I have a 86 Ninja 600R with 18000 miles that bogs heavily when I roll on the throttle. It seems worse when the outside temp is cooler. The valves were "supposeldy" adjusted when I took it in for a major $800 tune up at the local Kawasaki dealer about 500 miles ago. Could it be plugs, or carb jetting? The mechanic replaced the jets during the tune up. Any help would be greatly appreciated.