I have had the old problem of the bike hesitating when running at a steady 55 to 60 MPH. There are many threads that talk about about the vacuum problem with the petcock or maybe a dirty petcock. There are also many threads on jetting when you have changed anything (I have a Vance & Hines exhaust).
When we say an engine is running "lean" it means that it is not getting enough gas for a for a particular rpm. Running "lean" can also mean that it is getting TO MUCH air (it is a balance thing). You need the proper amount of both AIR and GAS.
To change the jetting on the carb you have to pull the carbs, change the jets or "O" rings or add washers. This is a fairly big job and may require several tries to get it right.
Here is what I did to check it without requiring the removal of the carbs:
Take a 12oz plastic water or soda bottle and cut the bottom 1/2" out. Clean it good.
Cut a hole in the bottom of the piece you have cut out. Start by leaving about 3/8" (bottom) around the edge of the hole.
Remove your seat.
Take the two bolts out that hold the rear of the gas tank.
Lift tank about 3 or 4 inches (I put a short section of 2x4 in to hold it up). You need to be able to get at the air cleaner inlet.
The bottle bottom will fit loosely over the air cleaner inlet.
Put a hose clamp of the proper size over the bottle bottom/air cleaner inlet and tighten (gently - don't crush the air cleaner inlet)
Put gas tank back in place and seat back on
Start and ride the bike (make sure you get it good and warm to test it properly)
So.....is you problem worse or better.
If the bike sputters and won't take the gas or won't run at high rpm you need to go back and make the hole bigger. let in more air.
After a couple of tries (changing the size of the hole in the bottom of the drink bottle). I got mine to run perfect.
I think this tells me that I have a jetting problem rather than a petcock problem. I need to jet the carbs so that I let in less air and more gas.
Would really like to hear comments on this test from some of you guys that have done the jetting changes to your carbs.