After browsing through the online resources I would have to slightly
correct myself. Proper term for what I experience beside stumble is
probably "detonation" rather then backfiring.

At WOT (measly 2000 rpm) the engine detonates like a cannon. In the
water it's not too bad but on land, boy...sounds like a shooting
range..

I'm using (as stated) BR6FS (couldn't get AC MR43T). The equivalents
that I found in Merc manual are AC-MR43T = Champion RV15YC4 = NGK
BR6FS... Is that your experience guys?