Based on what you said, I am suspect of the battery and/or connections. Get a volt meter and measure battery voltage at the battery terminals. You should have 12.6 volts. Much less than 12.6 will start to affect how the engine cranks.
Another possibility that is very common with Briggs & Stratton Over Head Valve engines is that the valves are out of adjustment and causes the automatic compression release to not function properly.
Check the battery voltage and report back with your findings.