First,make sure the prop is balanced and not just seem to be balanced with a good balancer.
After this,rotate your engine until the prop is at TDC.
Next,alighn your prop at a 90 degree to this or so its pointing at the 3 oclock to 9 oclock position and try again.
Also,what is the timing set at?Is it Synchro spark?Should be set at 28* degrees BTDC.
Sometimes if your low needle is rich it will vibrate.