If you have a good running 91 FX without any mods, consider yourself "lucky". If you do a search (here on RCU) you'll find I am not the only one who thinks they are junk.

The Jett 90L is available through:

http://www.jettengineering.com/

Not the cheapest around, but definately one of the best. You can get the info on them there.

Sorry to hear your (Super Tiger?) 75 is not performing for ya.

Gary