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91' KX125 Help Please

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We have been working on a 91' KX125. The compression stunk so we we gave it a fresh top end tonight. The carb has been cleaned. It runs 100% better for about 5 minutes and just struggles, bogs down for a couple of seconds, and just cuts off. We can let it sit until the engine cools and start it back up and ride another 5 minutes. I don't think it's the plug fouling since it will start again on the same plug. I don't think it's carb problems but maybe I'm wrong?? Could this be electrical, a bad coil, a grounding somewhere?? Any input would be appreciated!!

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If you just did a top end and it will only run for 5 minutes and then not again till it's cold is either a carb issue or a failing big end rod bearing. Did you check the clearence from side to side on the rod's big end or if there was any up and down play? It could also be the needle and seat are sticking shut due to varnish build up from old gas. The carb might look clean but without spraying carb cleaner through every port and orfice and seeing it blow out another hole and then compressed air ensures a clean carb. Another thing to check is the plug wire with the cap unscrewed. Remove the plug cap and see if there is wire ends over 50% of the bare wire end. If not cut about 1/4 inch off the end of the wire and screw the cap back on. I had a 250R trike year ago that had a bad big end rod bearing and it would run for so long and then shut down. It would not start and run again until the bike was cold and the piston shrunk back to normal size. You might also want to do a leak down test of the bike motor.

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Thanks for the info. We just needed to go with a plug that wasn't so hot. It's been running fine today after a colder plug.

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Good to hear that it was not as bad as I was thinking. Go Redskins!!! Hopefully we will get a real QB for the 07' season.

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