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I think I screwed up my fuel mixture

I recently installed an AFR gauge and thought it would be nice to see if I could optimize my fuel mixture. I thought I was running too rich at idle (~11.5) although the car ran nicely. I used to adjust the mixture on my SC and thought it would be the same but I'm not used to the spring loaded screw on the 930. I pushed down and turned counter clockwise and immediately the AFR went to about 14 at idle. So I turned it back and then it would go straight back to ~11.5. Somehow couldn't get it in the middle. I decided to drive it at the 14 setting and it ran poorly until all of a sudden it seemed to go back to its original 11.5 setting (idle) and started running better without me doing anything. I gave up and thought maybe the screw had been stuck in the long tube and had become unstuck and back to its original setting. However, now when the engine is colder it seems to run a little leaner (~12) and doesn't get to 11.5 until very warm. I'm also noticing less throttle response and power. I'm a little worried that I might have damaged something. I heard you are not supposed to push down too much on the mixture screw. I don't think I did too much, but want to know if the concern is warranted. Why would my cold idle AFR change and not the warm.
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Last edited by jwakil; 04-08-2015 at 05:53 PM..
Old 04-08-2015, 05:49 PM
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When you did the "initial" adjustment, was the car fully warmed?
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Old 04-08-2015, 06:10 PM
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It doesn't hurt to "push down too much"; i.e., it won't damage anything, but it will push the air/fuel metering arm down past the adjustment point causing a momentary rich condition until you release the pressure. Push down just enough so that when rotating your allen wrench you can feel it engage the adjustment screw. Done it a million times. Plus, that can be advantageous if diagnosing your mixture. If you push the adjustment shaft down very slightly to where it deflects the arm (thus enriching the mixture) and your idle changes one way or the other (if rpm's go up, then you were too lean to start with, and conversely if they go down you were to rich to start with and are now even richer).
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Last edited by mark houghton; 04-08-2015 at 07:35 PM..
Old 04-08-2015, 07:33 PM
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Quote:
Originally Posted by mooney265 View Post
When you did the "initial" adjustment, was the car fully warmed?
Yes, the car was fully warm when I set it.
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Old 04-08-2015, 08:07 PM
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so many rock apes and hung over morons have worked on these cars by now there's no telling what condition things are in...
Old 04-08-2015, 08:10 PM
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Quote:
Originally Posted by mark houghton View Post
It doesn't hurt to "push down too much"; i.e., it won't damage anything, but it will push the air/fuel metering arm down past the adjustment point causing a momentary rich condition until you release the pressure. Push down just enough so that when rotating your allen wrench you can feel it engage the adjustment screw. Done it a million times. Plus, that can be advantageous if diagnosing your mixture. If you push the adjustment shaft down very slightly to where it deflects the arm (thus enriching the mixture) and your idle changes one way or the other (if rpm's go up, then you were too lean to start with, and conversely if they go down you were to rich to start with and are now even richer).
That makes me feel better. I thought I might have messed something up. I think the tendency for my screw to get stuck and then pops up in the long vertical tube could explain the sudden jumps from one adjustment to the next, and the one time it happened while I was driving. The metering arm might have been held down and then released. Although when it happened when I was driving I went from overly lean to rich, which seems backwards to what should happen if you remove load from the metering arm? Also, I read somewhere that you need to let it go through a hot and cold engine cycle before things stabilize. Haven't really tested after the full cycle.
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Last edited by jwakil; 04-08-2015 at 08:33 PM..
Old 04-08-2015, 08:14 PM
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make small adjustments and drive it after adjusting no matter what it reads.. you cant let it idle for a long time and get a good reading.

glad to see someone else is idleing inthe 11-12 range.

what is you mixture at 3k rpm in second and 3rd.
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Old 04-09-2015, 04:49 AM
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Is your lambda system hooked up? I usually adjust with it disconnected, then hook it up. WIth it hooked up, 14.7 at idle is pretty easy to get .
Old 04-09-2015, 06:55 AM
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Quote:
Originally Posted by T77911S View Post
make small adjustments and drive it after adjusting no matter what it reads.. you cant let it idle for a long time and get a good reading.

glad to see someone else is idleing inthe 11-12 range.

what is you mixture at 3k rpm in second and 3rd.
Well today I drove it and it's pretty much back to its normal mixture readings. ~11.5 idle, and between 13.0 and 13.5 @ 3krpm, 2nd-3rd gears. After all that, I think I really didn't turned the mixture screw at all, just pressed it up and down and caused the metering arm to change temporarily. The car seems to be back to its normal good power and running, so I'm not going to mess with it any more.

I think it also depends where your O2 sensor is mounted in the exhaust system. My 11.5 might translate to someone else's 13.5 if they have it mounted in a different location. I have a cat bypass and the sensor is actually mounted in the exhaust barrel. Plus the sensor wasn't hooked up for a couple of years, so I don't know if that caused any abnormalities in the sensor surface. All I know is it is pretty consistent from day to day, and my idle of mixture of 11.5 works fine.
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82 - Austin Mini 998
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83SC Euro w/77 3.0 Carerra Eng.--sold
Several other daily drivers not worth mentioning...

Last edited by jwakil; 04-09-2015 at 09:38 PM..
Old 04-09-2015, 09:36 PM
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