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Xr 250r fouling plugs

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I originally thought the float level was off cause the bike was running so rich, however after setting it to the Honda manual, it still ran rich. I just rebuilt the eng. and installed xrs only cam and exh. sys. when I tore the bike apart I remember the burn was perfect, nice brown plug, min. deposits. After installing the cam, exh. and jet kit from xrs only it blew black smoke and fouled plugs after an hour or so. I played with the needle today and ran out of adjustments I then went back to stock jets, 40 pilot, 125 main. With the needle raised 1 clip it still seems rich. Blows some black smoke and a little stumbly. I'm getting confused because the bike ran fine before the mods and now with almost org. jetting seems rich. Logic would say that it should be lean if it ran correct before it got a cam and exh. The air filters a clean UNI, valves set correctly don't know what else to adj, and yes I cleaned the carb during the rebuild. I just talked to Xr's only today, and they said it should run at my altitude with their jet kit (42 pilot, 128 main). They said it's the one they send out with all their pipes and never have any complaints.The only thing they said was to rejet leaner. I'd like to know the real reason for this rather than putting a bandaid on it. I completely went through the carb today, I couldn't find anything wrong that would make a difference. I then returned the jets to the xr's only kit and fouled a plug within 20 min. I'm starting to wonder if it's an electrical problem. I pulled the resistor out of the plug wire and replaced it with a piece of copper. No change. any ideas?

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sounds like you have covered everything. are you using the stock plug? maybe its a heat range issue. how do you ride? do you get up on the rpms, or do you ride in tall gears and lug it around?

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did a new needle come with the jet kit? i have seen worn needle allow excessive fuel to pass.

although this could contradict the fact that it was fine prior to the rebuild. if you suspect the need could be worn, i would replace it.

how about the air/fuel screw? have you made any adjustments there?

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Have you double-checked your valve adjustment? Are you sure your cam timing is correct? A valve hanging open will cause your exact symtoms and your statement that the carb appears perfect would lead me to a base engine concern.

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I think I may have found it. It looks like the small screw that holds the spark plug cap to wire has been broken for a while. I think I was getting a weak spark and at higher cyl. pressures possibly no spark, wetting the plug. Unfortunately I never got to ride the bike until now, as I bought it as a basket case. I possibly would have found it sooner, but I was second guessing my work because I had my fingers in every part of that bike. A new coil from Bikebandit is only $60 I'm going to throw that on it and see how she does. Thanks to everyone for their feedback, Matt

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