Another jetting question

what are you current jets, pilot, main, needle, clip position, typical riding conditions, and elevation, i had a 91 and i live in NH which is pretty much sea level, i believe i had the stock needle in the 2nd clip position, 152 main and a 42 pilot, this bike had an aftermarket exhaust and silencer, boysen reeds, and the airbox mod

Assuming said street-bikes are in good tune and you put on a free-flow exhaust or air-filter system, you'll increase the amount of air getting through and have to compensate by making the mixture richer. Your engine sucks in air and fuel hopefully in the right mixture. If you make it easier to suck in air(free flow filter) then it will suck less fuel, and you will need to put in a larger jet to compensate.
As far as the advice you received to go leaner on your particular bike, I assume you gave details on how the bike is running and the problems you're having. If you got the advice at this site, you can almost surely count on it 'cause if it's wrong, someone will surely jump on whoever is steering you wrong. I personally only know the basics and have only touched on jetting, but if you give detailed info and have done your best to find info in a search(by far the best way to learn) then there are plenty of helpfull people in this virtual world.
Do an honest search, and if you don't find the info, at least you'll know what kind of info the jetting gods need to be of help.
Good luck. DVO

Another answer to his question: because generally, the factory jets the bike too rich to begin with. Yes, aftermarket filter and pipe will lean the jetting, but many times not enough to get that "optimal" setting. Plus everything DVO said.

if the top end was bad then it would have an effect on how the bike runs, making it seem like the jetting was off, when was the top end last done on it, take the pipe off and or the reeds, also take off the spark plug, and look into the cylinder with a flash light and crank it over slowly to look at the pistion to see if it looks scored, you could also take it to your local shop and have a compression test done,you didn't state the elevation your at but i would say your running too rich

you can get a compression tester at advance auto for 20 bucks. compression should be between 120 and 185 psi. screw the the thing into plug hole ,open throttle and kick it till the gauge stops going up. book says warm up engine first then check it but if your good cold its only gonna get better hot. mines 160 psi cold.anyway this will tell you how the rings are. hers the piston measurements standard 65.939 - 65.954 mm, service limit 65.79 mm. also check out this link it mentions the pro-x pistons and the honing.
http://www.eric-gorr.com/techarticles/twotopweb.html

Carbs mix fuel (and thus the arrived at air/fuel mixture) based on pressure differentials...not the amount of air going thru the carb.

That point could be argued semantically, but if you're going to argue, I'm fairly sure you understand my point.

Think of a plate choke operation. A plate closes off the airstream above the jetting orifices. That results in a richer A/F mixture getting to the engine. Because there is less air? No, because the pressure differential has increased. The engine is sucking at that closed plate, creating something of a vacuum (compared to ambient outside pressure). Said outside pressure then pushes the fuel into that 'less than' air pressure area (vacuum if you will). That's what the venturi of the carb does. It creates the pressure differential.

Nature abhors a vacuum! At least that's the caption on a larson cartoon that showed lil miss riding hood on the way to gramma's house...carrying a hoover!