High Altitude Fuel (2-stroke)

VP's MRX02 is usually the go-to fuel for Mammoth level elevations, some cut it with a bit of U4.4 or Pro6-HT as well. You're on the right track with lowering pilot/main/needle, but that will be bike/fuel/temperature specific type tuning once you get sorted. Cheers

I saw an email that you can pre-order VP fuel from them and they will have there on site for you. I imagine they would do it again next year and can probably offer advice as well. 

I think the bad VP was awhile ago and the listed the batch numbers. Probably shouldn’t be an issue now. 

avgas

Per MXA: MRX02 is a top-level two-stroke fuel. Although it has less oxygen than U4.4, it actually makes more power. Expensive.

https://motocrossactionmag.com/racers-guide-to-the-hierarchy-of-motocross-racing-fuels/

On my CR500 and CR250 I run U4.4 straight 40:1 Maxima 927. Keihin 38mm PWK drop 2 clips on CGN needle, Main 168 to 160, pilot 55 to 50. Subsequently switched to SmartCarb and runs way better. Still need to lean out despite claims of altitude insensitivity, but much easier to tune than twisting the carb around to access needle or main/pilots. Got my best running two strokes ever with SmartCarb. 

Back in the day we used to mill a head for Mammoth.

I used to run 88 octane with a lead additive in my stock (w/ pipe) KX500s. Seemed to work good, I’d drop the needle a couple of clips and adjust the air screw at the track. It wasn’t perfect, but it was absolutely good enough for my talent level. 

Unless you cut the head for the elevation you don’t need a lot of octane. Definitely lost some power, didn’t matter much for me on a big bore.

I run 91 pump in my 300. There’s a few VP gas stations in Southern cal, Or shell.

I am a woods/enduro guy but have a lot of time on turbo snowmobiles (2 stroke) over the last 15 years. Unless you've got a higher compression engine that requires something special 91 non-eth should be 100% fine. If you really want to geek out on it, most higher octane fuels will give you a lazier feeling throttle response and can even have less BTUs per unit which results in less power, too (unless you properly jet for this as well)e. Both are mild, but its basically a waste of money unless you are actually fighting det. The "non-eth" part is important however, especially for those racing (eth = lower BTU)

As to how much leaner you should run, I'd probably be looking at the jetting chart for your bike + your notes. I'm a fuel injected guy so can't comment here. 

 

VP MS109 REG has MTBE and 10% oxygen and zero ethanol. I pay $117.00 for 5 gallons from the distributor.

The MS109 that does not say REG has Ethanol instead of MTBE and it has less oxygen.

Yes, I have a Mammoth and race gas only specific head.