Small Engines (Lawn Mowers, etc.): Honda HT3813 surging power, honda ht3813, head gaskets

Question
I have a "very used" Honda HT3813, ser. 5100657, liquid cooled 2 cyl. lawn tractor.  When it idles, it will surge instead of running smoothly.  When in gear and under load (say slightly uphill), it runs very smoothly.  Going downhill makes it surge again.  I tried to adjust the idle screw but it would rev too fast at idle speed but it would run smoother.  I changed the oil, cleaned the air filter, put in new plugs but nothing seems to fix it.  Any suggestions?  Thank you in advance.  Chuck

Answer
Surging on a four cycle engine is caused by one of two things.  An air leak or a lean fuel situation as though you are running out of gas.  The air leak would be in the fuel system it could be any gasketed surface it could also be a warped intake manifold.  I would also check head gaskets to be sure you have a good seal between the water jacket and the cylinder.  Lean fuel can also be caused by plugged fuel filter were only a small amount gas is being allowed through.  if this was happening 10 minutes after you started mowing I would also check the fuel cap.

Lean fuel situation is caused by not enough gas into the combustion chamber.  This could come from a restricted fuel line or restricted fuel passage in the carburetor or fuel filter.  I do not think it is the governor but be sure it is set properly.  It could be mechanical because it does not happen underload.  I hope I have helped good luck in your project.