Ripped from Migcruisers.com
The PAIR valve on the Marauder and Intruder LC is used to burn off hydrocarbon emissions from incomplete combustion that occurs during deceleration in gear. It works like this:
When you open the throttle on your bike, manifold vacuum drops off since air is entering through the carbs. This air mixes with fuel that is burned in the combustion chambers, more or less completely. When you let off the throttle, particularly in gear, the throttle plate closes, restricting airflow to the combustion chambers, and the manifold vacuum goes way up. This is particularly the case when coasting in gear since the engine is being pushed by the rear wheel at speeds above idle. Since the throttle plates are closed, a high vacuum is developed. This vacuum pulls fuel from the carbs into the engine, but the mixture is too rich for it to burn. It's dumped unburned into the exhaust system.
This is where the PAIR valve comes in. The valve is controlled by manifold vacuum. When vacuum increases (pressure decreases), the valve is pulled open. On the Marauder, fresh air is drawn in through the PAIR's air filter and fed into the exhaust port of the cylinder heads. On the LC, the source of fresh air is from the rear cylinder airbox. That air provides the right mixture to spontaneously ignite the hot unburned fuel exiting from the exhaust valve.
If you have stock pipes, you won't notice this effect. However, change to less restrictive aftermarket exhaust, and the combustion takes place futher down the pipe, resulting in sounds that people describe as backfiring, burbling, popping, etc., during deceleration. The sound you're hearing is the sound of the exhaust gasses burning off. Some folks kinda like the sound, others can't take it.
What can be done about it? You can disable or remove the PAIR valve from your bike. This will in no way affect performance, even a little. All it will do is remove the backfire, and of course, increase hydrocarbon emissions. The valve can be disabled by disconnecting and PLUGGING the vacuum line that controls it, or by blocking its fresh air intake. The valve can also be completely removed from the engine, revealing those beautiful V-twin jugs hiding underneath.
In order to remove the valve, though, the vacuum lines must be properly plugged to prevent air leakage into the manifold, the fresh air inlets into the cylinder heads must be blocked, and in the case of the LC, the fresh air port on the rear airbox must be plugged. You can probably fabricate everything you need to do this, but there's a better way...
Check out Moccasin Mike's website for information on his PAIR valve removal kit. It's a great deal at only $15 and contains everything you need. Detailed instructions for the Marauder and the LC are also included. Mike's website is: