All of the above sound like good suggestions... easy on dropping the main too much as the main jet is for the 3/4-full throttle circuit, but the needle is for mid-throttle and air screw for 0-1/4 throttle. Does the 28mm carb have a pilot jet? If so, you may want to go one smaller on the pilot, too, if the air screw doesn't seem to have an effect. To adjust your air screw to the right point, you should let the bike idle, start with the air screw around .5 turns out, and turn it out slowly until you hear the engine RPM's peak. Let us know where you end up with the air screw (how many turns out). I usually do it 2-3 times to make sure and this is your setting. On plugs, I'm assuming you're running the stock R6252K-105? The plug is a 10.5 heat range and for trail riding I would suggest running an 8 or 9. So maybe pick up a BR8ES and BR9ES and give them a try. Also leaning your oil mix, as mentioned, may help the spooge, too. If the bike has a lot of time on it already, you may want to repack the silencer too as the packing may be pretty nasty.