So.. I figured out why.. and this was a funny one.. so I thought I'd share in case anybody else is having the same issue.
In short.. when I turn off the ignition, the engine keeps running for .. oh. about 3-4 more seconds. Any by running, I mean perfectly smoothly running. It's definitely not dieseling or anything like that.
Today after a few hours tinkering on the car, I asked my wife if she wanted to go for a quick GoKart spin around the block. "Hell Yeah" she said.
So.. fired it up.. backed out of the garage.. and shut it off to run and grab my phone... you know.. just in case.
And it shut off RIGHT away. No 3-4 seconds of extra running. ??
Interesting.. so.. went for a good run around the block.. took a couple of neighbors for a spin.. and back to the garage.
This time.. it did the 3-4 seconds of extra running.
Ok.. enough of this. So. I grabbed my Voltage tester.. put it on the coil.. flicked on and off the ignition... and it cuts power perfectly.
Then.. fired it up.. and did the same thing.
WTF... The voltage tester showed 12+ volts for that 3-4 seconds.. and then steadily dropped to 0 over another 3+ish seconds.
I noticed.. it was perfectly timed with the "Spin Down" of the radiator fan!
Aaaaaha!!!
And there it is. The inertial rotation of the cooling fan was acting like a dynamo and pushing 12v back into the positive side of the wiring. It was actually keeping the coil powered up! go figure!!
So.. I grabbed one of the extra 10A Diodes I had on hand that I used to wire in my Hazzard Flasher. Put the diode inline to the cooling fan.. and problem solved!! No more continued running.
And there you have it.. if anybody else is noticing that the motor won't die right away when the ignition is cut.. see if it happens to be timed to the cooling fan. If so.. an inline Diode will fix it.