Have a Skag 48 with 14 hp Kawa. Starts right up and purrs at all rpms - with no load. Last month or so started losing power, coughing and sputtering in heavier mowing conditions. Now it dies out just by engaging the blades. Also coughs out some black smoke. Checked the usual stuff. Cleaned carb, new air filter, new plug, fuel filter, fresh gas. My service man hasn't looked at it, but says that probably the intake valve hav been damaged by sucking in too much dust and grit. I hope its something more minor. Any suggestions from the experts out there?