On Sun, 24 May 2009 09:57:26 -0400, "Stormin Mormon"
wrote:

* If the spark plug is Champion, try a different brand. They
had bad sparkplugs in the past.

Plenty of good advise for the OP, so far. I would start with a new
plug with a hotter heat-range. Putting a "hot plug" in one mower was a
solution for me.

Some pre-gapped plugs are not accurate and should always be checked ,
before install.