A guy recently gave me a push mower because he bought a new one. At first glance, nothing was wrong with the mower. Being an ANTI-champion spark plug person, as soon as i got it, i replaced it with a brand i like. well, needless to say, i've never had a lawn mower get this hot when using. It gets so hot, that the head disfigures a new color, and it even gets the spark plug so hot the whole metal part, like where you slip a wrench onto to tighten it, gets turned a black burnt like, sometimes purplish in some spots. When i got it, i noticed the champion plug that was in it looked discolored as well. i thought that maybe he put in a hotter heat range spark plug, but i went online and looked and the plug he had on it was the correct one. RJ19LM. at first, i didn't think nothing of it, but when i put a new spark plug on it, within hardly no use at all, the spark plug started to change color at the metal. even the crush washer/gasket gets a burnt greyish look. I put it on by hand, and when i thought maybe something was wrong, i went online and got the correct torque specs, and torqued it to what it said, and it still changed. I even went to one grade colder heat range and still the same thing. I went and even bought a new champion and the same size champion, yet one grade colder, and both of them change color within a tank full of use. I took the cover off, and all the fins are clean and no grass or anything clogging them up. Before i go any further it's a B&S 5 HP. The deck says MTD. i also have another B&S 5 HP, and using my favorite plug on it, i've ran it for 3 years and the spark plug hasn't changed color, and the head on it's motor isn't the same color. On the one i'm talking about causing the trouble, sometimes when you mow with it, and you let go of the handle to turn it off, it will backfire, and still continue to run. Like you let go, it will almost stop running, then backfire, then continue to try to run. The guy that gave it to me, is the "spoiled" type and never took care of it. I changed the oil as soon as i got it, and put a new air filter on it. I have 7 push mowers, the oldest one i have, i've owned for 12 years, and have never had anything like this. The only logical thing i can think of is MAYBE carbon build up on the piston head or in the combustion area. I've heard that carbon can create hotspots and can cause one to continue to try to run, and i was thinking maybe the hotspots might be whats causing the cylinder head to get to hot and distort the spark plug color. As a test, i took my other 5 HP B&S and placed the side by side, and started them both up at the same time and let them run for about 10 minutes, then shut them both off at the same time. The one i had, you could touch the back of the block, around where the oil dipstick is, and you could keep a finger on it, for about a second and a half, maybe 2 seconds, before you'd have to pull it off from being to hot (reflexes) the one i was given, if you touch it, you instantly pull it off it gets so hot. I Put them both back into my shed and throught the hour noticed how hot the motor got when cooling off. the one i had, cooled off A LOT faster than the one i was given. after a couple hours, it was still on the hot side. by that time, the one i had that was mine, was almost ice cold again. any ideas on what else it may before i tear the motor apart and check for carbon?