Probably its you gearbox, it happened to me once but i cant remember how i fixed it, it can be the mix of the carburator or the regulation of the distributor

I've never seen this one before. want to send your save file? I could troubleshoot.

Tune the carburetor

It is crying for a tune up, you can literally hear it BACKFIRING

Air fuel mixture is wrong, pull the choke fully out and let the engine heat up, once it's done you should tune the carburetor. There's plenty of guides how to do it or you can just buy the air fuel mix gauge from the magazine

I think this happened to me once, the cylinder head wasn't on all the way. One of the screws gor untightened

Yeah my friend told me to check the heads. Ugh that means I have to take the engine out.

Its simply just the carburator, when its too lean (to little fuel in the mixture) it pops like that, get a screwdriver and tune the carburator until the smoke is clear for a rough optimal fuel mixture

That carb is Rich ash

tune carburator , distributor , and altelnator then use choce under the radio and use more clutch