Misfire Rotax Max Senior

My Rotax keeps misfiring, sometimes i can drive 4 rounds and nothing happends, then on different spots on the track it start misfiring at 80/90% of the max speed.
It stotters for 1 or 2 seconds, misfire and then it drives normal again

When it first started I did not made any significant changes

Things I already did:
-New spark plug
-New spark plug cap
-Changed battery’s
-Checked al electric cables, clicked them off and on again

I heard something about a wrong timing in your electric circuit…

I hope some one can help me

Double check the ground wiring. Then, clean and rebuild the carb. An ultrasonic would come in handy here. Check the reeds and the sensor on the back of the block. Now is when a good friend with the same motor is needed because purchasing one electron part at a time is going to be expensive so you’ll need someone to swap parts with. Also give the fuel pickup a look.
It also maybe cheaper in the long run to have a kart shop go over your engine.

To add to what’s listed here. What you describe could also be a “lean pop”. That could be from jetting, or an air leak somewhere.

Do you think it’s possible it’s started to run a little lean?

I experienced similar symptoms a few years back when using a first generation Evo wiring harness which had a faulty ground connection. Replacing it with the latest generation Evo wiring harness solved it. Which one are you using?

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I am using the latest Evo wiring.
I don’t think it’s running lean, we have two other Rotax Max Evo and I am using the same setup.

Thanks for the replies. I think I am just going to bring it too the shop and hope they can find it.

Problem is that it isn’t always happening, but they will have experience with it I guess.

Try new fuel pipe and filter , once I had one pulling air

I experienced something similar on my DD2. For me the issue was fixed by trying a new ignition coil. It’s a fairly easy swap, so definitely try that if the other suggestions don’t work out. Took us way too long to figure out what the problem was, and no online threads mentioned it as a possibility, but maybe try that?

It does sound electrical that’s for sure.

I agree with it being electrical. For me when I had my faulty ground connection it would only start backfiring, not lean popping, after a few laps then would get worse to the point of having to come off the track. New wiring harness solved it. There’s a slim chance that the issue is in the ECU failing, but I doubt that as they are rather bombproof.

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I got my kart back from the shop.

They said it was something in the membrame, the screws came lose that’s what started the problem.

They had to open the engine seal and fixed it.

I hope this can be helpfull