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KX125 won't start and stay running.

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I've cleared jets, cleaned the carb. only to have the plug foul. I'm using the same fuel that was mixed in 5 gal. The bike ran great for two and a half fills. It will pop when attempting to start. Other times it will fire only to rev then shut off. Any ideas?

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Remove the air filter and install a new fresh one.

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Put a fresh new(not cleaned up old one) plug in first. How do you ride your bike? 2 smoke motors are made for high revs. If you are putting around under 5k rpms you are more likely to foul a plug. What premix are you using? I used to run spectro and fouled plugs all the time. I'm using Amsoil now and have never fouled one.

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new plug BR9ES.NEW filter.32:1 BEL-RAY HR1 and 93 pump. I've just today put new ignition coil and Brain box, plenty of blue spark. 173 compression( checked today). The bike was running great, then just bogged and quit.

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and you have already determined your jetting is correct, right? Sounds like you are running too rich and fouling plugs. That was the problem I had with my 04 125 until I leaned it out to proper jetting. Stock jetting was way to rich. Go to PCs website and see what they recommend.

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I was going to say that you might be leaking oil through the rings but if your at 170+ psi that's probably not it. Definitely check the jetting. Move your needle jet clip up one to bring the needle closer to the main jet and see if that makes a difference. Just to warn you, if you hear a pinging/knocking or the plug comes out white the jetting is too lean. Running too learn is far worse for your bike than running rich.

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