85 fourtrax 250 stalls on heavy accel

Need help. I have a 85 fourtrax 250. It sat in a garage for a month with new gas before storage, went to start it, idled ok took it out for a ride went ten feet and stalled. I can ride it part throttle, once its in third or medium or full throttle it bogs down and stalls, no power. I changed the air filter, iinline fuel filter, plug hoping that something simple would fix it but the same thing happens. I use this quad for maintaining baseball fields, so this problem couldn't come at a better time than now. Part throttle in 1st or 2nd works fine. Please help!