I wasn't planning to use the 91 as the motor. The case sizes are the same so I was just using it to see how things would fit. I didnt have a problem with the needle valve with it. I have the rear cover plate bolts all the way against the engine mount and it was still too long. If I put the 91 in it I bet it would sure scream, but then it wouldnt be SPA legal. Weight wise, with the stock muffler the 91 is only .7oz heavier. They are the same without mufflers.