Small Engines (Lawn Mowers, etc.): B&S Quattro runs, then begins to misfire, briggs stratton, lean mixture

Question
I am an amature mechanic, but follow manual instructions well; I am planning to get a manual to assist.  I have a Briggs & Stratton Quattro model#10A902.  I have had the mower for 10 years with few problems.  The motor runs fine for a few minutes then suddenly starts to misfire and lose power.  I removed and cleaned the gas tank/carberator and air filter.  I put in a new spark plug (Champion J19LM), gapped to .30, put in new premium fuel.  The mower started fine, mowed for about 20 minutes then the problem resumed.  The old spark plug and the new plug are black/sooty.  According to a spark plug condition chart, it states,"Dry sooty deposits indicate a rich mixture or weak ignition.  Causes misfiring, hard starting and hesitation."  I suspect it could be an electrical issue.  What should be my next step in troubleshooting.  

Thanks,

Michael

Answer
We rarely find ignition problems.  Most of the time the problem is fuel related.

Do you have a mixture needle on the carb?  Many of the older engines do.  Have the checked the o-ring between the carb and intake tube?  Once in a while we find warped intake tubes...warped where it connects to the intake port.  This normally causes an air leak which results in a lean mixture.

Worse case you can re-jet the carb but this is very difficult unless you have access to various jets.  Briggs does not list their just seperately...you just have to find one that works.  I recently moved to a high elevation area and had to lean my carb by replacing the float bowl nut which contains the high speed jet.

Let me know what you find.
Eric