I got a 1998 Yamaha TDR 125 (4FU) 2 stroke motorcycle. It has a Mikuni TM28SS/1 flat side carburetor. I recently had to change the stator coil (the previous one burned down, due to wet conditions), which is from another 4FU TDR, so it's generating the factory values. However since I changed the stator, the bike keeps fouling the spark plugs. E.g. the bike fires up, and I ride it for 5-6 km, without problems, it's powerful, runs fine, but suddenly starts to bog, and then stalls. To me it seems like the plugs are carbon fouled, as I can't spot oil marks that would seem bad to me. Another strange thing is that the bike starts with choke on, but right after it bogs down. If I manage to start it somehow, and I start to apply the choke, the bike immediately stalls. However if it fires up, and I start applying the throttle it rungs. So I guess there is something wrong with the carburation. I took apart the carb and cleaned it, but the same thing keeps happening. The air screw barely has any effect when I turn it also. It's really strange to me, because only the stator was changed and these things started to happen. I also recently changed the exhaust to a Giannelli one that is specifically made for this motorcycle, and I already used it for ~1000km (ran fine with it), but I think it's worth mentioning. Also, when the spark plugs are fouled and I clean them they still don't give sparks, only if I hit them from 0.8mm to ~0.5mm. Anyone has any idea what could be the problem?

  • If you replaced the broken stator coil with a used one, and these problems started, I would suspect the stator coil. Jul 1, 2023 at 20:03
  • The stator has been measured, and it generates the factory values. I was thinking, that maybe something have happened when the stator got wet, since it was a really bad weather with freezing rain, lots of muddy roads. When the bike died, we also tried to push it in a couple of times, hoping that we could get it run again. Jul 2, 2023 at 21:29
  • Maybe it does generate the factory values under test, but that does not mean it isn't faulty under running conditions. If you have no other solutions, buy a new stator coil. Jul 2, 2023 at 22:03
  • ... or if you suspect other water damage, look at the other electrical components, not the carburation. Jul 2, 2023 at 23:26
  • Thank you for the suggestions! Sadly a new stator is not really an option, since I can't even buy it from the dealerships anymore. They ran out of stock. I'll check the other components as well. I'll update this post. Jul 4, 2023 at 7:06


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