Small Engines (Lawn Mowers, etc.): Power loss, briggs and stratton, hp briggs

Question
Background - Sears mower with 5 hp  Briggs and Stratton engine. Replaced the the deck with a used sears mulcher deck. Worked OK.

Problem - Mower has recently lost power. Bogs down in heavy grass, and loses power after running for a 1/2 tank of gas.

Actions - Replaced spark plug and air filter. Reconnected carburetor which had come loose.

Question - What can I do to diagnose or solve the problem?

Thanks

Answer
First I recommend you open the high speed mixture about 1/8 of a turn to see if this enrichment helps.  If this adjustment doesn't help and you continue to open it 1/8 of a turn at a time with no success, the issue may be related to compression.  If the engine is losing power because it is warm, it may need to have a valve job and the tappet clearance adjusted.  Hope this helps.