Probably not your problem, but I thought it worth mentioning for the smaller B&S engines. I've had problems with the governing due to an air vane getting too tight or stuck once the spindle is tightened down on to the coil. The only solution I've found is to file or sand some of the plastic off the bottom of the spindle until it swings free again. When stuck the engine runs just as you say, it just goes flat out and won't idle down. Another quick fix is to loosen the bolt on the coil a bit that holds the spindle, but then you could risk the coil coming loose.
Second this on the old 148cc ones vint mow, I have had to do this on a few occasions as well. its like the plastic expands as it ages, or maybe its something with the coil slightly delaminating.
Unfortunately, this one uses an internal governor - like a quantum. for what they lack in precision, the air vane governors do make up for in ease of service
I find a lot of the newer B&S are pains to get to idle e.g. Sprint series. As their name suggests, they do just seem to want to keep sprinting ahead rather than tune back to an idle.
As an aside, the main issue with the sprint not going to idle is both standardisation and emissions. Emissions meant the idle circuit was cut in the early 90s (ie pulsaprime carby) but still running fairly rich meant curb idle of 1750rpm. The twin spring came in in 1997 (California emission CARB tier 1 implementation) so to prevent surging at leaner mixtures they upped the idle speed to around 2600 rpm. It was also so machines like pressure washers didnt surge so much when you let go of the trigger.
Most machines in the usa were fixed throttle by that point so it didn't matter as much
