It is very possible for an engine backfire to cause a fire to start IF there is fuel for the fire. Your mower could have caught fire due to a backfire if: (1) It already had a gasoline leak when you loaned it to the neighbor, or (2) the neighbor could possibly have caused a gasoline leak, or (3) the neighbor spilled gasoline from the storage can while the engine was running, or (4) the neighbor spilled gasoline from the storage can while filling the tank and then started the mower again before cleaning the spill or waiting for it to evaporate, or (5) the neighbor tipped the mower sideways to access the blade area and spilled gasoline from the gas tank.
If a 15 year-old was operating the mower, then (3) or (4) or (5) is extremely possible and it would also be extremely probable that the boy wouldn't admit to it. A boy in our neighborhood destroyed a mower by error (5) - he stopped the mower, tipped it on its side to clear some damp clippings, started the mower again, and watched the mower catch fire.
Who pays to replace your mower? Conventional "good old boy" guidelines say that if it broke while somebody borrows it, then the borrower repairs it or replaces it.
I'm not that rigid on tools that I loan out. If I feel that the damage was not the fault of the borrower, then I feel that he has no obligation to repair or replace. If I know that my mower leaks gasoline and tends to backfire and I loan it to you, then I'd say that a fire was bound to happen sometime soon. Likewise, if you borrow my car and the fuel pump or the starter fails, you aren't at fault since that part was due to fail. If you borrow my mower and the throttle control cable breaks, I would refuse to accept payment for that part which died of old age.
On the other hand: If the damage occurred because the item was loaned out, then the borrower should offer to make restitution by either repairing the item, replacing the item with one equal to or better than the borrowed item, or pay me a fair replacement price.
If you borrow my car and you dent it, or it is dented by somebody else while you are using it, or you hit a curb and cause a tire sidewall failure then you pay. If you borrow my mower and pinch the throttle cable by folding the handle improperly, then I expect you to take care of the repair. I'd expect you to offer to repair it that day or deliver it to a repair shop that day. (Actually, I'd be happy if you went out and purchased the part immediately and the two of us replaced the cable in my garage that afternoon.)
If I were your neighbor, I'd offer to replace the mower, possibly with a $50 copay on your part since the mower was 3 years old. If you hesitated even slightly about paying a $50 share of the cost of replacement, then I would not hesitate to pay the entire amount. Without evidence to the contrary, I must assume that my 15 year-old son is at fault. How many 15 year-old boys take warnings seriously or admit to big mistakes willingly? Few.
FYI - I have offended several friends by refusing to loan them one of my lawnmowers. In fact, I generally will not loan any tool with a cutting edge - for both liability reasons and for tool damage reasons. For some of my moron friends, neighbors and relatives, I won't even loan them a screwdriver.
I assume that you are trying to determine who pays for a new mower. You didn't mention anything about your neighbor or his son offering to buy you a new mower, so you've already got a problem on your hands.
Good luck. Gideon
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m Ransley wrote My neighbor borrowed my 3 yr old Cub Cadet walk behind 6.5hp with Kawasaki engine. It caught fire while they were cutting the grass and has been junked , it is gone to the dump. My neighbor said it backfired then they left it and it burned. The kid cutting it was 15 and I figure he put gas in it while hot and it splled catching fire. But a different neighbor heard it backfire . I know a backfire can cause a flame out the carburator possibly causing a fire. But my question is this possible for a good mower to catch fire from a backfire , or is the only logical explanation is he was refilling the mower while hot and caused the fire. Im just curious and have googled for 'Lawnmower fires" and find nothing showing a possible defect to cause a fire.