Regardless of what brand tractor this may be, I would say that since the problem happens only when cold, that the carburetor mixture is too lean. The lean mixture does not burn properly, and the incoming air/fuel charge is ignited as soon as the intake valve opens on the next intake stroke. If the valve lash was too tight, the problem would be worse when the engine was hot. As a test, if the mixture is too lean, the problem will go away with a partial choke application. Other possible problems: the timing is not set correctly, the distributor shaft bushings are worn, bad condenser, bad coil.