rising idle with time

[rogwilson]

I have a 1980 sc that has had a perplexing idle problem for over six months now.

Engine starts fine. Idle drops down from 1600 rpms in less than a minute to around 1000. Then it continues to drop to around 500 until the engine begins to warm and the idle creeps back up to 1000 as I drive to work. As the engine warms the idle sometimes continues to increase to about 1200.

What have I done so far?

Checked Auxiliary Air Regulator. Seems to open and close properly.

Checked for air leaks with carb cleaner. None deteched.

Checked and cleaned Warm Up Regulator. 20 degrees C: cold control 2.2 bar, warm 3.8 bar, system pressure 4.9 bar, residual 1.5 bar at 10 min.

Checked CO mixture: 0.8% (Oxygen Sensor disconnected)

Checked deaccleration valve: no difference when plugged.

Checked timing: 5 degrees BTDC with hoses disconnected. 24 degrees at 3000 rpm. Hooking the hoses back up does not make much difference in rpm. Maybe need to check the retard function further.

Searched archives: Found several posts with similar symptoms. In particular, Aletech in 2003 had almost the same problem. I would like to know how he resolved it.

If anyone has any suggestions on what to check out next, I am all ears. Thanks.

roger

[MotoSook]

Have you tried just closing the throttle bypass screw a little to reduce the idle? 0.8% CO is low, so you're getting too much air. Since your checked the AAR and AAV, close the bypass when warm. Aim for 2-3% CO, at 1000 RPM idle.

Edit: You'll have to adjust air and fuel to get the desired mixture at idle while setting the idle. Make small steps and keep track of your setting changes (e.g. 1/2 turn in of bypass, then 1/2 turb in (richen) of the mixture, etc...). Seems like you just need to set you idle speed and mixture. Do it with the car WARM.

[rogwilson]

Souk
i run with the oxygen sensor connected and the Bentley manual gives the CO range of .4 to .8 percent. i have read numerous posts of people disconnecting their oxygen sensor and running a richer mixture.

i did try riching the mixture to 1.5% (with the oxygen sensor reconnected for driving) and that made no difference in the start up problem.

if i richen the mixture to 2 to 3% would you have to disconnect the oxygen sensor for driving? JW Walker and others post of better, smoother performance.

i agree the engine is too lean. using your method of manually pushing up the plunger gives a surge in rpms.

If ohecht reads this post, would like to know his solution to a similar problem he had a couple of years ago.

oh, souk, if i richen the mixture to 2 or 3% will the timing have to be changed as well. thanks.

[MotoSook]

I would disable the O2 sensor, then set idle and mixture. I would also advance the timing a bit.

Is your start up problem, the 500 RPM issue? It's a gap between the closing of the AAR and the leaning out by the yet to be fully warm WUR is my guess. As fuel reduced by the control pressure, the AAR should reduce air accordingly. If air or fuel are not in sych, your idle will not decrease linearly.

When you richened, did you do anything else?

I would set idle and mixture after a warm up drive.

Something else to consider since you've checked you AAR and AAV, is the deceleration valve. It too allows air to bypass the throttle body. A finicky decel will send random amounts of air to the engine. Try to disconnect the vacuum to it. Or better yet, disconnect and plug the air hoses to and from it so you completely block the flow part around the throttle body. You've got an inconsistant or unexpected internal air leak is my guess.