250-500cc 07 WR250, Fires up then dies, why?

Hey,

New to the forum, had my Husky for around 4 or 5 months now. Noticed it's had trouble idling so I knew the piston/rings were on their way out, and then one morning it just wasn't wanting to start, would fire up on the kick but instantly die. Dropped the engine out and the piston did indeed look like it had seen better days. The barrel thankfully looked in good nick and had no scores so that saved some pennys at least. So with this in mind i bought a slightly oversized piston them .35mm (B) Size ones. Fitted this today and also fixed the mech powervalve as it wasn't connected for some strange reason. Built back together everything all fine, it started 2nd kick on full choke and idled pretty alright for about 30 seconds knocked choke off, blipped the throttle and instantly just did what it used to do again and die.

Had the carb apart and everything seems perfectly fine and in order, a change of plug kick again it fires up and then instantly dies. Its like as if it's being fuel starved but I can't seem to find out where and why it's doing this. Just lost all hope with it now and completely stumped as to where to go from here.

Cheers.

Another thing to check, if you have a filter or check valve on the fuel filler cap i've seen those clog, also I had one of the tubes on the
fuel cock collapse once. The bike would start and run for a couple minutes and stall after awhile could be restarted but after
a couple minutes would stall again.

Did you check the reeds when you had it apart. My Wr started chipping on the ends, made it hard to start. Also if you think its a fuel starvation issue I'd take a look for an air leak, crank seals, base gasket etc.

Well it was neither of the 2 you mentioned. The reeds we're in good nick like.

Noticed today when kick it, the engine was sparking at the front mount!! Turns out this had worked its self lose and of course not been establishing a good earth. Still didn't wanna have it when we was kicking it over, I've only got iridium br8eix plugs and it was still firing up and dieing. Rooting through the garage and my old man had a br8es plug brand new...bare in mind this was brand new in 1988 when he used to race 125 national karts, tried that, and amazingly first kick fired up. I couldn't believe it. Only thing I can put it down to is that my bike doesn't like iridium spark plugs. Anybody else have issues with Iridium plugs?