Check the cutout switch rubber boot in the carby.
With these G4's, the rubber perishes in time, and lets the contacts stay in 'made' position, cutting out the spark.

Good rundown on this issue HERE

The ODK Online Shop has decompressor refurb kits available. If yours is the rebuildable type, it will have 3 clips holding the lid to the body.

Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."