MZ/MuZ Riders

1996 ETZ 251. This bike has run for about 5 miles to the MOT (the good news, it passed) and half way back before it stopped. Thought it was out of fuel but when started after refuelling it would run for a few seconds then backfire and stop. Did this twice but was becoming unpopular with the local residents so pushed it home. The bike has "almost" started once since then but now refuses. Carburettor has been dismantled and had the jets blown through with air and the float level checked. Flow from the fuel tap seems good. Compression seems good. The ignition timing is within limits (about 2.5mm BTDC) however the spark is a pitiful tiny white one rather than a nice big fat blue one. The ignition module (PVL module), ignition coil, battery (12.8V fully charged), HT Lead, HT cap and spark plug have all been changed but the spark is still next to useless. I'm out of ideas and things to replace. Any suggestions welcome.

Thanks for all the input. Ended up adding a 0.33uF 400V capacitor across the PVL. That and a megasquirt of Easy Start got the thing bursting into life. Shame MZ's don't have the crankcase drain plug that BSA Bantams do cos I'm sure some of the problems were due to crud in the crankcase that had to be blown out. Found out today that fork drain plugs would be useful too.

Hi all, I have the same weak spark problem:
I've added an earth lead to the cylinder head/frame mounting stud, and an earth to the PVL ignition unit on the generator, both directly to the battery negative terminal, and it seems to have helped.
-But I'm wondering where to connect a capacitor: should it go across the two wires from the PVL unit?
or should it go across the Black/Yellow wire of the PVL unit and the earth?