I had that problem a couple times but then i'd rejet richer and it would go away. As of now i have a pretty rich needle setup and it doesn't do that at all anymore. the needles have been where all my major gains and changes like the backfires have happened. so i tend to think it could be that a richer midragne by adding another shim might do it.trying different mains or turns on the screws never seemed to help that or change anything for that matter much at all. But of course thats what i experienced so YMMV.
