Keep an eye on that cylinder, it may be burning oil. May need new valve seals.Ok I put a spare plug in the car and it seems fine now.
I guess that thing on the old plug was an anti fouler that was stuck in the head.
OK! this is 100% the cause of backfiring. It is completely normal and every carbureted engine ever made will do that with a 1' exhaust pipe. Here's the deal: when the throttle is closed quickly from a load the engine goes way rich for 1-2 seconds. Too rich for much of it to burn in the cylinder. With a short open exhaust, when the very hot rich gases hit the open atmosphere, they ignite. The flame travels rapidly back up the exhaust pipe and manifold, making an explosion of pressure shoot out the pipe. No tune up or anything else will eliminate the deceleration exhaust backfire -except a longer pipe. Ever watch NASCAR? If so, you've seen this often. The throttle is snapped shut entering the corner, and half second later a blast of fire shoots out of the short, hot exhaust. That's the over-rich mixture reigniting when it hits open air.It happens if i'm driving and I abruptly let off the gas. The car also only has 1ft of exhaust