What is going on with your lawn motor right now is that the fuel pump diaphragm is not working good enough. The diaphragm works better to pump fuel at high RPMs.
What happens is the engine runs lean because the diaphragm cant pump enough fuel into the little indentation fuel bowl below the carb. That causes the carb to supply a lean fuel vs air mix, so the engine runs lean. When the engine runs lean the RPMs increase, and the bad diaphragm works good with the RPMs are high so the little fuel bowl gets enough gas for a short time, with enough gas the carb runs at the proper fuel to air mix and the engine no longer runs too fast, with the engine no longer running too fast the diaphragm can not supply enough fuel and the cycle of the little bowl below the carb not having enough fuel repeats.
When this cycle is in the too lean mode you are causing extra wear on the cylinder and piston because the temperature inside the cylinder is too hot during lean running.
----------------------------------------------------------------------------------------------------
To fix it you will require the Briggs cab rebuild kit (probably 795083 for about $4.99 at any motor shop, and most hardware stores) and a can of carburetor cleaner spray.
Go to YouTube and look at the videos for "how to replace the diaphragm on Briggs"
Then go on more YouTube about Briggs until you find and view some that include the spring and the filter.
When you remove the carburetor from the engine pay attention to the governor linkage, and the throttle linkage, so you know how to put it back on.
Look for the spring. It is probably best to remove the five screws and then flip the carb and tank over before you separate the two, so the spring will stay in the carb and not fall out. If it falls out by you removing the two with the carb upwards you may have some trouble figuring out where the spring belongs.
Be sure to clean the filter (if yours has one, it probably does) and be sure to put the spring back in where it was.
