Hey guys, new member here. A few months ago I purchased a 88 trx250r. Only mods it has (as far as I can tell) is a KN filter, and FMF header and pipe. It was jetted for sea level, took it to Winchester Bay, OR and it ran great. I live in Nevada (Reno area) though at 4600ft elevation. I just pulled the main and pilot jet, main is a 158 and pilot is a 55 for sea level. Any recommendations on where to start for both jets with an elevation of 4600ft?
Higher altitudes require higher compression and leaner jetting for top performance. If you don't increase compression, at the very least at 4600 FT Lean your jetting out one jet size for every 1000/1500 FT increase or per the refference Matuus provided.
There are also charts available from nearly every carburetor manufacturer that will give you a correction factor based on elevation and temperature. No guessing needed.
I have found over time most if not all charts are off the mark. The "one jet size for every 1000/1500 FT increase" seems to be a closer match to reality. However; at the end of the day, it's best to test run at Altitude, and adjust jetting as needed.
Carlos
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