89 Isuzu very slow, lacks engine power

Hello everyone! i have a good one for you! i recently purchased an 89 isuzu trooper rs 2.6l 4cy 5spd 4x4. 36 hours after i bought it the engine block cracked. i fell in love witht the truck so i found an old junkyard engine to
put in.
before i replaced the engine it was great (for that 36 hours), i was able to zip around with no problem. now with the new (old) engine, its very slow and lacks power. i cant get on the hyway with it at all because i cant get up to 65mph fast enough. i have to rev it to like 4000rpm in every gear before i shift up because at less than 3000rpm it doesnt have enough power to continue to accelerate. and hills?...forget about it. i have to keep in 2nd (or 1st) to even climb the smallest of hills.
in effort to fix this problem i have replaced the plugs, wires, rotor, distributor cap, air filter, fuel filter, fuel pump, and o2 sensor. i had the cat converter checked out and is working perfect. plus an uncountable amount of fuel system cleaners and engine restore crap. but all that did nothing for my problem.
everything sounds and looks healthy, the timing is good, and it revs up good while the truck is in neutral. it just barely has enough power to pull itself. i have stumped many mechanics with this problem. i realy dont want to replace the engine again. i hope i have given you enough info to give me a strait answer! thanks.
~Benjaminjamiahjammin
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Benjaminjamiahjammin wrote:

sounds like is needs a valve job
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Please post the results of the compression test you did (or should have done) and that should give you a good indicator of where your problems are. If all cylinders are showing good compression, then you have a slipped timing belt. If all cylinders are low, then you still may have a slipped timing belt, but if only one or two cylinders are low, you may have a blown head gasket or as another poster said, leaking or burned valves. A compression test will reveal the problem, I bet. r/ Steve
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