Hey there all. I have a YardMachines Briggs&Stratton 550EX Series 140cc
The old air filter had become was dirty and fuel soaked, so I replaced it with the exact part. When trying to start the mower, it starts immediately, but idles down and then stalls. Reading other posts, that situation pointed to a bad spark plug or dirty carburetor. Carb cleaned and spark plug replaced, but still having the same problem. Drained the oil and the fuel and replaced both, same problem. For the hell of it, I stuck the old air filter back in the mower and it ran fine (except for white smoke). What could cause it not to run with a new filter, but work with the old one?
Running too lean - not enough fuel. So with a dirty filter you're getting less air, creating a mixture that will burn. Clean filter too much air and not enough fuel. What did the plug electrode look like that you removed? White? It was probably running lean before but not so lean that it wouldn't run.
Let me guess... You just took this mower out of storage. It has the fuel in it from last September. And/or you stored it with fuel in the carb.
I'm with Darryl. The old, dirty air filter is making up for blocked low-speed jets... which have been clogged by Ethanol fuel during winter storage. The issue may be exacerbated by poor quality, out-of-date fuel still in the tank.
