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Idaho Going to Idaho for 3 weeks to ride and need jetting for 1999 yz250

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I am taking a trip o Idaho need recommendation for jetting when you go from 4000ft to 10,000ft, how do you deal with the altitude changes? What jetting will be the best "happy medium" 1999 YZ 250.

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for 6000' i would go 2 sizes smaller on the main, 1 size smaller on the pilot and drop the needle 1 notch, as a general rule. If you already run your screw at 1 turn out or less, you  may be able to just open it up 1/2-1 turn instead of the smaller pilot.

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I have the same bike and llamaface gave great adive to get you through your trip. I am 10 sizes leaner on the main, don't remember the pilot jet and stock needle in the first notch. My jetting seems crisp, but it still sponges a fair amount, it seems that the fmf gnarly pipes do that though.

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I am 10 sizes leaner on the main,

 

Good idea to clarify. When I say '2 sizes smaller', I mean going from a 175 to a 170, since jets are available in 175, 172, 170, 168, 165, etc....

 

All other things being equal, 2500' of elevation is worth about 1 main jet increment (175 to 172). Of course all other things are not equal, and if you are coming from the east coast, the drier air in the west probably means you should stay 1 step richer than you would otherwise calculate. also there are wide variations in temps in the mountains. It's not unusual to start off in the 40's or even 30's in the morning at lower altitude, but you still want the bike to run at 9000' in the afternoon when it heats up.

 

It can be helpful to look at the jetting specs in a ktm manual, look up the temp and altitude where your bike runs great now, and simply apply the same changes in jet sizes and needle position that the manual suggests. Obviously, you wouldn't pay attention to the particular jet or needle specs, just the difference between where you are and where you will be.

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Good idea to clarify. When I say '2 sizes smaller', I mean going from a 175 to a 170, since jets are available in 175, 172, 170, 168, 165, etc....

 

All other things being equal, 2500' of elevation is worth about 1 main jet increment (175 to 172). Of course all other things are not equal, and if you are coming from the east coast, the drier air in the west probably means you should stay 1 step richer than you would otherwise calculate. also there are wide variations in temps in the mountains. It's not unusual to start off in the 40's or even 30's in the morning at lower altitude, but you still want the bike to run at 9000' in the afternoon when it heats up.

 

It can be helpful to look at the jetting specs in a ktm manual, look up the temp and altitude where your bike runs great now, and simply apply the same changes in jet sizes and needle position that the manual suggests. Obviously, you wouldn't pay attention to the particular jet or needle specs, just the difference between where you are and where you will be.

good idea using the ktm manual that all have that same thing in the YZ 250 manual thanks for the help

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I have an excel spreadsheet that will calculate jetting based on temperature and elevation. You need to know what the STP (Standard Temperature & Pressure, I.e. 70 degrees F and sea level...usually the factory jetting specs) jetting for your bike to use it but from there is quite simple.

 

Just enter the STP jetting for your bike, the elevation and temperature you are expecting, and it will calculate the jetting you should use. Main & pilot only...you are on your own for needle type & position. You can fine tune from there with your fuel or air screw.

 

If anyone is interested send me a PM

Edited by Wiz636

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