Since we've not run the engine with mufflers, we don't have an answer about how it should respond. We'd think that bolting-on the muffler for the FS-91 Surpass shouldn't cause any problems. Slow acceleration is usually due to a too-rich idle mixture. It's possible that the fuel tank's too high. This is common with the twins because of the low carburetor.

We've never flipped the engine over and run it with the carb on top. The crankcase vent would also then be on top, and you might wind up with too much liquid building up in there.