History (or lack thereof) purchased this 85 camaro v6 2.8l mpfi, the person i bought it from said he had a used engine put in (gave me the old one that was locked up). Said it was running rich (it was), got it to my shop, swapped cold start injector as i suspected it may have been sticking open, adjusted the timing to where it needed to be 6 btdc with set timing disconnected, there are no misfires (checked with timing light). Swapped the iac cleaned injectors/maf, replaced plugs, drained fuel and put fresh in, new manifold air temp sensor (had code) etc... and ordered a new computer, seems to be running (fuel wise) much better. HOWEVER, once i got it running good, i noticed a vibration, fairly severe, around 800 to 2000 rpms, seems to subside slightly after that but still... I am a newb to the site but not a lazy one, as you see i did a good bit of troubleshooting and research before posting. No difference between park or in gear, same symptoms. Next i unbolted and backed off the converter to no avail. Checked motor and tranny mounts, nothing bad there. THE QUESTION ! Seems to me the only 2 things left are the flexplate or the harmonic balancer, flexplate is NOT warped (no wobbles) from underneath the car, seems immobile when running, I tried the old balancer (holes drilled in IT where slightly in a different spot but otherwise looked identical) furthermore, there was no obvious counterweight or excentric shape to the balancer so is it neutrally balanced or not ? if it were balanced why would there be a cluster of 3 holes half drilled in it (just curious). Back to my question, or list of clues, the flexplate does NOT have a wheight on it. Neither does the one on the old engine! If i am correct, it should, HOWEVER the previous owner said he had just put in a new crank kit before it started running rich and he parked it. CONFUSED YET ? So it could be many things, wrong crank kit for that year IE internally balanced vs externally, incorrect flex or balancert or a seemingly infinite combination of both. FEEL MY PAIN YET? NO, i do not know what year model the engine in it now came from, i am not sure if my balancer is neutral or not. RACK your brains as to what did, may might have happenned as i cannot get any m0re info. What is the most likely? thx FRED

Can you get ahold of him and find out if he has any receipts or remembers where he got the crank kit? maybe you can find out what crank is in there, or pull the pan and check part numbers, then compare them to part numbers on the flexplate and balancer. or i believe engine shops can check flexplates and balancers for you. how does the rubber isolator look on the balancer? it may have let go slightly and shifted. Ill keep thinking about this one, im interested.

Id just stop running it and pull it apart to find out.

There was a desing change to the engines in 87, they went from external balanced to internal balanced.

If someone got parts mixed up from diff years, you could be destroying the engine.

UPDATE !
Figured it out, the engine actually came from a 88 firebird, was running fine (explains the neutral flex/balancer), the previous owner wanted to put a new crank kit so he went to advanced auto, they asked him what year model engine, he wasn't sure if it was 87 or 88 and they told him it didn't matter and that they were the same; WRONG !!!!
I took a gamble and guessed they gave him the wrong one (with the counterweight between 3 and 4 and got replacement flexplate for externally balanced engines, dropped my tranny, pulled it out, swapped flexplates, reassembled, vibes are gone!!!!
lucky guess/diagnosing !
On to my newly acquired 85 Z28 305 HO.
Thx to the replies.