Like all the side valve lawnmower engines I can think of, the Briggs has a rather low compression ratio (6:1) and it also has a feature called EzySpin, which holds one of the valves slightly open during the first half of the compression stroke, to act as a decompressor and make it easy to pull over. If you want to check the compression, Briggs recommends that you disconnect the spark plug lead then turn the engine backwards (by the output shaft clutch housing in this case) until you find compression. Then flip the clutch to make the engine turn backwards until it hits compression. If it bounces back, compression is OK. If it soggily keeps turning through the compression stroke, it's not OK.
If you conclude that compression is bad, tell us about it and we can talk about how to diagnose the problem.
