high altitude jetting

2 replies to this topic
  • cohonda

Posted February 26, 2003 - 07:49 AM


I have an 00' with a white brother e series. I run this bike at 7000 ft to 11,000 ft. I bogged heavily with a 180 main jet so I changed to a 175 and it helped a bit so I went to a 170 and now it run like a champ. How do I know if its running to lean? thanks

98' cr250, 00 xr650, 02 xr50, 03 crf 450

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  • qadsan

Posted February 26, 2003 - 09:55 AM


A 180 main jet is way too rich for a stock uncorked XR650R at those altitudes. Even a 170 main jet may bea bit rich. You may want to try a 168 or even a 165. If your low end throttle response is suffering, then you might also want to try a 65s pilot jet.

Exhaust popping on deceleration is one indication of running too lean. If your engine is surging to running too hot, then it is likely running too lean. Another very good indicator of running too lean is the spark plug. Properly reading your bikes spark plug can tell if you there's detonation, preignition, whether its running too lean, too rich, etc. Several popular articles about reading spark plugs simply state that if you're spark plugs insulator is tan, then your mixure is correct, but there's much more to spark plug reading than looking at colors. The color of the spark plugs insulator can be misleading and is not always a good indication of your mixture, which is why you should not rely on it, but rather use it in combination with other clues such as looking at the type of deposits, location of deposits, the mixtre ring, etc.

Running too lean can lead to catastrophic engine failure, so if you decide to ride at elevations much lower such as sea level or experience the signs of running too lean, then make sure you rejet to a richer mixture.

  • billmunson

Posted February 27, 2003 - 05:19 PM


DO a spark plug read at 3 throttle settings: just cracked, 1/2, and full to test pilot, jet needle, and main, respectively. You need to travel along a flat surface for about 30 seconds maintaining that throttle setting all the way. Then pull in the clutch and quickly kill engine, coast to a stop, check your plug.

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