Home Forums 2-Cycle Racing KT100 running rough once reaches temperature

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    • #29924
      Joe Bowen

      So this past race weekend I was having issues with my engine stalling out and running rough. The engine would run good (13,000 RPM’s) up until it got close to ideal temperature 375*F  then it would start bogging down cutting out and eventually dying. I was running new fuel from the track pump that day. Any ideas on what could be causing this? I have a lot of support from the experienced guys at the track but we just ran out of track time on race day to diagnose the cause of the problem. my plan is to start with rebuilding and cleaning the carb, New spark plug and see what happens  unless you guys have some other advice.

      Thanks in advance

    • #30235
      Paul Lyda

      First try new spark plug.  If same problem do a rebuild of the carb and be sure that all screens are clean.  Most likely this will not be the problem… but good practice anyway.

      Next check to ensure that your fuel tank is venting well.  If it is not, then you may be creating a vacuum lock type of situation that just happens about the same time engine is getting up to temp.  To check, as soon as it stalls unscrew fuel tank cap slowly and see if it releases any pressure.  You want no change.

      If, those are all good, you most likely have a bad TCI.  We have had a couple time where were chasing what we thought were fuel issues but ended up being a bad TCI!  When they are going bad they start to show it when they are hot.

      If none of the above solve your problem, you may be having an issue with some part of the motor losing its seal when getting hot.  This could be caused by improper torque of cases or jug or head.  All or any of which could lead to an air leak and a lean condition.

      My gut says bad plug or TCI… especially if the slowing motor also made popping sounds.

      Hope that this helps.  Tried to take you in a progression of $$ you have to spend to test/diagnose.

    • #30239
      Joe Bowen

      Thanks for the reply I rebuilt the carb and new spark plug and plan to go test it today. Those are good suggestions to try I’ll let ya know what I find out

    • #30536
      Walt Gifford

      Could be a bad TCI or coil. Don’t bother going to the track without these spares. Also, clean your ground connections.


      FAA certified jet engine and aircraft technician,
      Nicholson Speedway class champion 2001,
      Yamaha KT100 Service Center,
      41 years karting experience

    • #31169
      Thomas Salandra

      You might even have a cracked fly-wheel. The same thing happened to me and that was the issue.

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