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Flosi77

High Altitude Riding

9 posts in this topic

Ok so i have a 04 yz450f and it runs like a complete turd up above 7,000ft the thing is a i'm going to be riding between 7-12 thousand ft next week and am lookin at trying to help it out a little so i can enjoy myself on the trail. ANyways i have started by dropping my main jet froma 172 to a 165....any other suggestions?

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my home is around 1,200 and i typically ride up high like i am going to this week or down in mojave

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For a 8000-10,000 foot increase in altitude, assuming the temperature is roughly the same, multiply your normal home/Mojave main and pilot jet sizes by .91 to .93, and use the nearest size (Your 172 would be a 160-157)

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thanks....yeah i thought 165 was still to big...guess i'll make some last minute changes before i head up tuesday

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Don't forget to drop the pilot by the same factor.

Good suggestions for hi-alt. one thing I notice you didn't mention is changing needle clip position. I have always moved it one clip leaner when going "up high" (home altitude only 500ft, hi-altitude 9-13k ft) but do you think that may not be necessary?

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Not always, and not even the majority of the time. The needle is a secondary metering device that modifies the flow of fuel after it is first metered through the main jet, so it has less fuel to work with when there is a smaller main.

However, every engine is different, and any given one of them may still be too rich at part throttle even with the smaller main. If it blubbers at a steady throttle between 1/4 and 3/4, then it's asking for less fuel, and you should drop the needle.

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Not always, and not even the majority of the time. The needle is a secondary metering device that modifies the flow of fuel after it is first metered through the main jet, so it has less fuel to work with when there is a smaller main.

However, every engine is different, and any given one of them may still be too rich at part throttle even with the smaller main. If it blubbers at a steady throttle between 1/4 and 3/4, then it's asking for less fuel, and you should drop the needle.

Hmmmm . . . next time I may leave the needle alone and change only the main.

I definitely don't experience any blubbering between 1/4 and 3/4 throttle.

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