4banger

NIGEL DABSTER is correct, whatever you do don't shorten spring. 'THINNING 'the coil with a grinder is only way to modify a spring, otherwise you risk slide sticking open..........trust me I know to my expense.!!

Check the disc isn't warped as well.

Hi Bonito. The 4 st engine is prone to air leaks on the exhaust joint, this allows oxygen in and causes the misfire at higher revs. Reseal using fireproof silicone and some aluminium foil wrapped around pipecovered in goo, make sure it cures before starting engine.The worst joint is between front pipe and rear silencer btw Hope this helps. ps also check for air leaks on inlet rubber, again prone to splitting.