POOR ACCELERATION 1990 chevy pu 5.0L 170,000 miles

Could be the throttle position sensor, a bad ect sensor (does the temp guage work), a bad intake air sensor and it could be a clogged exshaust...the lack of pwer (does it run good at really low throttle but bogs when floored?)

Yes, it bogs out when i try to give it gas.
But sometimes it runs ok,(that is more bad then good lately).

Remove your air cleaner assembly and hook up a timing light. Run the engine and shine the light into the TBI unit. Rev the engine and watch the spray pattern it should look like a nice round cone if not then your injector is clogged.

Yeah if the injector isn't clogged. It sounds like a sensor. Low compression is unlikely cause that would give a nock or such. And even a tierd engine with vacume at 14 doesn't run that bad. It's worth the time to chace a 15 dollar sensor...It's just the 79$ diagnostics that suck.
You could change evry sensor for the tune of more like 300 minus the maf and tune it up for 100.

The only other thing is make shure the timing advance is working if you own a timing light. If the mechanical advance fails the engine is stuck round 3500 rpm

People who throw in a 400 - 600 dollarjunk engine should just toss out the cash for a 1000 dollar rebuild anyways! There's a reson it's in the yard to begine with. You just shouldn't fix(mechanically) an old engine.