Have a briggs & stratton 4.5hp that jams up. Sometimes when i use pull cord and doesent start very easy, Sometimes not at all.have checked oil,Replaced diaphram, Checked its not grounding,Spark ok,cleaned carbuareter and tank. Fresh fuel in tank.Valves ok, Piston ok.

Bob, October 2008

When you say the mower 'jams up' when using the pull starter do you mean the engine locks up solid or catches on compression and 'kicks back'?
If the engine is 'kicking back' then I would take a look at the flywheel key. On some occassions the key partialy shears causing a slight alteration to the timing. This leads to kickbacks, poor running and in a lot of cases the engine not starting at all.
It may be worth a look anyway even if the symptoms dont match. Good Luck

smudge, November 2008