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930LDR 930LDR is offline
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Join Date: Dec 2004
Location: Danville - CA
Posts: 1,259
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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