I have owned several (actually, quite a few) motorcycles in my lifetime, the most-recent a 2009 Yamaha VStar 1300 Cruiser, the subject of this anomaly. I generally do all my own periodic maintenance and shy away from dealers unless the problem is beyond my perceived capabilities. This problem tested the limits of my capabilities.
Description of the problem: Motorcycle starts right up. Low speed travel is without problems; smooth engine performance. However, when reaching speeds ~45 MPH or so, and up, engine performance suffers noticeably. Occasional hesitation, inability to accelerate normally, difficulty maintaining speed with consistent throttle setting, overall poor performance. Conditions seem to improve whenever speed is decreased.
Possible causes: Bad fuel? Dirty fuel injectors? Bad spark plug(s)? Dirty air filter? Fuel line restriction? Timing problem? I won’t go into the ugly details of my troubleshooting process, however the problem has been remedied.
The fix: Replace the battery. Evidently the voltage was low enough that it affected the electronic fuel injection controller (but the battery was still capable enough to deliver sufficient voltage and current to the starter). My ‘old school’ reasoning skills had difficulty comprehending the fix, but, trust me, battery replacement corrected the problem.
Do you have a similar problem with your motorcycle? Does it have EFI? Battery replacement might be the secret fix to the problem.