Questionhi,i got 1991 kmx 125 for off road use,i was wondering what i can do to get maximum performance out of it,eg carb set up,thank you very much
AnswerWell, one initial thing to look for is the early kmx bikes had a restrictor in the exhaust chamber, of course by now it is likely to have been removed or modified on your bike, but if not, you can remove that. It depends where you are as well btw, USA or UK different countries had different restricting factors, but as I said, it most likely isn't there anymore anyway, though worth a look. If it is there,
No rejetting is required on any u.k spec and most european spec bikes. It will then have 24bhp or between 20-21bhp at the rear wheel. This is enough power to redline the kmx125 in 6th, and gives 80mph on std 14/48 final drive gearing. However you can fit a 15t front sprocket to up the top speed to about 85mph, which is better for road use, and still allows good acceleration. If you tune the engine closer to 25bhp at the wheel then it can pull 16/50t gearing to give almost 90mph, but with less acceleration. 15/48t is the best gearing compromise however!
Your KMX125 may also have a restrictive airbox lid however that is not a restriction as such, but it hampers airflow and by modifying the lid and re-jetting gains of 1-2bhp have been seen on KMX's that have been dyno'd before and after.
There's plenty that can be done to tune the KMX like any other 125 really, but when in full power form it's as quick as any of it's rivals and as fast as you would want from a trailbike 125. Of course you could fit a decent exhaust system many to choose from, and there are possibilities in the carb areas, high spec reed blocks for instance, but bear in mind the more you tune up the possibility of unreliability creeps in, I reckon if you try the gearing route, expansion box, check out the reed valve upgrades available, and that will be giving you the most hp increase in the first instance
