Modern Vespa

Hey all,
I've got a question that I thought I'd bring to the experts.
I've got a 1960s Vespa large frame that I stuck a 2005 LML 2T motor onto many years ago.
It's recently picked up a weird issue that I'm not sure how to solve.

Runs like a top. Kicks over on the first kick. Idles great. If I ride it (as of the last two weeks) I can get maybe 4 blocks and then when I come to a stop the idle drops down really low and lopes along for 30 seconds or so until it dies. I can't rev it at all when this happens. After it dies, sometimes I can get it to start up again and run fine for another few blocks... Sometimes it won't come back to life for 5-10 minutes.

I put a spark tester in-between the plug wire and the plug and I can see, when it's misbehaving, that the spark appears to only be firing half the time.

I pulled the flywheel and the stator is visually fine. The pickup coil (which is external) shows a reading within spec (109 ohms), as does the low tension coil (380 Ohms). I thought the LT coil was low, but I found a spot online that shows that this stator plate (with a red CDI should show 400ohms, as opposed to the 500 I've seen on my other scooters).
When I spin the cold engine with a drill, I see about 111 volts AC on the low tension coil.

Any guesses?
Any help would be greatly appreciated.

-Bob

With that fault I would change the CDI. These can be unpredictable when breaking down and often change habits with temperature.
There is the plug lead and cap that could also be swapped before hand.

A problem I had and I'm not at this point suggesting it is your issue, was the crank seals degrading and giving exactly the same problem you describe when running from cold to hot.

I appreciate the assist.
I guess it's not going to hurt to have a spare CDI laying around if that isn't the fix.
I also found a copy of the service manual this afternoon that mentions that the LT coil should produce 200-300 VAC when spun over. Is it more likely that my 18v cordless drill isn't spinning fast enough to make the voltage climb that high? or that the output of the LT coil is lower than expected?

Also, on the failing seals. Would I see oil on the stator? or leaking from the case? Just in case that's something worth ruling out.

Thanks again,
-B

Does it say if VAC is peak voltage or volts RMS?

Just says "…generates a voltage of 200-300 V A.C."
Then mentions testing by "connecting a neon bulb of 220v (in series with resistance) across the green and white wires. Kick the scooter to rotate magneto, the neon bulb should glow simultaneously."

My estimation is that my drill is spinning the engine at least as fast as the kickstart would.

Also, the plug is out…so without the compression resistance, I'm surely spinning faster.

I didnt see any oil as my seals were drawing air in and leaning the mix to the point it would not start. Changed the carb and all sorts before and old speedway rider said my crank seals were almost certainly shot.

But from your description it sounds electrical.

Just wanted to post an update.
I ordered an LT coil and a CDI.
Swapped out the CDI first (as it was the easier swap) and the strange halftime spark issue seems to have been solved.
Very much appreciate the help and knowledge.
Looking forward to a summer full of riding!

Spark plug lead is loose in cap or plug gap is too wide?

Check the magnetism on flywheel. It should be be able to hold a 19mm wrench. If not, it will give an inconsistent to no spark. Happened on my P2. Now I have an extra CDI that wasn't the problem