Well, an engine backfires because it has unburned OR excessive fuel and air in the combustion chamber. I would guess initially that you had bad spark plugs but you have mentioned they have been checked so I'll rule that out. A medicore spark plug wire could be to blame too. I would guess though seeming how the spark isnt the problem that when you shut it off that fuel continues to go into the chamber and that perhaps the machine is running ab it hot and the fuel is combusting from the hot engine. Tried letting it idle for a couple minutes to cool down before turning it off. Might not solve you problem but it might lessened how loud the backfire is when it goes off.
Try this, next time the engine is cool, turn it on, let it run for 30 seconds then shut it off. If it still backfires, it wont be cause the engine is running too hot since it wont have had time to heat up all the way yet. Just a thought.
We had a vermeer when I worked for another company. If I recall it too backfired alot. We just laughed and I'll be honest, I never bothered to look for the problem since I was underpaid. He owner didnt care about his equipment as long we could make him money and that they still worked good enough to get the job done. Preventitive maintnance happened when something already broke down. And by underpaid, I mean I was promised one thing and given another. In the end he lost alot more than the differencei n the wages promised I can assure you. It happens when you screw the estimater.
