Have you checked and cleaned your pulse generators?
I had not considered that yet. Wow! So thanks for the question. I will check that this evening! It's my suspicion however that either I have a sticking intake or exhaust valve on the left head, or possibly the left carb slide may be sticking and not opening far enough (or perhaps too far) under 2,000 RPM. I got it to backfire in the intake yesterday while snapping the throttle open and close several times when jerking. And then POP! Left intake manifold backfired. This seems to suggest a vacuum leak or a sticky intake or exhaust valve. The only reason why I'm suspecting the left carb-slide is because that slide has scratches and gouges on it. When I rebuilt the carbs, I noticed that the left slide doesn't transition in and out smoothly with a sticky point. I wonder if that could be the problem.