Sounds like the jetting to me.... Ever clean the air filter out? You should clean the air filter after every ride and don't use gas because it eat's away the material that they use on filters.... Get a new spark plug... maybe a hotter one, don't go too hot or else it will mess your piston up. Sounds like jetting to be though, but take your plug ouy and if it is burning black really bad then on your main jet needle, move the clip UP one because moving it up will lean it out. Try that out and let me know if that works man. Oh yeah and for the jetting it is either clogged or if the weather changes like it get's cold and it does that in the winter then that's what it is and I don't know if you will need to make the main jet or the pilot bigger or smaller because I would have to ride it to know. Call a local shop and ask them about the best jetting and tell them the year of the bike etc. I hope that was a help to you, just let me know if that helped the problem. Oh yeah talking about dirt bikes, wan't a 89 Husqvarna 510 Dual Sport?