Hi,
Can someone tell me what better muffler then the standard can be use with the 91FX ?

Shlomo,

If you need a replacement muffler, those that come with OS clones (ASP/Magnum/SC/SY), as well as anything that fits the .61FX, will also fit the .91FX. You can get any from [link=http://www.justengines.unseen.org]JustEngines[/link] for low prices, sent in a padded envelope.

Macs [link=http://www.macspro.com/onepiece.asp]one-piece[/link] is expensive (item 6860), but will fit.
It will not give any boost in power. It may even lose you some, because of its limited expansion volume.

You can also fit a tuned pipe, from Macs, or if you can get an appropriate header, or fabricate one, you can adapt other tuned silencers/pipes, like the MVVS, or the Rossi.
You can get it for less than the price of the original standard silencer AND get a healthy power boost.

If you opt for a tuned exhaust, you may have to reduce your compression ratio, with an extra head shim, or two (or use FAI fuel).

Ultrathrust from Performance Specialties. It's a bolt on replacement and will get you 600 to 1000 rpm depending on prop. Go to YS support forum and ask Dave Shadel . He makes them. He may be able to tell you how to get them there.

Davis Diesel makes a decent muffler as well and there queiter than stock mufflers. I've used them on some smaller engines with power gains. Only bad thing is that some are still strap on types. But they are supposed to be changing that.
www.davidieseldevelopment.com
[&:]

Close Doug...but this may work better...
http://www.davisdieseldevelopment.com/

Boss