what cc is your Rotax and what is not fast enough, we can get a 250cc to run 139 at sea level but do not know how to change the tunning to run at 4500ft above sea level,not sure on head volumes and compression ratio's.
There is a lot of learning on this available on the net and you can even buy density altitude computer programs to figure it for you.
The main variables are temperature and altitude. As a rule of thumb, within reasonable ranges, you need to drop about 3% in jet area (which equates to about one jet size) for each 1,000 M of altitude and one size for each rise of 10 degrees C in temperature. Barometric pressure and moisture also factor in, but are harder to factor, but directionally, the drier the air or the higher the barometric pressure, the bigger the jet. And all of these are additive, so you need to cumulatively adjust for them all.
In 2004, on a very hot day, a density altitude calculator said for me to drop 7 sizes from my baseline. I did not have cojones big enough for that (and, besides, I only had one spare piston and no spare barrel). I dropped 4 jet sizes from my established baseline (sea level at about 20 degrees C), and got very similar speed performance as at sea level, which seemed to indicate I was pretty spot-on. I probably could have dropped at least 1 more in the heat of the day, but did not want to risk frying a piston.
As others said, the only way to be sure is to try it. Drop 2 or 3 sizes for starters, and look at your plug and the top of your pistons between runs. Bring a pocket full of jets, as the temperature and humidity can be widely variable throughout the day and the course of the week.
And the final bit of advice, is don't listen to what anyone else tells you, even me.
Manhattan Mark
