I inherited an old Yard Machine mower with a B&S 4HP engine. I'd really like to get this thing running to use as a 2nd mower. It starts up fine and runs like a champ for about 60 seconds. After that it slowly starts to skip and puff out black smoke. The problem gets worse the longer it runs.

I put in a new plug after finding the old one badly carbonized. Same problem--it starts fouling the new plug after about a minute of running OK. The carb is a one-piece plastic unit with no adjustment screws whatsoever. All the holes appear to be clear and there are no visible defects except...

I'm wondering if the diaphragm is bad. I don't know what a good one should look like (Hey, I'm a V-8 guy, not a small engine champ), but I'm suspicious. The diaphragm has a "bubble" in it with a spring behind it. I suspect the diaphragm should be flat and that the bubble is an age problem, but I've never looked at a new one to know. Also, the diaphragm is separated from the gasket along part of the perimeter. This all seems suspicious to me, but again, I don't know what the correct appearance should be.

Can anyone tell me the most likely cause of the problem? Is the diaphragm the most likely part to replace? I don't want to send this thing to the repair shop because I'd probably spend more there than this mower is worth.

Thanks

2 to 3 bucks tops to buy a diapragm and gasket kit together and do it yourself, its a few screws to remove, but while your there, make sure the pickups are clean and the tank has no trash in it. plus the gasket and diapragm should be sandwiched tight, so make sure the carb is on tight. also since its black, that does mean a rich situation, white/bluish would be oil.

It was the diaphragm. Replaced it, and it runs like new.