if it runs well and only pops on FULL release decel, check the idle speed (too low), main jetting (too lean) and exhaust pipes for leaks. Also, pipes without a crossover are susceptible to this by nature.

 

little air leaks exhaust and intake first, then maybe enrichen idle circuit. But really, just give the old gal a break and forget about it. It's good enough...

Pure gas is getable, but not necessary. We must work with available materials...