Join Date: Apr 2015
Location: Murrysville, PA, USA. A pleasant, wooded suburb of Pittsburgh.
Matt, I thought about the "engine bogging"/perceived loss of top end speed that concerns you. A couple of thoughts:
I saw in the photo that you have a generously-sized tank bag on your bike. I surmise that on a camping trip you might also have had some additional luggage hanging off your Duke.
You say that you could not get the bike to run beyond 85 mph (as opposed to its normal top end of just over the ton). My thought is that you were retarded by aerodynamic drag rather than loss of engine performance.
To greatly simplify a complicated formula that accounts for several variables, let us just observe that aerodynamic drag increases as the SQUARE of velocity. For example, if you are riding at 60 mph, your aero drag would be 3,600 units above what it would be at zero speed. But if you increase speed to 85 mph, your aero drag soars to 7,225 units, about TWICE as much as at 60mph. Any extra obstacles to aero flow at this speed (ie, rider's body, external luggage) will create correspondingly large increases in drag.
Riders who like to do the ton on a Duke typically ride solo, with no external loads or attachments and they lie down on the tank to minimize drag. Clearly, with a large tank bag it would be impossible for you to assume a racing crouch. In addition, your external luggage would be creating heightened drag, magnified by riding at high speed. My supposition is that the combination of an upright/semi-upright riding position, along with additional drag from luggage, would be sufficient to create a level of aero drag that would impede you from reaching theoretical top speed. This is particularly true if you were riding into any degree of headwind, which would aggravate the bike's coefficient of drag.
I believe your engine is probably OK and normal. When you get your bike back from the shop, if you are worried I suggest you find an isolated road, then try a top speed run, with no luggage, no headwind and using a racing crouch, flat on the tank. I think you should be able to do the ton or thereabouts. Top speed is subject to variables such as rider size, weight, position and clothing, engine tune, optional equipment, fuel type, ambient temperature, wind direction, tire inflation, road surface, sprocket sizes, chain lubrication, tension and alignment, etc.
"If you don't know where you're going, you might wind up somewhere else."
2015 390 Duke
1980 Moto Guzzi V50 II cafe racer
1984 Moto Guzzi SP 1000