Here's a scenario for you. So for example you drag your Hayter Harrier fitted with a Briggs and Stratton engine out of the shed for the first cut of the season. Engine starts hunting and surging like mad. You clean the carb thoroughly, perhaps more than once, but still the damn thing keeps hunting. Here's something you should check. The float level.
Best way to check it properly is to remove the carb from the engine, and take off the float bowl. Then flip the carb upside down. The float should be sitting parallel to the carb body, as shown in the first image below.
In the first image you can clearly see the float level is spot on and doesn't need any attention at all.
In the second image below, the float level is way out, the rubber seat has expanded and pushed the needle out further than it should, thus pushing the float out of level. This results in a low fuel level in the carburettor bowl, and will make the engine run weak. Often mean hunting / surging. In some cases the rubber seats expand so much they restrict or even stop the flow of fuel completely, with obvious consequences.
In this case you would replace simply buy a needle and seat kit from your Briggs dealer and replace them. You'll be surprised just how many of these engines are running around with this problem, I've fitted hundreds of them over the last two or three years.
Anyway, thanks for reading, and hope this helps someone.