There have been a lot of posts concerning modified or aftermarket pipes snapping and popping during deceleration. Here is why it happens and why you shouldn't worry so much about it.
When decelerating, especially at high engine speeds, there is a considerable vacuum below the throttle plates. This pulls more fuel than normal through the idle jets and, at the same time, there is very little air entering the engine. The closed throttle position and the engine speed signals tell the igniter that the engine is under very little load and the max advance curve should be used.
The end result is that the spark occurs well before the maximum point of compression. Since there is very little air and not a lot of compression pressure, there aren't many oxygen atoms close enough to the gas to ignite. Very little, if any of the fuel actually gets burned in the engine. By the time the piston nears the bottom of what should have been a power stroke, there is nearly as much of a vacuum as near the end of the intake stroke. When the exhaust valve opens and you have relatively wide open pipes, the vacuum pulls air up the pipes from outside. This rush of air fills the vacuum, lots of oxygen is close to the unburned fuel and the heat of the pipes and exhaust valve lights it off.
So, what is to be done? There are a few choices.
1. Richen the idle mixture so that there is so much gas in the cylinders that you are sure to have some close enough to the oxygen atoms to ignite. The down side of this is that the plugs may foul during normal idle and, since the idle ports are used to richen the mixture when running hard, the mixture will be too rich and you will lose some high speed performance.
2. Tun the length of the exhaust system so that the sonic pulses keep the air from reaching the hot end of the pipes until after the raw fuel is gone. The downside is that this will only work over a limited range of engine speeds and you could end up with very long pipes.
3. put flap valves on the ends of the pipes to block the intake of air. Of course, this ads to maintenance and may look a little odd unless you are building a tractor theme bike.
4. Just ignore it knowing that it doesn't hurt anything and makes your bike sound like a dragster shutting down at the end of a run.
