Ask Dr. Dirt: Jet Rich To Run Cooler

Dirt Rider’s Chief Physician AKA Dr. Dirt is Scot Gustafson. If you have a question about bike maintenance, set-up, tuning, or if you need help diagnosing a problem or just want to know how something works, send your question, along with your first and last name and your city and state, to We may edit your letter and might also use it in Dirt Rider Magazine’s letters section.

Jet Rich To Run Cooler 

Dr. Dirt,

I have a 2010 KTM 125 EXC. I do a lot of trail riding in mountains ranging from sea level to 1000 feet in altitude. I do a lot of hills so I want to adjust my idle rich to try and save my engine from excess wear! What settings would you recommend? Thank you Jamie Fennell

County Waterford, Ireland

For off-road use, the recommended jetting for your KTM is: 48 pilot, NOZE needle on the 4th clip, 170 main and air screw set at 1 turn out. In cold weather use the NOZD needle and a 172 main. While richer jetting will help your engine operate cooler, there is also a fine line between throttle response and fouling plugs. To help preserve your engine I would recommend installing a radiator fan to keep operating temperatures down, similar to the one Ryan Dungey uses on his factory KTM. It is also especially important to keep small engines well lubricated so be sure you increase your fuel mixture ratio from the stock 60:1 to between 32 and 40:1. It is not uncommon for professional racers to run their engines at 20:1 because they use them so hard.