Lots of different issues will cause a backfire (or series of backfires) in the exhaust on decelleration:
(A) carburetor too rich
(B) carburetor too lean
(C) cooling system marginal
(D) camshaft with lots of duration
(E) sticking distributor advance
(F) other
The ethanol MIGHT contribute to either B or C, but as others have mentioned, probably not the issue.
Incidentally, my John Deere 420 did the very loud single backfire about 30 seconds after shutdown (idling did not help, but a carburetor rebuild did). The carb was partially plugged by the evaporation of modern fuel which contributed to C. And I detest working on small engine carburetors
.
The carb rebuild completely solved the issue.
Jon.
