piaggio nrg 50 electrical problem/ engine fault

[davethenorman]

my 1998 nrg is sick.
I brought it as a non runner, and found no compression. New piston later, we have compression and a spark.
Still not starting, so I took the reed values inlet and carb from a working nrg, and hey it starts, but runs for onlya few seconds, then dies. turning the accelerator makes things stall and die quicker.
Could the fault be with the magneto, which is not producing enough power for a good spark? I have also changed the coil, and what looks like some sort of electrical reg that sits on the side of the seat, again from a working NRG.
The last owner said that he cahnged the drive belt and then it would not start, but given the lack of compression, and carb problems he may be wrong. Does the magneto run from the crankshaft? If not below the starter ring, is a small cog which is linked to a smaller cog, but I have no idea what this runs.
Any one got any ideas on what else to look at?
Thanks
Dave
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Dave The Norman