Though much improved after syncing the throttle body butterfly valves, balancing the TB vacuum with the idle air bypass screws, and adjusting the valves precisely so that all intake and exhaust valves are consistent with each other, my '88 K75S still does not idle smoothly. The idle varies 200 or 300 RPM randomly, between 1,000 and 1,300RPM, and it backfires periodically (though much less now).
When I say backfire, I mean ignition occurs back up into the throttle body, not that it makes a loud noise out the exhaust from the ignition of raw fuel. It does not do that at all. I know this because when balancing the throttle body vacuums with my homemade device, which has clear silicon tube connected to the vacuum ports, I could see the light from the ignition in the throttle body through the tube when it stumbled. It is not always the same cylinder that backfires.
In addition to the steps mentioned above, I have also verified that the TPS is set correctly and checked for leaks with MAP gas 3 times. So now I am wondering if some of this is just the nature of the engine and occurs on all K75s to some extent. If not, what do I need to look at next? Other than an imperfect idle it runs great, accelerating smooth and strong from idle to 6,000RPM with absolutely no hesitation, no matter how fast I get on the throttle. And when decelerating, it does not after fire at all, not even a hint. Should I be able to get it to idle more evenly with no stumbles?
ETA: Also, Installed freshly rebuilt Bosch Type II injectors.
ETA: Pulled the plugs and they have the ashy grey color associated with a good combustion mixture.
