Was it sitting for any length of time? Could be plugged up carbs. The adj screws are hard to get to. You pretty much need a special tool to adjust them. Basically a 90 degree gear drive screw driver. K&L sells them. We have two of them. They could have dirt in them or water if the previous owner took them mudding or swimming. Also check yer airfilter. Is it a stock airfilter? If so I would put a K&N in it. I also have an 05 with some mods and I love it. I bought mine brand new and never had any probs. Keep the engine oil changed or you will be replacing cam chains lots. Also keep the front and rear diffs clean. These atvs are notorious for leaky diff seals. Mine seaps just a lil but I keep mine checked a lot. Yes they are wheelies machines. A stock one should pull a wheelie no prob. The real hot rods were the 06 and up 750's. 05 was the first yr and they had the timing retarded really bad. I advanced the timing and made a big diff. You might check your belt, could be slipping. Clutch kits make a HUGE diff with power. I recommend the Maroon primary and the almond secondary. Also, not to take you away from this awesome forum but since there isnt much atv traffic on here, you can go to mudinmyblood.com. That site is for all makes and models of atvs but probably 80% of the guys on there ride brutes that have huge lifts and tires. Im on there and I highly recommend them.
2010 KLX250S-Full FMF-Rejet-UNI-KX450 graphics
2008 KFX450R-Full Yosh-K&N-DG-ITP-Douglas wheels
2005 Brute 750-Full Big Gun-Rejet-K&N-ITP
1985 Bayou 185-Stock-
Kaw Valley Racing #68