Good job J-R.

There are two main differences between the FZ91 and the AC91. The FZ has a tapered collet retainer for the prop drive washer whereas the AC has a woodruff key type. Also on the FZ the carb body on the back of the engine is slightly larger but not enough to be apparently to the nacked eye.

In the beginning, the only way to tell a FZ from a AC was by looking for the collet driven prop drive washer because they both said AC on the case. Right before they quit making the FZ they cut off the AC part on the case and then, eventually, they started stamping FZ on the lug. But the majority of FZ's say AC on the case.

The engines run pretty much the same and it doesn't make much difference as to which one you have unless you are going to buy a crankshaft, prop drive washer, carb body, or barrel.

Enjoy, your engine

Jim