I don't know what your altitude is, but a rich mixture an be part of the problem. Also poor timing. It's also possible that you may need to deflect the air slipstream away from the diaphram opening. Basically, any one of the posts prior to this one, or all of them, could be correct. I'd start by leaning the high end mixture down to somewhere close to 1-1/2 turns with the low at about 1 turn and see what happens. Plan on a little experimentation depending on your altitude.

If you have a Syncrospark ignition I doubt you have a timing issue. Throttle coupled is another story.