Small Engines (Lawn Mowers, etc.): 20 hp model #461707, exhaust valve seat, spark plug wire

Question
motor starts but it runs and then slows down and now I may have pull the choke closed to keep it running

Answer
It sounds like one cylinder is not running.  There are two simple test to confirm this, one is a compression test.  Simply remove both spark plugs and insert a compression gauge (one cylinder at a time) and see if you have readings in both cylinders.

Another simple test is to remove ONE spark plug wire at a time and start your engine.  Does it run on one cylinder, but not the other?  WALA!  Cylinder problem!

The most common problem I see with this engine is that mice sometimes build a nest above the cylinder in the cooling fins over the winter, the engine is started in spring-overheats and the exhaust valve seat pops out of the casting.  This is not repairable...Replacement engine is needed.

If you have a cylinder problem, be very cautious sticking money into the engine as some parts for this engine are no longer available.

Hope this helps...drop me a followup and let me know what you find!