My bike developed an annoying habit... Starts fine but when revved loses all power and stops, even under no load. Try to pull away in gear and it stalls. Once coaxed past the off idle patch it revved fine but definitely spitting back at times on the overrun when out riding. Ticked over no problem.

Lots of messing about led to it running under no load but with regular spit back through the carb, including flames. Very exciting. It was also often reluctant to restart unless left for a few minutes. Applying choke seemed to improve this.

I decided this was a blocked pilot jet leading to a weak mixture. Cleaned it all out, no mess at all in the brand new carb. Jetting correct.

I looked briefly at the rebuilt magneto - no obvious issue.

Then I checked the plug gap - it was as set by the manufacturer and presumably for an alternator ignition system. Reducing it a little to 0.4mm (as by the book for a magneto bike) appears to have totally solved the issue.

Am I mad to think a mag should be able to cope with a few thousandths of extra gap? It was a new plug so had probably sooted a bit after 200 miles of use before the problem started.

Great that it's fixed but it leaves me wondering...