I have a handed down walk behind lawmower I'm guessing is 12 years old. It has a Briggs & Stratton engine. For the most part it is perfectly fine. However, usually once or twice while I am mowing the lawn, all of a sudden, the idle will drop. The engine won't stall but it barely sounds like it is running. After about 15 seconds the idle roars back up to normal and it is fine. Any thoughts what the cause may be?

 

Briggs & Stratton made alot of engines types on push mowers even 12 years ago

Make sure the fuel cap is venting...when the problem occurs loosen the cap.
If a diaphram carb it could be some debris in the tank getting caught on the pickup tube, or debris may be in the diaphram itself causing it to starve for fuel at times. Does it happen when you make a certain motion or certain area u mow...?
Possible the govenor spring is getting caught on something...