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Thought i sorted it out but the bloody thing still does it,when it does it blows rocker cover gasket out on the rear cylinder,tried disconecting PC111,the quickshifter,have just ordered a pair of coil packs and give that a go,if all fails dig a big hole and bury it

Fire up the JCB

Just thinking about this, For it to blow the rocker cover gasket it must be pressurising the crankcase.

could it be a broken ring or crack in the head

hope the coil sorts it, but after that I think it could be something mechanical, maybe a compression or leakdown test

Cant see why a dodgy coil pack would cause this issue. Yes a faulty one could cause a misfire but the excessive crankcase pressure is caused by like he said, broken ring/rings, a holed piston or a crack in the cylinder head. Must be annoying!

the weird thing is guys i can start it up as normal go out and ride it for a couple of hours like i did saturday runs like a dream,turn it off start it up ten mins later then it backfires as soon as it starts,so if it was mechanical surely would do it all the time where as this has been an intermitant fault which leads me to think its electrical.....

Any smoke on start up etc? Valve stem seals?

no smoke what so ever mate bloody thing does my head in,like i said runs like a new bike and goes like stink defo electrical good friend of mine builds race bikes and it's one of those things that you find by a process of elimination

done nearly 300 miles last weekend and it looks like the backfire issue is cured,ran some injector cleaner through with 2 tanks of fuel and have had nothing since and she seems to tick over a lot smoother too,a car mechanic friend of mine after i explained what was happening said it was defo a fueling issue and was probably a blocked injector(he has seen this sort of thing on cars) hence the cleaner.so touch wood happy days....

Nice one, glad to see/hear it's finally been found/cured.

nice and cheap fix too