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Join Date: Dec 2004
Location: Danville - CA
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Exclamation Need some thoughts on Idle and Mixture Adjustments

So I had my car in the shop last week because it was running like crap above .5B of boost. My mechanic went though and found that the distributor was loose and screwing with the timing. Tightened that down and it seemed to solve the problem. Then he went through and double checked for any vacuum leaks and tighted up all the hose connections.

The car shows up at the track on Friday morning and is running great. My LC-1 had been needing a recalibration as someone left the key in the on position too long at the shop and it calibrated itself to dirty air. I go through the excercise trackside and get it working again. My initial reaction to the readings I was getting was that they couldn't be right as it was showing the car idling with a mixture around 17.0 - 18.0. I called my mechanic who thought that if he in tightening up the hose connectins he fixed some misc vacuum leaks that this could effect idle mixture.

I adjusted the mixture on the fuel head down to around 14.0 and went out on the track to see what was going on under boost. AFRs under enrichment seemed spot on to where they had been before the car went into the shop. Low 12.0s at the highest.

Bring the car back into the paddock and its idling really low. Like 600 RPM. Damn thing is fighting just to maintain idle. So I adjust the idle up using the idle screw on the throttle body. This fixes the idle but also leans the mixture back out. Play around with the combination of mixture and idle speed and just can't get it to where it was before heading into the shop. Try really cranking up the mixture and then adjusting the idle up to see if I can get it to the right place and must have momentarily over adjusted the idle set screw as I heard a hissing coming from the throttle body. Backed the screw back down and the hissing went away.

Ended up just saying the hell with it went about the day continuing to watch boost mixture.

Again the car is running well on boost but still just seems off at idle and while cruising under partial throttle. Where it used to cruise at 14.0 or 15.0 and part throttle (no boost... 60mph) its now down at 12.0. When you let off the throttle, almost all of the crackle and popping is gone. It used to pop really loudly cruising around in second in the city when I let off the gas and now it barely does it at all. When I do let off the gas in those situations the AFRS flight way up to 18.0 where I think they used to bounce only as high as 16.5 maybe.

Idle is a little high now but has also began to hunt a bit moving in and around 900 - 1200 rpms at a stop light. AFRs at idle are somewhere arounf 15.0. Las time I tried to bring the AFRs back down it had the impact of lowering the idle back down to where the car was stumbling a bit.

Any thoughts?
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Rob Montgomery
'88 Blk/Blk 930 ('Lucy') - Not Stock & Not Running
Old 08-25-2008, 04:50 AM
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I would pm RarlyL8 .He would be the one that could help you.
Old 08-25-2008, 06:37 AM
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There is a sweet spot in the distributor timing at idle that may be off now. What is your timing set at? What was it before the dizzy got loose?
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Old 08-25-2008, 08:31 AM
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Quote:
Originally Posted by 930LDR View Post
. . . Idle is a little high now but has also began to hunt a bit moving in and around 900 - 1200 rpms at a stop light.
Hunting at idle is a sure sign it's too rich, not too lean. AFR of 15 is lean. Suggest you check AFR using a secondary AFR reading source.
I have had similar issue controlling AFR (stock 30-year old WUR). One minute it's lean, the next it's rich, some days it's ok, the next day it isn't. In process of installing DWUR, crossing fingers the WUR is the problem.
Old 08-25-2008, 09:24 AM
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Did you check to mak sure the AAR valve is opening/closing properly. I experienced a similar problem after a track day where the car idled extemely high when I had done and touched nothing to the fuel system other than drive the car hard.
Turns out the AAR valve was not closing down because the electrical connection to the heating element was full of oil and not heating the internal element. Cleaned it off and she runs great.
Process of elimination is key.
But you need to have some baseline settings written down to go back to where it sounds like you changed alot of items up to now.
Set your dizzy timing then try WUR pressures back to nominal settings and see from there.
Make sure your O2 sensor is not overheated and damaged, running too high a temp on the sensor will destroy it and give false readings.
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Old 08-25-2008, 12:14 PM
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Quote:
Originally Posted by fredmeister View Post
Did you check to mak sure the AAR valve is opening/closing properly. I experienced a similar problem after a track day where the car idled extemely high when I had done and touched nothing to the fuel system other than drive the car hard.
Turns out the AAR valve was not closing down because the electrical connection to the heating element was full of oil and not heating the internal element. Cleaned it off and she runs great.
Process of elimination is key.
But you need to have some baseline settings written down to go back to where it sounds like you changed alot of items up to now.
Set your dizzy timing then try WUR pressures back to nominal settings and see from there.
Make sure your O2 sensor is not overheated and damaged, running too high a temp on the sensor will destroy it and give false readings.
+1 On checking AAR valve.
Also check cold/warm control pressure settings after that.
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Old 08-25-2008, 01:34 PM
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