1988 244 DL, B 230F

What did I do or did something coincidental happen to alter the IAC operation?

In preparation for timing and idle I listened to the IAC through a long screw driver. I could hear if ticking.

I borrowed a peripheral tachometer to check my idle speed.
I set it up, grounded the IAC at the test point (BL-W wire), the RPM dropped.
The RPM is on target at 750 PM.

I ungrounded the IAC and went to ask my friend about a calibration question on his Tach.

I re-calibrated the Tach and grounded the IAC - BUT this time it does not slow the engine RPM.
The engine sounds faster. The Tach is reading 800. The engine should still be warm (?).

I again listened to the IAC through a long screw driver. It now sounds like it is buzzing (?).

I disconnected and shut down everything and tested the IAC resistance.
Pins 5 to 4 (20.9 Ohm); 4 to 3 (22.7 ohm); (spec for both is 20 ohm).
I'm thinking something happened to the IAC.

I set everything back up and restart the engine.
I look at the timing. It is 7 degrees +/-. I know that it was 12 degrees a few days before when I looked at it without getting involved with the Tach and IAC grounding.

I moved the timing up to 12 again and drove 17 miles home.
The engine feels much smoother and has improved power. Now it sounds like a Singer sewing machine w/o the putter I have asked about and tried to understand for years.

The next day I pull and clean the IAC (0 280 140 520) and test the valve by putting 12 Volts to the center pin and alternately grounding the side pins. The valve opens and shuts as anticipated.

I reinstalled the IAC, started the engine. The engine sounds fast. I hook up the timing light and it shows 20 degrees +/-. The RPM does not drop when I ground the IAC at the test point.

?? What is the RPM when the engine is warming up?

?? Ideas, theories and funny stories welcome.

1988 244 DL, B 230F