I started the engine from cold with choke on. Engine warmed up and as I start to close off the choke the revs start to drop and engine stops. I've wound the idle up but made no difference. Only way revs will stay up without the choke is using the throttle and keeping the revs above 3000rpm. Almost seems as if the engine isn't getting enough fuel. Plugs and air filter are fine. Anyone had this problem before?