Scooterotica

Recently rebuilt Rally engine, been fine for a couple of hundred miles. Topped up with 4 litres of Shell petrol and silkolene 2T. Set off down the dual carriageway at a fast cruise at about half to three quarter throttle. About a mile later I felt it begin to nip up. Pulled the clutch and the engine stopped. Two minutes rest later and it's fine. Plug a nice healthy colour and no problems since.

Any theories to what might have happened?

as you have rebuilt your engine just recently ,check tighten everything especially cylinder head .

Waaay too little info here to gain any replies other than speculation / guesswork.

Rebuilt, as in changed bearings, seals and rings or rebuild as in rebore, new piston etc?

Run in fully or not?

Using mineral, semi or fully to run in?

Did it get jetted?

Changed the exhaust since rebuild?

etc etc

Had all the bearings replaced and a new piston and rings. Everything else remained as it was for the last several thousand miles.

The only thing I can think of is that I also fitted a new air filter and the holes in the "heart" were a bit bigger than on the old one. That said, the plug was a nice colour so who knows? I know it was a new piston but the bore was same and I've done a couple of tankfulls of petrol with no issues.

What was the piston to bore clearance between the old bore and new piston?

I got a mahle 3 ring piston from SR to replace one that was damaged,both were 66.2,but the newer one was a fraction bigger so I had to have the barrel honed to get the correct clearance (3.5 thou )

Gran sport piston replaced with another one of the same size. No idea of the clearance as I didn't build it. I'll pop the top end off and have a look.