250-500cc WR 250 boggy motor down low

Bit of a weird one but was at the end of a ride today going back a main dirt road heading back to the van & the motor almost died & became boggy like the fuel was cut off, I hit reserve & it was the same (plenty of fuel in the tank) had to stop as there was no throttle response, so when home I
Changed the plug & it is better but down low it bogs down, any ideas?
Oh & the fuel tap leaks a bit.........

Dirty/ blocked main jet maybe. If its starving for fuel I'd be looking at carby first..

Seems that way.... starving.....that is

Was the plug oily? If the carb isn't dirty, I'd say you were trying to foul a plug from to rich jetting. Spend a lot of time plunking around at low speeds just before that?

Funny thing the plug was wet & before it happened we were in transit down a main dirt road so going fairly fast, probably 3 miles, prior to that yeah some slow stuff.

Your probably running a tad too rich between low to mid throttle which exacerbates the problem when make that transition from slow speed riding to open road..what carb mate? If it's a mikuni, have a look at lankydoug's post in the jetting database as he's been playing around with this carb on his WR250 with great success.
Leaky fuel tap...it's probably a bit of gunk, you could pull apart the petcock and clean it/blow it out.

Like he said, you are probably a little rich on the bottom or using heavy oil and when you where going down the road you were running on the clip and needle nozzel ( Mikuni ) or Diameter ( Keihin), which are a size fat. Anyway, the bottom line is it was trying to foul a plug because it was all loaded with oil in the bottom end from riding slow / idling and the RPM of the road ride started to move the oil out of the crank and mix with the rich fuel condition at the givien throttle stteing you were cursing at. There wasn't enough heat in the rich part of the throttle to burn the extra oil in the rich mixture and it began to foul the plug. It's sort of a snow ball effect.

Give us your oil, mix ratio, carb type and jetting specs.

Was the bike running fine before this? on previous rides? if this just started towards the end of the ride I don't think it would be a jetting issue.
I do agree with motorhead though but why didn't it run after a fresh plug?

Hey, I could be wrong. I might be mistaking what he's saying for something else. You have a valid point. I could try and throw logic at it all day, but it's not that big a deal.

Thanks Guys, to answer some of the questions, it was running perfect on the last ride without a hickup, same jetting, (dont know what the jetting is, I haven't had it long enough to have a look) same 50:1 premix as last time & fresh plug at the beginning of a 25 mile day, Carb is a Mikuni, & yeah put in afresh plug & it was still boggy down low, oh & pretty much same altitude same temp as the ride when I got it home & put the new plug in, also checked the filter & it is fine.
So .... My local mechanic thinks it might have nipped up as the engine has cooled when I got out of the bush & onto the main track, I hope not :-0
Me .... I am thinking of cleaning out the carb & give it the once over & see if that solves the problem.
But the thing that has me worried is it that it totally died as if the fuel was shut off when traveling about 30 miles an hour with frequent changes in throttle position............surely that cant be a jetting issue?

Check compression,check your reeds(easy when you take your carb off to clean).

Will do for sure, carb is off so I will be having a look at the reeds, but realistically I will only change 1 thing at a time so hopefully I can narrow it down

You could have a piece of dirt stuck in the pilot jet. If you want to easily inspect the piston for damage pull the pipe and have a look in the port. If it went lean and hurt the piston there will be heat damage and scoring on the skirt. You can also pull the intake manifold and look at the intake side as this is the side that sometimes gives it up when it's overdue for a fresh top end.

Make sure the power valve is not sticking open.

Thanks Heaps guys I will rip in in the next few days & let you know what I find.

Might be 1/2 the problem right here:
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good job,
i'd say you found the problem

Shit!!! The question is where did all that carbon bits go??? If a large a piece makes through to the piston then it will score it... best to do a top end.

Are they VForce 3 reed cages in the WR250's or stock...either way, that's the main cause of your issues as it would be a prick on idle.

It is strange.

About 3 yrs ago there were a rash of V-Force reed failures.

The reeds in my 2004 are original and are fine.