When I bought my 1991 GS500E it was equipped with an ill fitting K&N lunch box filter that often fell off the carbs and a V&H exhaust system with dented headers. I've since replaced the air filters with better fitting K&N pod filters and a Yoshimura exhaust system. With the new parts, the bike appears to backfire more when closing the throttle at speed. Does the bike need to rejeted after this work?
It is normal for a K&N air filter fitted bike to back fire a little on sudden de acceleration.
Without knowing your jet sizes however, its a touch hard to say if your bikes setup is too lean.
Look at the jetting matrix on the Wiki to see what others have used successfully. You may have to pull the carbs and inspect the jets to see what is on your bike.
Okay here's a silly question. What the difference between backfiring a lot and backfiring a little? If the bike truely is running lean what other symptoms should I be seeing?
If I accelerate hard on my bike then just snap close the throttle, I'll get 4 or 5 small backfire crackles as the bike slows down due to the rapid leaning of the mixture. That is normal!
If it does it every time you de accelerate even gradually, that is not normal and your bike is running too lean.
The easiest available method to see if your bike is running lean is to pull the spak plugs our and look at the electrode and surrounds. If its white = its too lean!