“I own a 2018 KTM 250SX two-stroke and am running VP C12 in it, but I can’t seem to get it jetted properly. It doesn’t run well down low, and I wonder if you have some jetting advice for me. It is a stock engine with a Pro Circuit pipe. I read on the internet that I should be running MS109 or MRX02. What should I do?”

Are you really running 100 percent C12 in your stock KTM 250SX engine? It doesn’t need that much octane, can’t use that much octane and will run ragged on that much octane. When it comes to horsepower alone, stock engines do not benefit from high-octane race gas. Pump gas and 112.5-octane C-12 produce the exact same power. Why? The pump gas, which has a 91-octane rating, is right at the base level of a stock engine’s needs. On the other hand, 112.5-octane C-12 burns too slowly and too cool for the stock compression. The conclusion? You are burning money in your stock KTM 250SX engine. The manufacturers have no intention of building stock engines that require expensive or exotic race fuels. They know that the average rider wants to fill up his gas cans at the corner filling station. Your bike, and every modern engine, was designed to run on the octane found at the pumps… read more >

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