My '87 Samurai has a newly rebuilt engine with a Weber 32/36 carb conversion. (I am not a fan of this carb from experience with it on other vehicles). It runs smooth and quite well 1-4 gears. In 5th gear it does great until about 55 mph/3000 rpm then it just quits accelerating-no sputter or issuing as if fuel starved it just will not go any faster or above 3000 rpm. (In 1-4 it will go to 4000+ rpm). I have adjusted the carb, set the timing at 10* BTDC with no vacuum. Valves were set at rebuild which was 100 mis. ago. any suggestions as to what might be causing the problem?