Small Engines (Lawn Mowers, etc.): engine noise, engine model type, briggs and stratton

Question
Eric,

I have White Outdoor push mower (Model 11A-54MB090) with a Series 450 139cc 4-cycle OHV engine that I bought new in 2006.  It recently started running louder and sounded different as well. The noise starts when the mower is first started and continues even when the engine is up to normal running temperature.  When I checked the valve lash, both valves (IN and EXH) were OK.  While turning the motor by hand I noticed that the EXH valve opens slightly then snaps/clicks closed during the first half of the compression cycle.  During the exhaust cycle it opens and closes normally without clicking.  

I was told that the EXH valve opens during the compression cycle to make pull starting the engine easier and that at higher rpm (normal running speed) there is a mechanism that disengages the EXH valve from opening during the compression cycle via centripetal force.  Is this true?  Is the noise I am hearing caused by a failure of the mechanism not disengaging the EXH valve during compression?  I haven't noticed any significant loss of power, it just sounds different and is a little louder.  

I should note that the engine always seemed a little under power, even when it was new.  Could this problem have always existed and is now getting worse?  I don't have anything to compare the engine's performance to, so maybe it was fine when new and I just expected something more.  My previous mowing experience has been with 2-cycle engines.  

Thanks in advance for your advice on this matter.  

Adam

Answer
Is the engine a Briggs and Stratton or Tecumseh?  The website for parts is currently not working so I can't look up the information.

On the engine, either on a sticker or stamped in the valve cover, you will find the engine model, type and code or model, spec and serial number.  

Please send me the numbers off the engine.

The compression release, to make starting the engine easier, is usually on the exhaust valve as you described.  Several years back some compression releases had a spring on the cam that could break causing some extra engine noise.

What engine oil are you using?  Let me know the engine information.
Eric