I had a 2002 KX500 street legal that surged so bad that it was nearly impossible to ride at highway speeds at a constant RPM. I tried jetting, checking for air leaks, KIPS operation, etc. I tried a flywheel weight which "seemed" to magnify the effects of the surging...I was about ready to give up on trying to dual sport on it, though it was tremendous to ride off-road. Everyone has their theory of what makes big-bore two-strokes surge (ie. mixing of burned and unburned gases, intake pulses, exhaust pulses, combustion chamber design, blah, blah, blah blah). I researched all types of engines and their operating conditions and one consistent cause for surging always seemed to be found: a constant or TRANSIENT lean fuel condition! I had tried all kinds of slow, main & needles (PWK 39) to no avail. My k5 had an FMF Gnarley & a RAD valve, and was geared pretty high 18/47 (don't ask!). I noticed that the big KX motor, going down the highway at 50-60MPH, was spinning pretty fast but required hardly any throttle to do it. I stumbled on to an Italian Keihin distributors web site who showed that a PWK is much more sensitive to the slide cut-away and over a broader range, up to half the the throttle opening according to them, than say a Mikuni. I called Carb Parts Warehouse in Cleveland and they suggested slide #4 or 5 instead of the stock #7. I went with the #5 and IT WORKED! It went from unrideable to taking 50 to 200 mile trips; could have went farther if not for the seat & the vibration. You'll have to rework your other jetting because it richens the mix everywhere, and the fuel mileage suffers--mine dropped to 33MPG on the highway (measured!) but with my 4 gallon tank I could safely go 100 miles between fuel stops. Another unexpected benefit was that it pulled much harder off idle. With my tall gearing I had to slip the clutch a ways to get it rolling; after this change you could let out the clutch and roll on the throttle and away she'd go! I hope this long post will be helpful to some of you--this problem was maddening.
