Home. Back. ...... The Gas Gas Guru. Technical Information.
I bought a '94 JT25 at sea level and (no surprise) it doesn't run well near home, which is at 6500 ft. I generally ride between 6500 and 9000 ft. - sometimes down to 4500 ft. I know jetting depends upon the condition of the motor, but can you recommend a starting point for these altitudes?

I think the JT25 came with jetting around the 95 main and 33 pilot sizes but don't hold me to that. A VERY general rule of thumb is to decrease the main jet size one step for every 2000 ft of altitude. Sometimes a reduction in pilot jet size works but there are instances where an increase in pilot jet works better at higher altitudes due to the old Law of Physics. Be aware that ambient temperature and humidity have an effect on jetting needs. Higher temperature and humidity sometimes call for smaller jets. ALWAYS err on the side of running rich and jet for the altitude where your bike will run the leanest. Your Gas-Gas dealer is usually a good source of jetting information as he/she will often be familiar with the area where you ride and can supply you with the necessary brass to get your bike running at it's best.


Professional tuner and 'Trials Competition' writer Jon Stoodley has very kindly stepped up to the plate to answer your questions. If you're having problems, or need some advice just send an email to Jon. If it's something that could be useful to others, we'll post it on the site.
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