When I fired up my Baby Ringmaster I had a bad vibration  I believe I was using a 5x3 3-bladed prop.  I put the prop on the balancer and sure enough it had a heavy blade.  I read what I am about to describe somewhere else-so not trying to claim credit for the procedure.  Just passing it on because it works.  The first step is to lighten the heavy blade by removing material off the back side.  I did this until the prop stops turning with one blade pointed straight up.  I now have two heavy blades and one light blade.  Now comes the clever part.  When putting the prop back on the engine I place the piston at the top dead center position.  When installing the prop I put the light blade of the prop up (same up as the piston).  This puts the two heavy blades opposite the piston.  As it turns out pistons are never completely counterbalanced.  The slight weight offset of the prop helps with the counterbalancing.  Anyway the 3 bladed prop now runs smooth as silk so the above seem to work.