Whenever I back off the throttle or down shift with no throttle the exhaust pops excessively.  My Harley will do this but only once or twice.  My buggy will pop dozens of times until it slows down or I give it throttle.  It seems to pop too much.  I have a stock 1967 1500cc engine, single solex carburetor with an oil bath air filter hanging off the side, along with dual exhaust.  One of my friends said its the oil bath filter causing too much restriction which is choking the carburetor off and causing excess gas to enter the carburetor and the unburnt fuel is popping in the exhaust.  I have a free flowing foam air filter in the garage which I have not put on the engine.  Does anyone concur with my buddy?  I think it sounds logical.  I am going to try the other filter and see if it stops the pops but I wanted to hear from the VW engine experts here.  Other than some occasional stalling when I come to a stop the engine runs and starts up fine.

Popping out of the exhaust is often a signal of an exhaust leak.

I had the same problem with backfiring while decelerating and stalling at idle.  Turned out to be dirty idle jet/dirt in idle circuit in carburetor. 

Is the exhaust stock, or glasspack, or what? Is it new or 40 years old.
sounds like the exhaust has lost its muffling ability
not sure what an air cleaner has to do with exhaust ... sounds like a wild ass guess to me

The exhaust is brand new, I believe it is glass packed but not 100% positive.  Its a dual exhaust from JBugs.  I will check the carburetor again.  It has been cleaned twice but it sat for 20 years, its quite possible that all the debris has not been cleaned out of it.  I put all new fuel lines, gas tank and fuel filter on it as well as putting seafoam in the gas.

I just put the free flow air filter on the buggy and took it for a long ride.  It still pops when I back off but actually its noticeably better than with the stock oil bath filter on.   It didn't stall once this time out and it seems just a tiny itty bitty bit more responsive and quicker revving now.  I will try to clean the carburetor again and check the idle circuit for debris in the passages next.

I just wanted to follow up on this thread because I solved the problem.  I found a leak in my carburetor at the accelerator pump so I ordered the carburetor rebuild kit and cleaned and blew out the orifices.  Put in the gaskets, float bowl valve and diaphragm for the accelerator pump.  I set the idle air screw and idle.  Now ALL the popping when I back off the throttle is gone.  The engine sounds much better too!  Something in that carb was not right and it was causing the popping in the exhaust on deceleration.