I have a problem that has cropped up. My 1655 gas has been coughing when it starts to pull harder. Doesn't seem to be getting enough gas. Tank, settling bowl and fuel lines, to the carb, are all clean. Have had a new carb kit installed but still having the problem. I have noticed a layer of frost on the section of the carb where it's bolted to the manifold. Seems when the engine warms up, 30 minutes or so, the frost is gone and tractor runs fine. This has been happening all spring and summer, to a varying degee. I'm thinking carburetor icing. I understand how this happens on an aircraft, but what would be the cause in a tractor and what may be the cure? Obstruction, bad float, needle valve, etc.