i have just had bottom end rebuilt and all new vavles in head.put it all together it run 4 bout 5 sec made a bang and stoped.wont run now.im sure timing is bang on not 2 sure on dizzy arm.plugs leads r all ok.any 1 got any idea im not that clued up on these.

 

Double check the timing. Make sure the belt hasnt jumped and bent any valves. Check for fuel and spark. If both of these are ok and chances are you havnt fiddled with the metering head adjustment then whilst winding it over try moving the dizzy. Mark it before you start moving it so you can out it back if needs be. It'll only take a small amount of movement aswell so take it slow and try not to touch the leads while you there or you'll get a jolt