DC310r

i have a 86 tri z 250 Ever since i got it it hasnt ran quite rite,In lower rpm runs ok and free revs great but anything past 3/4 it acts like it has a rev limiter?Its all stock with a pipe,I thought it mite be carb so a i put one my 38mm carbs on and it still does the same thing just gets there faster know lol.This machine should be right up there with my 250"s just not running a maximum.please help.