Modern Vespa

The scooter starts up cold and idles nicely. Runs great for first ten minutes or so then it starts to studder but still idles fine. Never stops running but just seems like it is missing. New plug and the tip is snug. Compression over 100. Clean carb with new gaskets. I have tried both advancing and retarding the fuel mixure to no avail. Dash lights, turn signals, electric starter dont work. Gas guage spikes. Has to be kick started but starts on first kick. New starter motor. New ignition switch. Brake light works but tail light does not light when I first turn the key on like it does on my other et2 on which all the lights and starter motor work. Alternator is putting out over 14 v when reving engine. Any ideas? CDI? Coil? Thanks.

That is a head scratcher. If you are idling fine then the Air/Fuel mixture screw should stay where it is. It sounds like fuel delivery is not the problem because it runs fine for 10 minutes. It sounds like a heat issue. It might be related to your electrical problems. Check your fuse and see if it's good. The ignition system, headlight and the rear light are not fuse-protected so that might be part of your problem. I would swap out known good parts from the other ET before buying new ones. Do one at a time until you find the offending one.

Fuse is good. I checked compression today after driving it for a while and it was about 95 psi. I'm thinking that might be my problem and lowish compression gets worse when it heats up making it run rough. Is that possible? Thanks.

Usually the compression goes up when the engine is warm. 90 is kinda the breaking point, it will run, but not well.

Yep. The fact that it starts easily and idles great all the time and runs great for the first 10 minutes tells me that compression is good enough. Something is heating up and causing it to studder and run rough. Can the CDI do that or how about the magneto? Can these things fail intermittently?

I think you found the cause. Another possibility, if the owners have been using the cheapest possible oil they can find, is that a lot of the exhaust port is carboned up. This usually takes 3-5,000 miles to show up though. But that is very low compression, so that's what I would address first.