I've got a 14 HP K series, side shaft on my log splitter. I've worked on my friend's K series as well. One on his Wheel horse. 12 HP. The other on a big Ford LGT 14 HP. They are true work horses when they're running right, but I have cursed them as well on occasion. Bad condenser and bad points arcing could cause a backfire like that. I'd check for clean fuel in the carb, spray it out, then do the points and condenser at the same time. Check for fire with the spark plug, see if it's strong. New plug is a good idea too. I've seen plugs fire great outside the engine, but put them under compression and they'll fail. Those old Kohler "K" series are getting hard to find any more. I saw one Wheel Horse either 8 HP or 10 HP sell at an auction here in Knoxville, TN about a month ago for the unheard of price of $35 bucks plus 10% commision plus 9.75% local and state tax. I was going to bid on it but my friend couldn't keep his hand down, so I let him have it. More than likely I'll have to get it running for him. Coil allegedly out on it, but could be anything really. Good Luck.