i had the cam sproket shear off and damage the head,so i brought a new head androcker box(which did not have a decompressor????off a dominator?) now the bike wont run,my mate put it together so im not sure whats he's done,when you kick it over it seems to lack compression,althogh you can hear the induction and feel tdc,i know the piston is fine as i saw it before it went back together...my thoughts are that the timing is out and a valve is still just open before tdc? maybee valve clearances,not quite shutting? help!!!!!