I have a 420 that surges at anything other than wide open throttle, and my dealer has cleaned the carburetor, replaced fuel lines, and last year replaced the ignition coil behind the flywheel. The dealer says he can’t get a new carburetor. It’s running pretty good at wot but when you go downhill it will die and then maybe come back before it cuts off - sometimes it will backfire. The dealer says it probably a piece of trash floating around in the carburetor causing the problem. Does anyone have suggestions of what to try next - or know where to get a new carburetor? Thanks in advance for any suggestions!