Hesitation/stumble at partial load/throttle

[zaffer]

I have a 1983 320i 4-cyl. K-jet car that is driving me nuts! Originally had issues with cold starts, warm running, etc and after any headaches, finally figures out it was primarily due to the valves on cylinders 2 and 3 being too tight, leaving the valves partially open. Valves are now adjusted to spec., set the timing (re-checked multiple times), new WUR, new aux. air valve, injectors are spraying nicely and are uniform (measured output), no more air leaks, new plugs, rotor, and cap, and wires are good (checked resistance). I also went through and checked fuel pressures and they are all within spec.

The car now starts and idles well on cold and hot starts, WOT seems to be pretty good, but partial/cruising load yields a hesitation/sputtering. The car also sputters lightly when idling. I have messed with the mixture and idle screws to try to get a good balance by ear, but the issue still exists. I checked the plugs and the color is good. WOT is smooth throughout the rev range and vehicle settles into idle pretty well at the moment.

I'm starting to get frustrated with not being able to get rid of the hesitation, and adjusted the mixture 1/16th turn at a time until I feel I have a pretty good balance. I don't have a dwell meter or CO meter, so I can't adjust those items. I did attempt to adjust the mixture by reading the O2 sensor and got it to about 0.45V at temperature, but still the issue persists.

It seems to me to be a mixture issue and everything I've read points to a lean mixture, but when I enrich it, the car won't idle and WOT suffers.

I've read a fair amount on this issue, but none of the solutions seem to correct the issue I'm having.

Thanks!

[Jeroen]

Sounds weird indeed, especially if the car is running fine and feeling strong enough at WOT. So you're absolutely sure static and dynamic ignition timing is ok? In that case CO is the first thing to check, or even better, a full exhaust gas analysis (CO, CO2, O2, HC, AFR) as that will give you a hint to what might be wrong.

Also a thing is that the mixture adjustment screw is fairly sensitive, so even 1/16th of a turn makes a huge difference.