from when you first ran the motor. All it takes is a few marginal or bad plugs created by an overly rich carb to start with to create your situation. Let us know.