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We were training at Kerpen with my son's senior Rotax and had a persistent engine problem we've never encountered before. Any suggestions appreciated!

All was fine all day Saturday. (BTW, it was sunny and 15 to 20 degrees both days.) Sunday morning we got on track, did  a total of about 20 laps with tire temp breaks. Then he went out again and suddenly the motor was sputtering all the way up to 7500 to 8000 rpm, then it would pull cleanly. 

Checked the plug, and changed the battery -- same issue. Put in a spare (old) wiring harness, and the kart was fine for about 5 laps, then the same sputtering. Had both batteries checked  - the second one has a dead cell, the first was fine. We put the first one back in, got on track -- same issue, except maybe 7 laps before it started. Went back to the original wiring harness -- same thing. (We also cleaned the airbox and carb and put in a larger jet (168 instead of 165).

At this point I don't know what else to do besides get a new harness. 

Any ideas?

 

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Carl,  Tim is in Germany . . .

We let all motors on Energy Karts here mate . . . hahahahahaaha

Ben, can you help this fella out??

Phill, with the drubbing I get about my sea doo engine I wouldn't know about that!
Didn't pick up the German connection, mea culpa..
The time difference would slow communication down a bit. Ha ha.
Ben is one of the good guys and he's a little nocturnal which might help talking with Tim..
Lol.

Sorry guys i went to sleep for once yesterday haha

Tim, its always hard to diagnose these ones over the phone or over the internet without me being able to hear the kart at the track..But here goes:

what you are describing does sound more on the electrical side of things to me. My reasoning for this is how you described that it was ok for 7 laps on that last session. My way of testing batteries is to put them on a load test for 1 hour at a certain amp load. A lot of batteries will show great voltage at rest but fall over when they are under load. This is sometimes the reason that a problem will develop over a session as the battery may start at 12.5V but by lap 3 it is 11.5V then lap 5 its 10.5V and so on.. you come back in the pits and check it after wheeling the kart back and she says 12.5V..

But if we eliminate electrical all together... it could be your needle and seat in the carb.. or fuelpump.. or an air leak in the engine..

It is unfortunately a bit of a process of elimination. Im happy to continue this via email if you contact me via that link carl provided.. I just haven't been getting on kartbook as much lately so its the best chance of me following it up with you

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