I've got a 6.75 HP Briggs that will not run smooth. It revs up and down constantly. I discovered today that it will run perfect without the air filter. I've cleaned out the air filter but that didn't help. The mower is almost new. It's only been used about 10 times. It ran really good until the last time I used it. Any ideas??

Clean the carburetor. Buy a new filter and see what happens. The old filter may be giving away.

good advice oscaryu1 :wave:

If the carb sits on the fuel tank, try replacing the diapraghm and cleaning the carb as well as changing the filter.

Model and code numbers would tell for sure, but if you have an internally vented carburetor, then a dirty air filter will cause a problem like you describe. Replace the air filter as oscaryu1 suggested, if that does not take care of your problem then look to the carburetor for additional service.

I replaced the air filter and the thing runs perfect again. I couldn't believe the filter was clogged because it's not very old at all. Thanks for the help!!

just wondering: is it a paper element or foamy type?