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04 rm 125 miss fire

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I have a 2004 RM 125, that miss fires really bad when under load. Almost like as if the plug is starting to fail. I have changed the plug but still cuts out really bad. standing still with no load the bike revs up into the higher rpm but starts to miss a little towards the top end. When climbing a hill and the power is needed the bike starts to cut out and all power is lost. Clearing the cylinder before a climbs doesn't seem help. Any suggestions?

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Is there a way to test the coil? I don't want to just through parts at it. Could it be a jetting issue?

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primary resistance .15-.45 ohms this is measured at the 2 small wires

secondary resistance 12-21 ohms this is measure at the plug wire to the metal at the coil body, or little wires to spark plug wire

an intermitant coil may read good when checked and act up when running.

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exhaust valve dirty thats your problem. also make sure there is tension on the exhaust valve spring... should be 1/4 turn clockwise. this in turn actuates the valve. no tension no action.

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I messed up, the exhaust valve tension should be a full turn clockwise.

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exhaust valve dirty thats your problem. also make sure there is tension on the exhaust valve spring... should be 1/4 turn clockwise. this in turn actuates the valve. no tension no action.

Where is the adjustment made? I got my son a 04 rm125 and did the top end right away, but saw nowhere to set spring tension.

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The dial on the right side of the cylinder, if the exhaust port is facing you. Left side if your sitting on bike. If you did a rebuild and did not touch it, it probably is set properly if no one touched it prior. If you check it the manual for an '01 says full turn clockwise after you reinstall the cap on the spring. Might be a good idea to check and clean the exhaust valve if you didn't when you did the piston replace. I've used carb spray and it does the job.

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