Small Engines (Lawn Mowers, etc.): JD stx 38 No Fuel Flow, valve lifters, head gaskets

Question
I recently tuned up my stx38 with a kit from the dealer and it may be coincidental but soon after when I am cutting the grass or just running the machine the fuel filter will empty and loose its prime, it almost seems to create a vapor lock. Work done so far remove tank, replace gas line (complete),replace fuel filter, replaced needle and float in carb. replaced carb with new one from dealer. Pulled off head and noticed a large buildup of carbon on intake valve. Don't know if this could be problem or not. Head gasket was discolored by intake but didn't seem to be blown out. Have always used Mobil 1 synthetic with oem deere parts. It seems as if there is no vacumn in the carb circiut. This one is beatin me up. Leaves are getting pretty bad here and I need to get this machine back in order. I have been in the auto field for over thirty years and am at wits end. Any help would be greatly appreciated.

Thanks, Steve
Kohler cv13s (gravity feed fuel system)

Answer
If you have already tried cleaning out the carb and this still doesn't help, a new carb or full carb kit should help. If there is no vaccuum still, then you may have a loss of compression in the cylinder, and the air is being sucked into the cylinder through another source other than the carb. Replace head gaskets and/or valves/valveseats/piston rings/valve lifters/breather line/valve cover. Any of these could be the problem.