Small Engines (Lawn Mowers, etc.): JD runs 1 hr and quits, john deere l120, old john deere

Question
QUESTION: I have a 5 year old john Deere L120 with Briggs V-Twin 20HP engine. The engine runs fine for approx 1 hour, begins to sputter and then dies after about 30 seconds. I can manage another minute or so of run time by applying choke. If I try and start again right away it will run for a minute or less. If I wait a half hour, it will run for approx 5-10 mins. If I wait a few hours, it will run fine again, except to repeat the above cycle.

I have replaced... Fuel cap, fuel filter, both coils, both spark plugs, and fuel pump. I have blown compressed air through all the fuel lines including the fuel "in" at the carb with the cover plate off. Also replaced air filters, oil and oil filter.

ANSWER: What brand of spark plugs are you using?  Have you checked the valve clearance?

I'd start by getting another set of spark plugs, NGK brand if possible.  When the engine stops next time have spark plug wrench handy and change out the spark plugs.

The fist thing to help trouble shoot the problem is to identify the main system malfunction.  I typically start with the ignition system and then the fuel system.  Once you know the ignition is functioning then move to fuel.

A set of new plugs may help isolate the ignition system.

Eric

---------- FOLLOW-UP ----------

QUESTION: How do I check valve clearance? Why do you suspect this as a possible problem? What should I expect to find if clearance is "off"? Why would changing the plugs again accomplish anything since they were just changed and the the problem remains(They are NGK). Any other suggestions?

Answer
Below is a link describing how to service the valves:

http://www.briggsandstratton.com/engines/support/frequently-asked-questions/Serv

The clearance is usually too close but you may find it too wide.

I have had a lot of bad spark plugs, brand new out of the box.  It is very cheap insurance to try new spark plugs.  I once spent as least 30 minutes troubleshooting a brand new, out of the box Homelite weed trimmer.  Installed a new plug and it started the next pull.  My rule now is to always install a new plug then troubleshoot.

Eric