#1208, Karmann Ghia Backfire Is Not Backfire

Flames out the exhaust is not a backfire. Flames out the carburetor is a backfire. The caller has a late combustion, known as “popping”, where the mixture burns in the exhaust system rather than the combustion chamber.

Flames out the rear indicate a severely rich mixture and a very hot exhaust. Too much raw fuel coming out the tailpipe.

An air leak in the exhaust near the head is a likely contributor of exhaust popping. Without air the unburned mixture could not burn, would cool by the time it exits and not self-ignite.

Lean mixtures often pop on deceleration when the mixture is not rich enough to burn when sparked but eventually burns when it hits a hot exhaust pipe which is exceptionally hot because the mixture is lean (Catch-22). A sloppy weld leaving a tang in the exhaust, or carbon buildup can create a hotspot to ignite unburned fuel that wouldn’t otherwise burn. But this all occurs near the head long before flames out the tailpipe.

Your probably right, but we always used to refer to it as a “backfire” when it came out the tailpipe too. Mainly because the fire came out the back maybe?? My Dads old 61 Ford I drove as a teen would produce a loud bang when coasting with the ignition switch off, and then turning it back on after a few seconds. I caught hell from him when I blew the muffler off one night doing that.

I had a 65 'Giha about 20 years ago which I bought with a bad engine. I rebuilt it myself, and while it ran ok, it also backfired quite a bit. (not sure about flaming exhaust though)
A friend who was more familiar with VW engines eventually dIscovered that I had installed the distributor drive gear backwards. We reversed the wires to two cylinders and the backfire was fixed. Since the caller mentioned he was working on the distributor, this might be something else to check.