Ford Festiva Mystery Misfire and Smoke

Car: 88 festiva with 227K miles For those unaware this is a Mazda B3 SOHC carburated 4cyl

Symptom: Misfire and Blue (oil) smoke.

When: Only happens at first startup after car has sit for a while (at least 8 hours). It starts ok, runs fine on all cylinders, for about 15 - 30 seconds. Then it starts to misfire. If I try to drive away I get a cloud of blue oil smoke. It then clears up and runs fine. No oil smoke, no misfire. If I just let it idle, it will misfire for a long time, and then blows even more smoke when I finally hit the throttle. It really only seems to clear up if I rev the engine a bit.

I have adjusted the valve lash, changed the oil and filter, replaced the plugs with NGK copper. Dist cap looks newish as well as wires ( but I have not changed them)

When I pulled the plugs, they looked fine for the most part. no fouling, color looked normal, they weren’t worn horribly.

I initially thought that it was a carb issue, but the smoke has gotten worse, so I am thinking maybe something is sticking? I get some clatter at startup sometimes (not valve noise) but it goes away as soon as the oil gets pumping (like 1 second) and it is pretty inconsistent when that happens. I am still getting over 30mpg in mostly city driving, and power levels seem ok most of the time

I am leaning away from valve seals, because in my experience they dont cause misfires and for the first 30 seconds there is no smoke at all, but am I in the wrong here?

Any thoughts from you master mechanics? I feel like a compression test might not help because it only happens in certain circumstances…

I suspect that you have a lot of oil leakage past the valve stem seals when the engine is parked and it has to burn off when you start the engine. That would account for the misfiring as well as the blue smoke. It’s possible that plugged up oil returns are contributing to the problem by restricting the ability of the oil to flow back through the returns, thus making more oil available to leak past the seals.

There’s no real test for this that I’m aware of, but I’d look into changing the valve stem seals.

If the valve seals were leaking that much, wouldn’t they show some sign of this happening if it sat for 4-6 hours as well? At least some smoke?

What you described ARE the signs of bad valve stem seals!!!