Charles is right on for gearing. His jetting advice is very sound too, for MOST Rotax engines. We’ve found over the years that a few Rotax engines like to run different jets than is usually considered normal. We had one engine that was an absolute dog until we put a 140 in it, and we were running 148-150 in all our other engines!
The MaxJet is useful as a baseline tool, but you will still need to experiment to find the optimal jetting for your kart.
In this case, gearing was most likely your biggest problem. If you haven’t thoroughly cleaned your carb and power valve in awhile, I’d do that too just to make sure everything’s working the way it’s supposed to.
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