My mc18 used to do the same thing, it also popped differently on each pipe when left to tick over. I also had a problem where the revs wouldnt drop back down properly once the bike had been revved up.

I had the Carbs check for balance with some decent honda vacuum gauges and guess what? They were miles out. Balanced the carbs and since then its been perfect, All the problems i had have now dissapeared.