100LL is Avgas, Low Lead 100 octane.
You are using too much avgas. I run a 13.6 head at 2200' and use 30% 110 race fuel, at 8000' i would be lucky to need 10-15% (but i do anyways).
Try reducing the avgas a little at a time as suggested above.
Gotta chime in here at the risk of getting a bit off track.
The above quote is the worst piece of advice that you can follow.
The number 1 cause of engine failures is insufficient octane for the motor.
You are running a 14-1 compression ratio. If you know someone who is into hot rods, ask them what octane they would run in their 600hp car with a 14-1 compression ratio.
Think of it like this-you have a 2 cylinder motor that makes 150 hp. An 8 cylinder engine is like 4 of your motors put together in one block. 4 x 150hp=600hp.
A car motor spins at 6000rpm tops. Your motor spins at 8000+ rpms. The car motor has nice rolling wheels and you would be hard pressed to hold it to floor for more than 10 seconds. Your sled, well, if your climbing, it can be up to a minute with way more more load on it.
You can never run too much avgas. Period. Beware of people that mix 30% avgas with such and such octane. They may ride like a granny and maybe they don't hold it the bars, ever.
Anyhoo, the advice of the head company could be right to say you can run 91 at 8000ft.
Unfortunately, you don't start out at 8000ft. You start at a much lower elevation. You will need more octane than a cheap *** mix of avgas and 91.
To try to save $10 a sled tank of fuel when you have a $15,000 sled and a $30,000-$60,000 truck is just ridiculous. Consider the fact that you could lose a day or week sledding, a rebuild is anywhere from $1000-$3500 depending on damage and spending $10 a ride for good fuel seems like chump change.
Thats my take with fuel.
Wheel house hit the nail on the head.
If your sled is a 2011 and you haven't cleaned or replaced the clutches, then there in lies your problem.
Screwing with mixing fuel isn't gonna bring you another 1000rpm.
A good tip is to take the secondary clutch off of a friends sled that is running good and put it on yours. Do this on the hill. Ride for a half hr and let him ride with your clutch. If his clutch works better, then don't give it back to him till you get back to the truck. Simple. This tells you what is or isn't wrong.
I'm gonna go take my meds now. Blood pressure meds too.