Just did topend, now fouling plugs

I just got everything back together with a new topend on my newish-to-me 01 KTM 250MXC, and I can't keep it running for more than about 15 sec before the plug fouls. Clean plug and it will fire immediately, bog badly under throttle, and stall. Lots of smoke too. Clean the plug, and it does the same thing over again. I've done this about 6 times now.

It was running well before the rebuild, never fouled plugs, but it was definitely time for a topend. The bore honed out nicely, and everything mic'd fine. I put my compression tester on it after a couple plug cleanings and I'm getting 210 psi, which should be good I'd think.

Could this just be from assembly oil? I tend to use a lot of premix when reassembling a topend, but never had this problem. Anyone had this problem?

Think it would be ok to really rev the hell out of it next time it starts, to blow it out?

Well, a new plug and repacked silencer did the trick! It's running great now. Hopefully the old plugs will clean up enough to act as a spare, since the damn things are $12 each.