I don't understand why you would call 8.5 to 1 'low' compression as this is the original compression ratio in many Datsun models & is more compatible with the lower octane fuels. Something like 7 to 1 might be regarded as 'low' compression.
Have you given the car a proper tune up as per the book? [I assume that it is a stock engine] To set the idle, once the dwell, timing & tappets are right, you will benefit greatly by having a hand held tach & a vacuum guage. Just 'fiddling' with the idle screw [mixture screw or idle speed screw?] does not usually produce the best results.

Once everything there is right, you can start trouble shooting the problem if it still persists.