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Race fuel with no jetting? possible but painful?

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Ok so my buddy is running a CR125R and was talking about running race gas, i informed him he needed to re jet it and he looks at me and say's "It'll be alright" Well im not really sure on how to jet his bike or mine for race gas which is why im not running race gas yet. But my question is how bad is it on the engine to run race gas if he does it anyways without re jetting it?

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Straight from VP's website about the U4.4:

Designed as a direct, pour-in replacement for pump gas, it will require modest jetting changes, i.e., +2 main jets or +2 pilot jets at most. With its higher octane rating, U4.4 is designed for use in stock and modified 2-stroke and 4-stroke applications.

However, this is not always the case and use should have your air/fuel ratio checked to ensure you are within the correct range

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