Hi Dave,

Yes well known RR that set it up changed and gapped plugs, fitted new dizzy cap, rotor arm and leads as spark was blowing out at 10 psi. This improved matters but obviosly spark is still blowing out once in a while (I assume) causing the big bang. Checked out electrics as said above and they seem resonable, hense why I am thinking about the coil.

Cheers for trying to put my mind at rest about the overrun noises !!