ok, my 1990 mazda b2200 pickup truck is running awful, when i bought it it would miss and sputter at low RPM's but run fine at higher RPM's. so we tuned it up, spark plugs, wires, distributer cap and rotor button and fixed a big vacum leak. now it runs fine at low RPM's but when it hits about 3000 RPM's it starts sputtering and wont rev up any farther. we thought it was a clogged exsaust but i took the catalitic converter and muffler off to see if that would help but no change. any ideas?