Engine vacuum is not RPM related, it is load related. The higher the load the lower the vacuum and vice-versa. But I am thinking that valve would be designed to open on very little vacuum since it needs to be open at WOT which is your lowest vacuum event.
I took the valve apart and found the problem with the fuel flow not shutting off after shutdown. The forward seal was stuck (likely from time) and would not move enough to fully open or fully close. I ordered the kit this morning, but it turns out the rubbers were just fine; the rear larger seal was a little oily or fuel soaked, but I cleaned it off. I'll go ahead and replace it when the kit comes in.
I also noticed that the little air hole on the bottom was clogged up from corrosion. So I cleaned and air-blew it dry. The valve works like a charm now. Start the bike and then disconnect the vacuum line (plugging it of course), and within about 2 to 3 minutes, the carbs run out of fuel and the engine shuts down. Re-insert the vacuum line and fuel flows as it should.
But now I know without any doubt that the carb float on the left side is not closing. I replaced the fuel filter and now that the fuel is flowing well to the carbs, it only takes a few minutes of idling and the engine begins to chug and want to stall. Gas it a few times and the exhaust smells like raw fuel. So the carbs will have to come off.
Now I'm convinced that my jerking issue is fuel/spark related. The left carb is dumping too much fuel from a failed float needle, and fowling out the plugs (possibly). Whether the plugs are firing sufficiently or not, it is clear by the way it behaves that the engine is running too rich, which would explain the 28mpg that I'm getting. Once you get going down the road, the rich condition is being blown out the pipe and seems to run normal; especially well above 2,500 RPM's. Come to a stop light and she idles perfect for about 2 minutes, then starts chugging again from too much gas.
So, next week my friend and I will remove the carbs and I will rebuild them, change the float needles and needle seats, replace the jets with the correct ones (someone put larger main-jets in the carbs), and see how it does. However, with the carbs out, I will also remove the coils and wires to ohms test them and also inspect the coils for damage/cracks.
I'm certain that this will solve the jerking problem.
On a side note, when I had the carbs out last year, I noticed that one of the CV cups was scratched/chaffed. Wouldn't this cause a drivability issue if the cup was sticking?