Sam

Pre-ignition, or "run on" or "deisling" are the terms for what you describe. Basically there is a hot spot in one of the cylinders causing a spark as the last of the fuel exits the carburetor. Once it happens, the engine keeps trying to run.

You either have plugs that are rated too hot for the engine ( wrong plug) or you have a lot of carbon buildup in your cylinders and a a carbon ember is staying glowing hot, causing combustion.

Could also be a sharp edge ( caused by scoring, wear) on one of the pistons. These can be ground round again.

Could also be too lean a fuel mixture--was your carb rebuilt as part of the tuneup?

Could also be a hot engine--have you ever had your risers and manifolds changed? You don's state the year of your engine, but about 7 years is typical for this needing to be done.

Bottom line: You need a tech to troubleshoot for the cause--and likely you are in for some major work ( But maybe its just the carb)

Good Luck

Kevin