1988 Isuzu Trooper Trooper takes for ever to excelerate

I bought an 1988 isuzu trooper 2.6 4cyl on the cheap, for a work truck/ resto project. Everything works great except for the engine take of responce.
When I excelerate it sputters and picks up speed very very slowly. I have to down shift just to get it up to 45 mph. I have replaced the spark plugs, distributor cap and rotor, fuel filter exct, and cleaned out the throttle body.
I was wondering if anyone else has had this problem, and what was the culpret? Please respond. MUCHO THANXXX