77 model yz250 spark problem

[blinky]

Hi, I have an old D model yz 250, about 77 model.

Got it going again after sitting in a garage for years.

It stopped on me relatively suddenly.

Took out the plug and replaced it with a new one.

Got it going again and it stopped soon after, within 10 minutes.

Cleaned both plugs and cheched them, they were working fine.

Put plug back in bike, started it, again it shuts down.

Checked plug and it appears to be sparking from the white insulator inside the plug to the outer casing. It is not sparking between the electrode? and the pin. Would this be a problem with the CDI or is it a plug/fouling problem. The plugs are not oiled up after use.

Thanks.

Blinky.