Small Engines (Lawn Mowers, etc.): B&S 3.5hp Black Smoke, piece of dirt, rich mixture

Question
Hi Michael, I've got a 3 year old mower with a B&S 3.5hp motor on it which has been fine up until today when after running for about 10 minutes it started running rough, missing and producing quite a bit of black smoke. Air filter is clear and I've run some carb cleaner through it. Plug was a touch black and oily which I've cleaned up. Oil levels are fine. Any ideas what would make it suddenly start acting up? Many thanks in advance! Jon

Answer
Hi Jon,

Black smoke usually is a result of a rich mixture.  I suspect that either a piece of dirt has gotten stuck in the float valve or the hugh speed mixture screw has gotten opened up too much.  Both would result in too much fuel getting to the engine.

Let me know how you make out.
Michael