I've been lurking for a while and finally decided to join the thread.
My immediate reaction, on reading the initial post, was valve timing. I agree with Graham on that. It's difficult to check this without a degree wheel. One thing you might try--and I don't know for sure if this will work--measure the amount that the #1 valves are open at TDC on the exhaust stroke. They should be the same. The intake valve will be opening and the exhaust closing, but at that point they should be the same distance from fully closed. Be sure to approach this point turning the engine CW as seen from the front.
Another possibility: adjusting the carbs should be done with the engine fully hot. Just warming it up in the driveway generally is not enough. But, I suspect that would leave it too rich, and the popping indicates a lean mixture.