Hi, my Briggs & Stratton 19.5 HP engine starts OK, idles low then high then low idle again repeatedly. This is without the mower deck engaged. I plan to adjust the carberator. But has anyone have this problem before?

Dirty jets causing a lean condition can be one of several causes of this...

A partial can of either Seafoam or BG44K mixed in the tank will cure it if it is... you can dump the rest in the tank of your car but its good to clean up the carbs of other gasolene yard tools too.

Avoid the other so called miracle in or bottle or can. They are little more than a waste of money... these two actually do as advertized. And the only ones that do based on my own experience over the years.

Thanks. I'll give it a try.