Ok I think there's something the others missed in pointing out to you. There is fuel/air ratio rich and there is also fuel/oil ratio rich. You don't want to run fuel lean on either as that will overheat and ruin your piston, because the fuel actually helps cool your engine. You want to run at least 32:1 on your fuel oil mix, some like 25:1 and stock is more like 20:1. Do not mix 50:1 or 100:1 as some oil directions suggest, and make sure you use oil for pre-mix, not for injection.
Your jetting will control how much fuel (and oil) are getting into your engine, for you fuel/air ratio. Jetting must be done to compensate for the temp and amount of air. Hopefully whoever was racing your quad jetted correctly, but more often than not racers run on the lean side to get a bit more power. The trade off is a much, much shorter engine life. You said you put the stock pipe back on, do you mean the silencer or silencer and expansion chamber? If I were you I would first do a compression test followed by a leaktest, just to see where things stand. Mix 32:1 as that's what most of us do, and get back to us on your jet size and also what mods are done such as is there an air box lid, filter type, exhaust, and your elevation