Motorcycle Repair: vulcan 750, bike mechanic, unusual sounds

Question
My bike has a knocking sound and no mechanics seem to be able to figure out what it is.  The bigger problem I have however is that when I go on the highway once I reach 57mph the bike starts jerking and the exhaust starts to almost sputter.  It keeps going but not like it should and it just jerks the whole time.  When I bought the bike it had a ton of power and no problems other than it needed to warm up for like 10min. and then it ran great.  Only after I had the carbs cleaned did it lose most of its power. It basically runs like a 650 now.  I would appreciate any advice.  Thanks

Answer
If it ran poorly after the carbs were cleaned, that's where I'd start looking for the cause of the problem. If they were cleaned properly, it means that they were removed from the bike and disassembled, then cleaned and reassembled with new seals and diaphragms. If any parts were missed on reassembly or it they were reassembled incorrectly or a synchronisation wasn't done afterwards (or it wasn't done right), then the bike will not run properly. I would try a carb synch first to see if that helps any, if not you should have the carbs removed and disassembled again to see if there are any problems.

As for the knocking sound, do you know where it is located at? Any reputable bike mechanic will have a stethoscope similar to the one your doctor uses to check your breathing; when placed at various points around the motor and transmission it makes it much easier to locate any unusual sounds. Once you've located the area of the sound, then you can determine the most probable cause and begin the teardown to repair it.