I read that thread, thanks for sharing. Maybe its the balljoints.(sorry).
My convertor is a b+m holeshot,, which is really irrelevant ,because a convertor is just a huge mass of steel being whirlled off the end of your crank. either its dialed in and balanced, or its out of synch. Mine has a raised ring on the snout that goes in the crank end , and it appeared to be worn on on side.....
This convertor was in a neutral balanced ,, cast crank,, 440. It shook, and I raced it. (the conv, and eng, are fine.)
Long and short of the story is, I'd like to see the convertor in a lathe and checked for concentricity. And made sure that the snout is true , and the right dimensions...