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I have a craftsman push lawnmower with a B&S engine. About a year ago, when I was 20 minutes into mowing, the engine would begin to slow down and power with it. If I didn't do anything, it would stop in about 30 seconds. However, if I pushed the primer button in several times as fast as I could, the engine would recover - for a while. This went on for a few weeks. I took out and cleaned the carb - didn't help. I got a new carb, and it seemed to fix the problem completely - the mower roared to life. However, after several mowings this season the problem returned, eventually to the point where the engine wouldn't run for more than 5 seconds unless I was pushing the bulb as fast as possible. I put the old carb back in and it seemed fixed once again! But this only lasted two lawn mowings. Does anyone have any idea what's going on?? Thanks!

starting to sound like overheating
Is the rope harder to pull when the engine is hot & loosing power ?
When was the last time you changed the oil ?

Sounds like you have water in your fuel.

Thanks for the helpful replies. I emptied the tank and put fresh from the pump gasoline in, but that didn't help. It has been quite a while since I changed the oil - I'll give that a try. It isn't any harder to pull when it is having it's problems. Forgot to mention that I changed the fuel hose, just in case. No improvement, and the hose I pulled out looked clear.

I'll give the oil a try - that is overdue, anyway. Thanks again for the good suggestions.

Did you drain/empty the carburetor?
water will sit in the bottom of it and stay and cause trouble.

Take the engine cover most have 4 bolts, make sure you don't have a mouse nest on top of the cooling fins, clean the fins, I use brake cleaner, when finished let it sit in the sun if you can at least 15 minutes.

Thanks for more good thoughts. No to the mouse, and I do empty the carb each time. After it's quit and I leave it for an hour or two (until it's cold), sometimes it has recovered but just as often it hasn't. After several days between mowings it always has recovered. One more observation - this has a choke that is connect through a spring. I can't normally adjust it, but if it is acting up and I thread my finger through to the choke, I can adjust it and the engine roars to life. I'm not sure what the spring attaches to (I don't have much engine experience), but it is under tension and so it is connected to something at the other end.

Thanks again, and any other suggestions are welcomed!

iit sounds like air filter is letting dust get into carb and clogging it up, check filter housing for damage or cracks and damage to air filter.