Ok i just bought a 02 yz250f ran great for a while then started fouling plugh now it wont run at all ive done everything i can think of spark plug oil gas oil filter air filter etc still nothin it started once with the gastank of but then died i checked the breather hole in the tank and its all good my last idea is valves but i wanna make sure before i buy shims

Pull the spark plug and test for spark, also look and see if it got fuel on plug. Sometimes when they tip they suck fuel in the breather and foul plug. Also check carb, so times the needle sticks if it was jarred much on the ride. happen to units that sit up awhile between runnings

Hi,you have done quight a few things to the bike wich aliminates a few of the problems,first since replaceing the burnt wire did you check that numer 4 cylinders plug was the write coulor,if its still sooty it could be down to a blocked air jet cousing a rich mixture,this sootyness on spark plug is normerly a sign of to rich mixsture or electical falt.
as for the exourst if the bike was running ok when you first put the exourst on it should be ok,some exoursts do fit strait on and some need up jetting,I would ask where you bought it from.
after saying all of this personaly I dont think it would effect just the 4 cylinder if I was you my first job would be to strip the carbs down and clean them up and go for another test drive,I hope this was some help as a ex-biker I understand how frustrating this is all the best hope you sort it from what you have already done I think you will get to the bottom of this and have it running properly in know time,again all the best.

turn gas on under tank ,get new spark plug, gas mite be bad.

Compression, spark , timing , fuel for any engine to run. Don't forget a stopped up catalytic convertor will have the same effect as you described. Backfiring of course is fouled plugs, bad plug wires, look for arcing from wires when it is dark outside. If it has a distributor check for cracked cap, had my 91 GMC start backfiring then die due to ignition module inside distributor going bad. It would start back up after it cooled for about 10 minutes.

Have you changed the oil and cleaned the air filter, after the fuel tap problem.
fuel in the oil can cause this symptom. the fuel ends up in the sump when the taps fail as yours did
also check choke is not stuck open, and battery is in good condition

I would start by switching plugs. Then does the wet warm one get hot and does the hot one now stay only warm. If the same plug stays wet, replace it.
If the same cylinder is not firing, check the wiring to the plug. Check the coil wiring and coil for spark to that cylinder.

recheck spark plugs , gap and for cracks

These things are really temperamental.The plugs are fouled again..I had the same bike and went through the same thing..It doesnt take much to foul them..Get some new plugs and she'll run like a beaut..Just remember not to choke it to much..Thats where the plug fouling happens the most..Always keep a spare with you and you'l always be on the good..It will work!!Have a great night!!!!!!!!

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fouled plugs and or faulty plug wire