Jetting Q on rotax max pre evo senior

Hi guys, hope you are all good. I have some questions about jetting on a pre evo rotax. I have a kart as a hobby, not racing. I took it out yesterday for a spin and found at 13k it was quite hesitant, if i backed off throttle slightly it would pick revs up and lurch forward a little. As i was having fun i didnt look into it but last night i did some looking.
I have a 160 jet in it (first time i have looked) temperature were 12-14 and a mixture of overcast and sunny. I was expecting to see a 170 based on looking at those charts on google.
To clarify a higher jet, eg 170 runs richer than a lower number like 160?
Secondly have i got some strange things going on in my carb?
Im a total noob with this stuff so hoped to get some words of advice from you guys who know your stuff.


  1. That sounds lean
  2. Jets are calibrated by flow, a 110 jet will flow 10% more fuel at the same pressure than a 100 jet.
  3. 170 on a 14 degree day with oxygenated pump gas would’ve been perfect on my old pre-Evo Rotax senior engine

I don’t think you have anything strange going on with your carburetor


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Thanks for replying. What would running lean cause in regards to the engines behaviour? I have never heard it pop ever, and i havent changed the jet before.
I finally got my data onto the pc i can in fact see i was hitting what i guess is its limiter at 14.1k, which i think is why it did the lurch thing.
So going up does make it richer, thanks for clearing that up.
Just regular shell fuel i believe, my mate was in charge of the fuel run. And 2%ish oil mix
I’ll give the 170 a go next time and ill do more reaearch into jetting for these engine’s. As a noob i have lots to learn. Thanks!