My 98 Suzuki VL1500 has a weak spark to the rear cylinder. i've replaced plug, plug wire, coil, inition module, and stator. I've switched coils and plug wire from one cylinder to the other. The weak spark remains with the rear cylinder. Whats left?

Make sure the battery is fully charged and check your charging system to make sure you are putting out 14 volts or so. If yes then check the spark plug wires for defects, then check the spark plug itself if black or wet looking then it may be from a fouled plug or weak spark. If you can switch the coils to see if that has the opposite effect.

change candle, and regulate the carburetor. the excessive gasoline in coming damage the candles.

Carb or fuel injection? Spark or no spark? Check for spark by putting a new plug into lead and laying on cylinder head. Leave old plug in engine. Start bike. If carb and no spark then plug or coil would be first suspects, then ignition module, then wiring from coil to ignition module. This is assuming you have compression on rear cylinder and all mechanical checks completed. If fuel injection and good spark, then ECU most common problem. Then injector, then wiring. If no spark, check coil, then wiring frpm coil to ECU, then ECU.

I just did one the other day. There are three coils. You can determine which one is bad by using the number one spark plug wire and an old spark plug. Put the spark plug in the wire ( while it is still in the coil) and ground the plug against the engine block. Make sure it is grounded solidly to metal or you could get a nasty shock. Have someone start the engine. Look at the spark. It should be fat and blue. Turn the engine off as soon as you have seen the quality of the spark. Pull one of the wires from each coil in sequence and move the number one wire to that coil. Proceed as above. The weak coil(s) will have intermittent spark, yellow spark or maybe none at all. Once you know which coil(s) are bad remove the two small screws at the ends of the coil. They hold the coil down. Pick the coil straight up, pulling the contacts out of the ignition module. Set the new coil in place and screw it down. Both towers spark every time the coil fires so reinstall the wires in either spot on a given coil. Good Luck

Could be a bad coil, coil wire or module. The module converts the pulses of the stator so the coil knows when to spark. Bad coil = no spark. Bad coil wire = no spark to plug.

The first thing I would do is replace the spark plugs again. You can foul a plug and it will cause you more problems than you can ever imagine. I see you have done all the logical things that I would recommend but I did not see you have replaced the plugs after the 200 miles. You would be surprised at how many times a $2.00 part can cause fits beyond belief.