12 HP Briggs carb help

[dtsr]

I have Simplicity 2012, Briggs 12 HP with a two-piece flo jet carb. This engine speeds up and slows down constantly until it is under load with the mowing deck and then it runs smooth. I think I have seen this addressed before and I have searched the forums but can’t find it. Any ideas on where to start looking for a fix to this problem will be appreciated. Thanks. David

[Kent]

First thing I'd check is for vacuum (air) leaks in one of two areas: 1. Worn butterfly shaft and/or bushings 2. Gaskets between intake >> heat shield >> engine block

[Allis_B12]

I had this problem about two weeks ago on my engine. If there aren't any air leaks where Kent described, check your carb mixture. I played around with mine until it would run at a steady speed without backfiring. It was running too lean. Hope this helps, Joel

[dtsr]

Thanks Kent and Joel, I will try that today when I use it with the Johnny Bucket Jr to move about 6 yards of mulch to the the flower beds. I'll let you know how it turns out. Many thanks. David

[HubbardRA]

Lean condition usually causes the speed up and slow down. Richen the carb a little. Worn throttle shaft usually creates an intermittent missfire that cannot be adjusted out with the mixture screw.