Most of my motor repair experience is on cars and motorcycles, which seem so much more complex. Yet, here I am struggling with a little single-cylinder lawnmower. It's a four-horse Briggs and Stratton (model 10A902) and it has run pretty much flawlessly since I bought it in 2000. It's a stone-simple model with a plastic carburetor mounted on top of the fuel tank. One week I mowed the lawn and everything was fine. The next time I started it it ran fine for a minute or two, then started loping badly. I checked the air filter, which was fine. The plug was carbon fouled, so I figured it was running rich. The thing is, it's got a one-piece plastic carburetor with no mixture adjustments anywhere. It kept running, badly, for awhile, then smoothed out for about a minute as it was running out of gas. I headed to the gas station and got a can of new gas, then refilled the tank and now it won't even start. I cleaned the plug, which didn't help, then put in a new one which also didn't help. I took off the carb, cleaned the heck out of it and put it back on with a new diaphram/gasket. Still no dice. The air filter is reasonably clean, the carb seems perfect, the primer bulb is fine and I can take out the plug, connect it to the magneto, ground it to the block, pull the cord and watch it spark. When I try to start it and then take out the plug I can see it's soaked with gas as if were flooding instantly. I can't find anything else to adjust and am out of ideas. Got any suggestions?

I'm still trying to start it, but not in any enlightened sort of way. Yank, yank, check the plug again...

Anyhow, I've noticed it's occasionally backfiring through the carb. Does that provoke any ideas?

Try another carb cleaning, did you blow through all the holes with carb cleaner? clean the pickup tubes and rinse out the gas tank?

I did everything you listed up to emptying the old gas, but did not rinse the tank with anything; just added new fuel. However, I have an update. The replacement spark plug I'd gotten was not an exact match, though it looked pretty similar and the guy at the hardware store insisted it was equivalent. After posting about the backfiring I reconsidered that decision, and went to a store where I could get the exact right plug. Voila, it started on the first pull and cut the whole lawn. Since this odyssey began I've changed the gas, the spark plug and the carb diaphram, and I have no way to know exactly which of these was the one that fixed it, but something seems to have worked.

Thanks for the thoughts!

on that model what caused your carbon on the plug is it was starving for air ( I.E. your re-usable air filter is dirty ) ............simple fix, wash it out with a little dishwashing soap , completely wring it out then put a couple of drops of 30 weight oil on it and give it a couple of squeezes to disperse evenly. the oil is actually what catches the dirt.

Thanks for the followup. I already did wash the filter but I wasn't sure if it ought to be oiled.

Oiled or not oiled wouldn't matter (if its clean)... on the running part of course (but do oil it because it does catch the dirt)

i have seen many mowers come in with no oil in the filter, this causes fine dirt to pass through the filter and into the carb..........the results are pretty nasty.

i have seen many mowers come in with no oil in the filter, this causes fine dirt to pass through the filter and into the carb..........the results are pretty nasty.
exactly..... sometimes you can even see the grit!