Just ran the Mantis for nearly an hour nonstop wide open. The "new" vent in the gas cap seemed to have done the trick. Idles smoothly, throttles up and down, and starts on 1-2 pulls.
And still no earthly idea why it decided to do this after 10 years.
Put an OEM stock tank vent into the gas cap. The vent hose only extends inside 1/8". The vent piece sticks up outside and I'll have to be careful yanking the rope but I think there's just enough hose to give it a little bit of flex.
It's been an interesting (albeit frustrating at times) repair. I did get into a small argument with the Mantis tech that swears my problem is/was low compression. When I emailed him about this repair and the fact that it fixed it, he said it wouldn't work! He also said it would void my warranty. A warranty after 10 Years? Right.
Thanks folks, for all your suggestions and comments. Great forum