Have a Briggs & Stratton 600 series Poulan Pro engine on a lawn mower. It starts and runs but only about half speed.
Make: Briggs & Stratton
New spark plug, new engine oil
The problem is that your carburetor diaphram is bad and is allowing too much fuel into the engine. This is a VERY COMMON problem. These usually have to be replaced about every 2 years.
Here is the part number of the diaphram.
Here is the parts list for your carburetor. You can also order the part here.
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I Own a Lawn Mower Sales and service business (37years). Have 2-cycle and 4-cycle certification.