Had an 8HP Briggs that did that and hard to start. Finally figured out the intake valve was sticking open slightly. A heavy dose of Marvel oil thru the intake while at full throttle cured the problem.
Mike

Sounds to me that your carb is running real lean...remove the bottom bowl bolt or adjuster, and clean the tiny holes in it..

Well there are a number of things which can cause this, do the cheap ones first. Could be water in the carb float bowl. Could be the carb throttle butterfly shaft is worn out causing excess play where it exits the carb body. Could be it is missing the spring wrapped around the governor to carb rod which takes the slop out of the linkage. Could be the governor "Static"setting is off. Could be the carb just needs a good cleaning. Could be loose carb jet in carb and more.

Walt Conner