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I'm wondering if it will be okay to run 100 octane low-lead aviation fuel in my 2007 RM250 motocross bike (two-stroke). Half of the people I've asked say yes, while the other half say no. Will it be okay to run aviation fuel in this high RPM two-stroke engine?

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You can run that fuel. I've run a 100 hours of aviation fuel through plus another 200 hours of race fuel that tends to destroy FI fuel pumps in various years of RM. I have a friend that is currently campaigning one in a vintage class race series. It's a 1978 RM250. It's fine. Two years since the last rebuilt.

But you don't have a fuel pump so you don't have to worry, if you did, then you drain the gas after each use and get the tank dry and then put in a little regular gas to let it sit until your next use. AV gas eats fuel pumps if left to sit. I don't know the exact degradation process from a chemist view, I just know from experience. Most high octane race gas does this as well.

The fuel will not hurt your bottom end. They are lubricated by oil in you put into your fuel. The fuel will not effect your oil mixture. Oil is miscible in gasoline. It mixes into evenly like vodka into cranberry juice.

I have raced two strokes or have been apart of a club effort racing them since I was 17, I'm 48 now. I've run higher than what you propose octane fuel in RG500 Gamma's, NSR 250R's, CR's and RM's. Your bike will be fine.

The myth that a higher octane fuel will damage your bottom end is exactly that, a myth.

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I can't recommend it. I was told that 100LL has about 4x the lead that the old regular leaded fuel had. Even on aircraft (which run at or near 75%+ power continuously) lead fouling of plugs is a major issue with frequent cleanings required.

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    And airlines are allowed to foul up the air with leaded fumes?
    – Cynthia Avishegnath
    Oct 30, 2013 at 7:17
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    Airlines don't use leaded gas. They use Jet A (fancy kerosene really). It's the small airplanes that use leaded fuel. It's a rather unfortunate thing, small airplanes were slowly migrating to unleaded. However, unleaded started getting ethanol added to it. Ethanol is illegal in airplanes. So, once ethanol became the big thing in unleaded fuels, airplanes were forced back to leaded...
    – Brian Knoblauch
    Oct 30, 2013 at 15:43
  • Lead fouling of aircraft engines is seldom a problem with proper mixture and spark plug selection. Even in aircraft with extended ground idle times, mixture adjustment prevents lead fouling in almost all cases, with some exceptions being selected engines where there is not per cylinder fuel injection and mixtures vary cylinder to cylinder at idle. These are rare if the power plant is properly managed.
    – mongo
    Feb 4, 2020 at 17:46
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You can, but you'll significantly reduce the life of your engine. That's the short and to the point answer. I don't know too much about bikes, but if this were a car, I'd at the very least have someone tweak the carbs and adjust the timing somewhat.

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    How specifically will the life of the engine be reduced? Do you have references on this?
    – mongo
    Feb 4, 2020 at 17:48
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AVGAS 100LL has TEL (tetra ethyl lead), locally about 1.3g / US Gal. The TEL may cause some fouling of the spark plugs. It will not shorten your engine life. It is used extensively by motocross and other enthusiasts here, and their engines die for reasons other than 100LL.

You will not need to re-time the engine, for normal racing, however some people will choose to do so.

The octane rating of 100LL is approximately 100, which is above the rating of normal, frequently containing ethanol, automotive regular grade gasoline (87).

Generally the 100LL does not degrade and /or gel the way that ethanol containing automotive gas will. For that reason, I tend to make my last fuel of the season 100LL in my small power equipment.

The biggest drawback of 100LL is the cost, and availability, since you will have to go to an airport to buy it.

The vapor pressure is lower, so it will keep a bit better.

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enter image description hereenter image description here i have ben running 100LL in my 1972 xl250 for about 2yrs now. Starts on first kick and runs extremely well. It wont harm your motorcycle. I also add marvel mystery oil to gas to add more lubrication to the top end. I live couple of miles from my local airport so its very convenient.

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    Are you allowed to use aviation fuel for road going vehicles - is there a tax implication?
    – Solar Mike
    Feb 4, 2020 at 11:40

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