P85 still won’t fire

The spark plug would be the only thing a backfire might, and that is highly doubtful, effect. Have you unpluged the wires going to your kill switch. That would have been the first thing to do.
I was thinking a backfire might have run the engine backwards and blown up the cdi by running the coil backwards but I don’t feel like that’s how it work idk
Off eBay about a month ago i am not the type of person to send something back anyway I’d rather figure it out just so I can get that knowledge
The seller probably is bicycle engines
This is probably correct since bicycle-engines.com also has an ebay site as well as an Amazon Partner site....If you call them on the phone direct and speak to "Molly" she will be able to pull that info right up for you.

Here is their phone number:
(406) 586-5970