Modern Vespa

I have a 2018 Sprint 150 with a Malossi big-bore kit and accompanying Force Master unit; it's got about 12,000 miles on it, of which about 9,500 were put on before the big bore kit.

Sometime in the last 500-1000 miles, I started having a problem: when I start the bike after it's been sitting a while, it starts fine and runs well until I have to stop at the end of the driveway or down the street, and then it quietly dies. When I re-start it, it is reluctant and takes a while to get going, but inevitably it eventually does. Sometimes I hear a detonation in the (Malossi) pipe when it's trying to start. It runs fine after this initial stall; I've never had it stall a second time. If I run the engine for five minutes or so before I get going, that seems to do the trick.

I have run fuel injector cleaner through the fuel system, checked the electricals, and tried adjusting the fueling on the Force Master, with no apparent affect on the problem. What else should I be looking at?

Sounds like a valve clearance issue. Have you necked your valve clearances?

I have not. I will look at that. Thanks.

That did the trick. Thanks!