Motorcycle Repair: lean carb problem?, vacuum petcock, air leaks

Question
I could use an opinion on a what I think is a carb problem.I just bought a '99 Kawasaki KLR650.  It has 5000 miles,and has not been abused.  I'm the 3rd owner.
It's a single cylinder 4 stroke, with a Keihin CVK40 carb. It's my 15th bike in 30 years, and I
have always done my own work.

The bike surges at freeway speedsin high gear, idles a bit rough (idle swings up and down 200 rpm total) , overrevs on shifts, and takes a long time for the revs to drop. If I rev it to 5000 rpm,pull in the clutch and chop the throttle, the revs drop quickly to 3000rpm, but stay at 3000 for 2-3 seconds before very sluggishly decreasing to idle. The plug is white.  I figured this is typical lean pilot circuit.  Most people recommend drilling the pilot plug and opening the screw to 2 -3 full turns out.
However, I looked at the pilot screw, and it has already been drilled and set at 4 turns out, which is very rich.  The jets have also been modified.  It has a #40 pilot, which is stock size, but a #150 main jet which is one size over stock.  Needle is stock.   So, the bike acts lean, but is set rich.
I couldn't detect any other air leaks in the carb, the intake manifold, or from the vacuum petcock. I have adjusted the throttle and choke cables.  The throttle and choke cables are not binding.  Air filter is clean and stock.  It is an oil type filter and has been re-oiled.

I have bought new stock jets and will be installing them.  I found the inside of the carb to be clean, with no debris.  Float is correctly set.  I haven't checked valave clearance, but doubt it is relative.
Any ideas on what I'm overlooking?

Thanks in advance.  

Jim in MN

Answer
Hi Jim,

I appreciate your thorough description and experience.

The carburetor is set a bit rich, but air-fuel mix indicates lean. This is indicative of an air leak.

The erratic idle is consistant with an air leak.

Was a checmical spray such as WD40, brake cleaner, or other used to detect air leak?

What is the condition of the air cut-off diaphragm?

Respond to my email: mshively1@woh.rr.com

Respectfully,
Mark Shively
Dayton, Ohio