RPM SURGE

[fastcat]

I've got two pushmowers . 3-1/2 H.P. Briggs and Stratton engines . After about five to ten minutes of mowing the engine rpm will nosedive , not stall just slow down . I wait just seconds and the rpm's return and black smoke comes out of exhaust .This keeps repeating until I'm done . What is the fix ? Thank You !

 

[slomo]

Failing ignition coil or dirty fuel tank with a partial fuel line clog?? Carb needle and seat or float adjustment needed??. Air, fuel, spark and compression need verified.

slomo

 

[Charlie8d]

Fuel system problem. You may have to clean/replace fuel lines, carb, & tank, then add a filter.

 

[bertsmobile1]

I think we need to know the models so we will know which style of carb you have .
Some can flood & some can not flood.
Could be as simple as a water build up in the bowl
If it has a choke, could be a faulty choke return spring
But was are guessing untill we know what we are looking at .