Loses power at 5500 rpm
I have just returned from a trip. On the road my 2006 DL650 started to lose power at about 5500 rpm, but only with a load on it. In neutral is was fine. On the highway I could barely keep up with traffic. I took it to a shop where they figured it was a problem with fuel delivery. So they cleaned the fuel filter and installed a new air filter. They checked compression which was fine, and electrical connection from the throttle which was also fine. At that point they were stumped.
I took it to another shop the next day. They also believed it was a fuel delivery issue. They replaced spark plugs just to be sure. And they said the old plugs indicated that the fuel was running lean. They drained the fuel tank in case it was some bad fuel. The mechanic concluded it was a vacuum problem because, he said, without the air box in, it ran great. He decided the problem was with the plenum boots (under the air box). The rubber was hardened and cracked at the ends. So he cut off the cracked hose ends, and on the bigger one, he tightened the bolts. It seemed to run a bit better, but it still struggles to make much more than highway speeds.
Lastly, let me mention that I can now hear a high-pitched whine periodically, like the sound of a slipping belt on a car (although not that loud; it's just enough to notice).