Dude, I cannot open your links to hear what your bike sounds like. But, here is what you need to do. Inspect the intake manifold and the rest of the induction system to make sure that you don't have any crack/rips/splits/loose clamps. If that all checks out move on to the carb.
Now, the carb. First and foremost, get a Clymer repair manual, or, download it from here. Take the carb off, remove the float bowl, float and fuel needle. Also, be careful to not loose the accelerator pump spring and rod. Set everything aside in the order you removed it. If you took the 3 phillips head screws off the top of the carb, and pulled the slide out (it has a needle in the middle of it, the jet needle it is called) when you pulled the carb from the bike, then you can skid having to do it now. If you didn't, now is the time to remove those screws and the slide. Once you have that all removed, look into the top of the carb. You'll see two tamperproof torx screw. They have a little "tit" in the center so a regular torx driver cannot be used, hence why they are called tamperproof as most people don't own them. Remove those two screws and get ready for the next step. Do everything as I tell you and you will be fine on this!!! Grab the carb in both hands, hold the main jet squarely against a flat surface, and press down very firmly. If will be a little difficult, and you may need to strike the top of the carb with a rubber mallet, but, the grey plastic "Y" insert that the slide goes in will pop out of the carb. UNDER NO CIRCUMSTANCES ATTEMPT TO PULL THE INSERT OUT FROM THE TOP WITH PLIERS!!!!!!!!!!! IT WILL BREAK!!!!!!!!!!!!
Now, once the insert has popped out, take it in your hands, and look at the tube on the bottom of it. There are going to be two grooves with two O-rings on it. Those O-rings are square edged, and will become dry with age. Once that happens, they crack or shrink, and it allows fuel to flow past them and the engine runs super rich. They may very well be good, but, it pays to check all the bases. You should replace even if they do look good to be safe. When you go to reinstall the insert, put some Vaseline on the O-rings so the slip in the carb and do not roll out of their grooves.
Other things to check are the following: idle mixture screw setting. They guy who rebuilt your carb may not have adjusted it to spec. On a 1st gen it should be 1 & 3/4 turns out from it's seated position (all the way in). Most quads run fine at 2 turns out, so it shouldn't hurt to set it at that as a baseline. And, remember, one turn is 360*. The idle mixture screw is used to fine tune the idle circuit, which is primarily controlled by the pilot jet, and it works at idle to about 1/8 throttle. To adjust the idle screw, turn it in to lean the idle circuit (lean means less fuel in the air/fuel mixture) and turn it out to richen (more fuel in the air/fuel mixture). Never turn the mixture screw more then a quarter to a half a turn at a time when adjusting.
Next thing to look at is the needle jet. That is the brass needle in the slide. Read the manual about how to get the slide apart to get to the needle. Once you've gotten to the needle, check to be sure the little clip is in the 3rd notch, which is the center notch, on the needle. If not, put it there, and reassemble the slide.
Now you are ready to do some plug chopping. With the mixture screw set at 1 & 3/4 turns out, the needle on the 3rd/center notch, and the O-rings replaced, put the carb back on, start the engine and allow it to idle for a good 5 minutes WITHOUT touching the throttle. Only adjust the base idle if needed. Shut it down, pull the plug and check to see if the center and ground electrodes are the color of a brown paper bag. If they are black, your idle circuit is too rich. If they are white(ish) or "ghosty" looking, your idle circuit is lean.
Now that you know the color of the plug you can make whatever adjustments necessary to achieve a paper brown plug. Start by adjusting the mixture screw. If the plug is black, turn the screw in a quarter turn and retest at idle. If it is improving, but not quite brown enough, turn it another quarter turn. The same goes if the plug is white(ish)/ghosty, only turn the screw OUT a quarter turn at a time. Rule of thumb, is you have to turn the screw two comple 360* turns, in either direction, you need to change the pilot jet by one size accordingly. That means, if you turned the screw out 2 full turns, go up one pilot jet size because it is telling you that it needs more fuel. If you turn the screw in 2 full turns, go down one jet size because it needs less fuel. To change the pilot jet you need to remove the float bowl, BUT, do not do that just yet. First, do another plug chop for the main jet, so you can change both jets (if necessary) at the same time. No one wants to take the carb off more then needed, lol.
To plug chop the main jet, ride the bike (on an easy incline) at wide open throttle, in 4th or 5th gear, for about 7-10 seconds. Keep the throttle pinned, pull in the clutch and shut off the kill switch at the same time. Just don't let go of the throttle until the engine has shut down!! Come to a stop, pull the plug (keep your tools with you to pull it) and check to see if it is "golden brown"/brown paper bag. If it isn't, make the jetting change necessary to achieve that. Again, if the plug is whit(ish)/ghosty, install the next largest jet and retest. If it is black, install the next smallest jet, and retest. I have found many times when it comes to tuning the main jet circuit, going two sizes (up or down) usually works best. Give this a try and report back.
Lastly, when you have the carb apart, check to see what size jets are already in there. Also, what mods have you done?
