Alright 1st things 1st, im not that mechanically inclined. anyways to the issue. I have a 1994 S10 blazer with a recent engine swap. its still a 4.3l. most days it wont crank. it will just spin and spin. on the rare days it cranks, the engine acts fine till i guess about 3000 rpm (i dont have a tach so im going by sound here) where it will start losing power severely and acts like it wants to cut off. it will jerk and pull really bad. my uncle used a code scanner and couldnt get any codes. id take it to advance or a shop if it could make it but sadly it cant. so can anyone give me any opinions on what this could be?

 

these cpi engines have a bad issue with the spiders going out. (main injection system)

They start leaking really bad thus causin these issues. Drive to autozone and rent a fuel tester for free. Test fuel pressure at the fuel port behind the motor to the right. you'll see it with a black cap. put tester on it and turn the key to the on but not crank position. look at the guage. It has to hit at least 60 psi when you turn the key. If it did,fuel pumps probally ok. now if it hit 60+ then started dropping down past 55, the spider is indeed leaking. It could also be the 1 way valve in the pump,but its not as common as the spider leaks.
Do this and let us know the specs. Dont spend a dime on anything till you do this.