can anyone explain this facet, or why rejeting is needed when i remove or change exhaust. i can't guess on my own.

With a more free flowing exhaust, the gasses leave the engine faster. More gas out = more air (and fuel) come in. That's the basic rundown.

Now is the exhaust system moving so much more volume that you NEED to rejet? It's possible. But it wouldn't take much to balance the system. Still have the stock intake? If so, I would bump the main jet up a size and adjust the air / fuel mixture screws.

Exhaust design does not effect compression, it does however impact on the scavenging effect. In simple terms performance exhausts are less restrictive and allow more exhaust gas to escape the chamber more quickly. Therefore more air/fuel can be drawn into the chamber during the overlap (the time when both exhaust and inlet valves are open). Often carb adjustments or a rejet is needed to benefit or compensate for this effect.

Search "motorcycle exhaust scavenging" on google 

thanks for info. does increasing valve clearance  to 0.1 for exhaust  increases scavenging?

I doubt it....In fact I can see it having a negative effect.

Increasing the valve clearance will reduce the amount of lift and cause the valve to open later in the cycle and close earlier, this in turn will reduce the amount of gas able to escape 

Here is an interesting link that explains in detail what is happening, it's also fairly easy to understand without getting overly complicated:

http://www.austincc.edu/wkibbe/camtheory.htm

It also confirms what sledge just said.

when i got my bike it has very leak of power. backfire in carbs, cant' adjusting the idle.
then i measured and replaced valve shims. it had backfire in right carb and when i checked the clearance right exhaust shim was out of spec. then everything suddenly start working perfectly but in cold weathers or when the engine is cold bike also has leak of power (i understand that it will be, but it has such of leak of power that it is even had to move at low revs) and sometimes backfire in carb. what do you think what could be the problem

Quote from: GS500F2004 on September 11, 2013, 03:52:42 AM
Here is an interesting link that explains in detail what is happening, it's also fairly easy to understand without getting overly complicated:

http://www.austincc.edu/wkibbe/camtheory.htm

It also confirms what sledge just said.

interesting and helpfull information )