The B&S 190cc "8.75" Intek in my 4-year old Snapper had recently been smoking a bit on startup, and more recently started to backfire (not a pop, more like a .22) after every shutdown.
I had changed the sparkplug late last year, but didn't think to check the gap. I'd several time knocked the dust out of the air filter and washed the prefilter, but hadn't ever managed to buy a new air filter yet.
So I finally managed to coincide remembering the filter, being at an appropriate store, and knowing which filter to get.
I used some carb/intake cleaner on a rag to clean the debris off where the filter seal sat, and since I'd been playing with Seafoam on cars recently, I poured a little down the airhole under the filter. I sucked it through gently by pulling the starter a little, and then ran the engine briefly to clear it out.
This reminded me of looking at this forum about Seafoam on lawn mowers, and I saw B-12 recommended.
I got some the next day, took the filter off again, and poured rather a lot into the air hole.
It was so much that I couldn't pull the starter much, so I took out the spark plug to relieve pressure.
(I also checked and set the gap at this time, which had been .030 instead of the required .020.)
The first slight pull sprayed a bunch of B-12 out of the plug hole, so I put a paper towel in front of the hole.
(B-12 strips paint fast. How do you prime a mower deck?)
After letting it sit a little while, I pumped out the remaining wet B-12 by pulling the starter. On every pull, vast amounts of tiny carbon chunks sprayed onto the paper towel which I kept replacing to see if it was done yet.
It seems to start a little easier now, and after a while it occurred to me that it's no longer backfiring!
The air filter had previously been dirtier than it was just before replacing it now, and the spark plug gap had been off for much longer than the backfiring (but was causing the startup smoke, perhaps?), so I'm guessing that the cleaned-out carbon (which might itself have been caused by the gap or the dirty filter?) fixed the backfiring.