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High altitude jetting and air screw.

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Ive been trying for A while to find a "formula" or way to calculate the necessary jet size for high altitude. The factory stock jet or recommended size is 380 at sea level with high humidity. How ever I ride anywhere from 6,250-9,000 feet, what should my jet size be? Also how to I measure my airscrew position, when it says 1.5 turns is that measured by fully tightening the bolt than unscrewing it 1,5 times? Also this may be a dumb question but is the air screw the same as the idle screw?

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Even before compensating for the higher altitude,

a big bore usualy requires leaner settings than stock.

(more vaccum signal from the added displacement draws in more fuel)

 

So in theory a 280 likely needs even leaner settings on all fuel circuits (pilot jet, needle's profile, main jet)

than what the OEM chart suggests at all elevation and temperature variables.

 

Yes the air screw setting is calculated from the fully closed position,

air screw and idle screw have different functions but setting the air screw will affect idle speed (which must be re-adjusted).

You normaly set the air screw to obtain the highest idle, then re-adjust the idle speed with the idle screw.

 

Simply explained the air screw fine-tunes the pilot jet circuit, consider it like a 'half jet' adjustment to finely tune

when an ever so slightly smaller or larger pilot jet would be required without stepping to the next size up/down of pilot jet.

 

If the air screw needs to be set outside it's normal range of adjustment (within so many turns of it's nominal setting)

it then indicates the pilot jet is too lean or rich for the ambiant conditions and can't be compensated for by the air screw.

Edited by mlatour

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Ive been trying for A while to find a "formula" or way to calculate the necessary jet size for high altitude. The factory stock jet or recommended size is 380 at sea level with high humidity. How ever I ride anywhere from 6,250-9,000 feet, what should my jet size be? Also how to I measure my airscrew position, when it says 1.5 turns is that measured by fully tightening the bolt than unscrewing it 1,5 times? Also this may be a dumb question but is the air screw the same as the idle screw?

 

I believe this is the type of chart that you are looking for.

 

http://www.ktm950.info/how/Orange%20Garage/carbs%20and%20FI/Jetting/jet_correction_chart.html

Edited by rtv

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Ive been trying for A while to find a "formula" or way to calculate the necessary jet size for high altitude. The factory stock jet or recommended size is 380 at sea level with high humidity. How ever I ride anywhere from 6,250-9,000 feet, what should my jet size be? Also how to I measure my airscrew position, when it says 1.5 turns is that measured by fully tightening the bolt than unscrewing it 1,5 times? Also this may be a dumb question but is the air screw the same as the idle screw?

 

I remember in flight school, long ago, I was taught that the mixture control was important, I'm close to sea level in DFW, so on the ramp we use full rich mixture, then the higher the altitude we were taught to lean out for peak engine performance, the air screw is different than the idle screw, heres a handy chart explaining carburetors I like to use.calchart.gifand here is the type of bird I used to fly.1112284.jpggood luck on your studys!Beavis-and-Butt-Head-Season-8-Episode-2-Daughter-s-Hand-Tech-Support.jpgyeup.

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