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# elevation change

## 6 posts in this topic

I am making a trip to Arizona with my bikes (yz450, yz85, crf230). The bikes are jetted fine for Florida, but I'll be running them around 5000 to 6500 feet above sea level compared to the 100 feet that I'm riding now. I am sure I'll have jetting issues with the yz's (stock jets). I am not going to race, just trail ride with the family so optimum performance isn't a major factor. I just want to keep them running decently without causing damage.

Do you think I could richen the the mixture enough by adjusting the carb for the altitude?

If I need to get some bigger jets, what would you recommend?

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You need to go LEANER for high altitude. You have to cut back on the fuel because there is less air. For a move from sea level to 6000 feet, multiply the size of your current main jet by .94

That, would, for example, take a 175 to a 165.

Also, look in the jetting DB at the top of the index.

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Where did you come up with that figure? (.94)

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It's based on the following formula:

Correction Factor (CF) = 1.0778-.00111*T + (2.38554*10^-10)*A^2 - (1.0777*10^-5)*A

T is temperature in degrees F; A is altitude in feet.

If you plug in A=0 and T=70 you get CF=1. This is your correction factor at standard temperature and pressure (STP). If you put in 6000 ft and 70 F, you get CF=.94. So your bike that was jetted for a 175 at STP now requires a 0.94 * 175 = 165 (or the nearest size).

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It's based on the following formula:

Correction Factor (CF) = 1.0778-.00111*T + (2.38554*10^-10)*A^2 - (1.0777*10^-5)*A

T is temperature in degrees F; A is altitude in feet.

If you plug in A=0 and T=70 you get CF=1. This is your correction factor at standard temperature and pressure (STP). If you put in 6000 ft and 70 F, you get CF=.94. So your bike that was jetted for a 175 at STP now requires a 0.94 * 175 = 165 (or the nearest size).

Is any factor in this formula specific to a YZ? I have a TTR250 and I'm trying to determine what to change when I go from 1600 ft to 4000 ft.

If I use 70 degrees (good guestimate), and I plug 2400 ft into this formula and get a factor of .9756, that would tell me to change my main jet from 140 down to 137. Am i following the logic correctly?

I would assume a little adjustment of the fuel screw might be necessary, but do you think the pilot jet could remain the same (52.5)?

Looking forward to any replies...

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Is any factor in this formula specific to a YZ? ...

If I use 70 degrees (good guestimate), and I plug 2400 ft into this formula and get a factor of .9756, that would tell me to change my main jet from 140 down to 137. Am i following the logic correctly?

No, and yes, respectively. But keep in mind that this formula makes two assumptions that you have to adjust for:

> Your current jetting is perfect, and you just want to know how much you should change it, and,

> Your current temp and altitude is 70F at sea level.

What that means that you have to do one of two things:

Imagine your current jetting is a 160 main, and that works good at 80 degrees at 2500 feet. Calculate the correction factor for 80/2500 (0.96) and then Divide your current jet size by that (yields 154). Now you can use the actual temperature and altitude of the place you want to correct for by multiplying it against your adjusted base jetting of 154 instead of the 160.

Assuming the same conditions above again, say that you want to know what jet will mirror your current setup at 70 degrees at an altitude of 6000 feet. Here, you take the difference between your base temperature and the standard 70 (-10) and add that to the target area temp, making it in this case, 60 instead of the actual 70.

Next, subtract your current altitude from that of the target area to get the net change in elevation (here, that would be 3500 feet). Now run the calculation for 60 degrees at 3500 and multiply the correction factor against your actual current jetting.