A little help please. I have a Courage 20hp model SV601 that will run fine for about 30 seconds and then starts to get rough, backfires and dies. If it sets long enough for fuel to fill bowl (5-10 seconds), it will fire and again and repeat. I have gone through carb twice, new fuel filter, checked valve lash, and just clipped off end of fuel shut off solenoid with same results. Fuel filter is staying full of gas during the process, and I have kept fuel tank cap loose to make sure air is getting into tank. Is there something else I might look for?