Motorcycle Repair: Fouling plug

Question
QUESTION: I have a 1988 Kawasaki Vulcan VN-1500-A. It has two plugs per cylinder and the rear cylinder, throttle side plug fouls out regularly. The engine idles well and runs good above 40 mph, but pops and jerks until 40 mph. I have new plug wires but they did not help. I thought maybe flooding or a sticky valve but as soon as I replace the plug the bike runs great for another 150 miles.

ANSWER: Hello Russ,

In order to help you 100%, I need additional info. That is, the VIN# and the millage.
Due to local emission control regulations and restrictions, there are several variations in the carburetor setup, intake and exhaust layout and ignition and/or camshaft timing.
That's for the VIN#. As  general, the carburetors VNs are equipped, are somewhat susceptible
to throttle valve wear. That causes the mixture to be far from ideal, up to 10-15% throttle
opening, which leads to plug fouling. Other reasons are, valve clearance, wrong spark plugs (depending to the local climate it may require one number hotter)and charging system.
More accurate and model specific when you provide me the additional info I need.

Best Regards,
Lazaros.

PS: Average local temperature and altitude will help.          

---------- FOLLOW-UP ----------

QUESTION: Thank you for your very fast and helpful response. Here is the additional information you requested.
!988 Kawasaki Vulcan 88 -VN1500-A2
VIN# JKBVNAA17JA002091
Milage: 34,291 miles
Altitude: 223.279m/732 feet
Average riding season temperature is 66.8 degrees (F)

Answer
Hi Russ,

Really sorry for the delay...
First, make sure of the following, just for peace of mind.
A)Spark plugs must be NGK DP(R)7EA-9.
B)Fully charged battery and charging voltage at least 13,4v at idle and no more than 14.8v
at any engine speed.
C)Ignition coil wiring polarity: red or black/red wire (same in both coils) goes to (+) or BLACK terminal. With the ignition switch ON, this wire must show battery voltage, with no more than half (0,5) volt drop.

Now for the main troubleshooting.

Check throttle valve synchronization.If you don't have vacuum gauges or an external tachometer,
you must pay your local workshop a visit. If you use vacuum gauge, it's easy, just keep the difference under 2kPa/15mmHg. If this is just too advanced for you, you can follow a more
practical way. Rise the idle considerably, by the idle screw. Disconnect the red or black/red
wire from the rear cylinder ignition coil and observe the rev drop. If the engine stalls, turn in the throttle stop screw until idle is maintained. Now connect rear and disconnect front ignition coil. Observe rev drop. Locate the screw that's on the throttle butterfly next to the closing cable. Turn it clockwise to decrease, counter-clockwise to increase rpm to match
the rpm when front cylinder only was running. Connect everything, readjust idle and take it for a ride. Let me know how it runs. Next episode, fuel level and idle mixture adjustment.
Let's take it step by step to avoid confusion and brain cell burn-out!!!

Best Regards,
Lazaros.