Gas On/Off

Has happened a few times. I load my Honda on trailer to haul to camp and turned gas to off. When I unload Honda and turn on gas, it spits and sputters like it has water in line. Finally settles down with no problem. Did it again a few days ago and now it doesn't start or run when it is turned on but when I move gas to reserve, it runs just fine. I switch back to normal gas position, it runs for a while (clears carburator) and then dies again and will not start. Acts like gas line is plugged or the gas position knob or valve is bad. Like I said, it has done it before and eventually quit, but this time it doesn't quit. Since it worked fine when I turned to off, and now doesn't run when knob is in normal mode, woudn't think gas line is plugged and works fine on reserve which is first time I ever used reserve and would think this would be first thing to plug. Could this be valve instead? Anybody else same problem in past???