god damn misfire

My 08 fe550 has developed a rather bizzare misfire, after warm up and off choke after about 5 mins riding it will start to misfire heavily, it wont take *** throttle at all but will allways idle, the only way to fix it is to gently coax the revs up and let it run at revs until it belches out a heap of blue smoke for about 30 seconds and then it runs fine all day, i'm doing the Orkney beach race this coming weekend and i would rather it dint do this as it will cost me a lap.
i have tried;
new plug,
checked carb for water and debris, it was perfect,
no oil lying in chassis rail,
breatrher pipe is clean and not blocked,
oil is slightly below level (it does use a small amount of oil, but not excessive),
air filter is always clean,
tappets are spot on,
any ideas?
bike has about 70hrs on it and this is really spoiling an otherwise good bike.
Cheers

Ah ha, on closer inspection of bike and forum, i can clearly see a puddle of oil sitting at the carb belmouth/plastic intake area, i have now convinced my self this is the problem. well see what happens.

Thanks for the replies,
the more i think about it, i'm convinced its oil level related and although the frame isn't full of oil, theres must be enough about for it to settle down in the belmouth area over night. it then lurks about until it gets dragged through the carb causing the bother, like i say once its coutghed its guts up it runs like a dream all day. last time i checked the plug it was ok Taffy but i'll take another peak and i'll do a compresion test while i'm at it, what sort of readins am i looking for?
Thanks.

Q, Donald were's your troosers?
A, stuck to me header pipe.