Engine will run, but it surges, hunts for a constant RPM
1. Partial Blockage of Main Jet. This is for bowl type carburetors only. Stop the fuel flow to the carburetor, either turn off the fuel valve or pinch off the line with a pair of fuel line pinch off pliers or a pair of vice grips. With a pan suitable of containing gasoline, loosen the fuel bowl nut and catch the gas. Now remove the bowl nut and the bowl. Blow out the holes in the nut with compressed air or carburetor cleaner. Some Tecumseh carburetors have a tall bowl nut. Some will have a pin hole in the side of the nut, and some will have a pin hole just above the bottom set of threads. This hole must be clear in order for the engine to run properly.
2. Leaking Intake Gasket. Check for a loose carburetor or mainfold connection at the block. All Briggs and Stratton engine 3 to 6.5 HP, check the intake manifold for cracking (may have to remove carburetor) or loose connection at the block. You will need to remove the air shroud to get access to the intake manifold.