how much fuel is getting into the crankcase? If a lot, check engine compression. If a little check engine compression. 450 psi+ is what I like to see. They usually won't run very well once they get much below 400. I think low spec is 380.
If it's low look for valve clearance issue first. Then pull the head and look for signs of bent rods. If they're all ok, pull the pan and look at rings. An oil ring will seal up just fine but if the top ring is worn/broken/cracked, or if the ring LAND is broken/cracked, it will have lots of blowby and never smoke a bit.
ZD21 has an electric fuel pump as I remember, so no mechanical pump on them.
Only other thing would be injection pump related although even that would be a remote possibility.