[Mike Martus]

Your repair is easy.

There are two possible causes.
1. The ball-check valve is stuck open. This is located at the end (inside) of the tube that extends into the carb (assuming that you have the older suction feed carb).

A little carb cleaner will do the trick or replace the tube. You can test by sucking and blowing with your mouth ( boy gas tastes nasty and is poison to boot) I still use this method to test.

2. A leaking head gasket is also very common. Easy fix is to re-torque it to 150 inch lbs. You will find the bolts near the exhaust are loose and the engine is sucking in air at low conditions. Better fix is to replace the gasket.

Since this is an older engine you may want to check the level of compression - the easy test is with your baby finger. Put it in the spark plug hole (no I am not joking) and pull over the recoil. If it blows your pinkey out it has enough compression to start the compression cycle. Note: Low compression (poor) improves the faster the engine runs.

I would be happy to assist further if needed. Just contact me. I have been teaching Small Engine repair for 33 years. I am certifird by both Briggs & Stratton and Tecumseh.