It's been awhile since I've been here and you guys have always fixed my wagons in the past so I hope you can do the same this time.
Well, I have this Masport mower with a B&S 550Ex engine. The problem is surging and I have thoroughly cleaned the carburetor but the problem remains. I tried a second clean carburetor and got the same result. Normally on other engines the source of this problem is lean mixture and a carby clean fixes it but this one has me stumped. I have also changed out the spark plug and the problem, remains. Any suggestions what else I should try?
I worked it out! I have been test running the engine without the air filter. That hasn't caused me problems in the past with other engines when testing them but this one was different. I also don't have the proper cover for the air filter ( recycled mower with bits missing) and the one I have has a hole in it for the primer bulb but the mower has an auto choke and its proper cover doesn't have that. Anyway, when I covered the air intake with the filter the mower ran fine. I hope this helps others from wasting their time.
This unit should run regardless of the air filter fitted or not. The governor should compensate for the extra air to fuel ratio if air filter is not fitted. This type of engine has a known issue with throttle touching the front bracket of the carburetor assembly as shown on the photo. I suspect when fitted with air filter, it caused the bracket to slightly flex and free up throttle from touching. Surging is likely due to the governor linkage being not responsive because it got stucked. Worth checking what I mean.
