Originally Posted by mtothef
shave a bunch of eight off the flywheel. all of which will probably make it a whole lot faster but also less reliable, and will considerably unweight my bank account.
The F81m , which of course came with no lights, had a very small internal flywheel, that is, internal to the coils. Yes the bike was a faster and when it came up on the pipe it was explosive. TO the point it was difficult to ride at times. At the same time I also had an F8 enduro. Working on the bikes one day, we pulled to ignitions off and replaced the F81m's rotor/ignition with the larger, heavier F8 ignition. Remember, this was in the days before flywheel weights.
The heavier weight smoothed out the ferocious hit and though the bike may have been a bit slower, I was actually faster on it. Ended up ordering another complete F8 ignition to leave on the F81m. THe other thing it did was provide the needed lighting coils which enabled me to install barely legal lighting, thus enabling me to ride the F81M in some enduros.
As for rotary valves, as I recall, the Bighorn stayed a rotary, but the 250 changed from rotary to piston port in either '72 or '73.