well the timing doesnt go out on them its preset into the cdi unit and it doesnt have valves that need timing up either,it sounds like either the person who has rebuilt it has maybe done a bad job and broken a piston ring or two or something else which has lowered the compression ,if the bike was ticking over fine before it had a rebore ect...then it should still be fine on the settings the carb had then ,the mixture screw should be 1 and a half turns out after scewing it all the way in and the tickover screw should be wound in as far as it takes to make the bike idle at a reasonable speed ,also a blocked exhaust pipe will cause the symtons you mentoined so check its easy to blow through when taken off,ok good luck ill check your reply and try to help more if needed ,adam
adam
February 2006