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CIS Mixture screw question

Hello everybody, I have tried this question on the 911 Technical forum thinking they might have more CIS traffic? They sent me back here.

So it's a stock 76 ROW 930. It has run rich since I have owned it.

I am getting to know my way round the whole CIS thing. I suspect my mixture screw has been tightened up all the way? and damaged?

In unwinding it it seems to make no difference to the mixture, and it has no detectable click that I have read about. Too worried to unwind it all the way at this stage.

What should I expect from this screw? I am just asking for any pointers before I pull it apart.

Secondly, in the center of the right hand half of this picture there is a screw with a white paint mark on it. Next to the regulator. Is this an adjustment screw? It is locked down so hard I feel I will damage it if I try any more to crack it off.


Old 10-08-2014, 04:50 AM
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OK trap for young players. That is not the mixture screw. It's simply it's access hole. Doh!

I can now access the mixture screw, though shes pretty tough to turn.
Old 10-08-2014, 05:14 AM
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By your questions... DON'T mess with that adjusting screw until you learn more about it. Much info on this forum, if you search for it... I believe your picture shows a cap on it - not clear - but normally the screw stick out a couple of inches supported by a spacer spring.

BTW, the adjustment of it is very sensitive, it is NOT, an all the way here or two turns back... NO, we're talking 1/8 to 1/4 of a turn CW or CCW to rich or lean the idle - not much effect while cruising or boosting.

Do yourself a favor and search before playing with it... may be too late by now.
Old 10-08-2014, 10:28 AM
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No, I have not messed with it yet. I'm just going through the system to familiarize myself with it.

Thanks for your advice on the sensitivity of it.

Cheers.
Old 10-08-2014, 12:55 PM
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Feel free to adjust the idle CO adjustment screw. You won't learn how and what it does if you don't.
It's good to have a wideband AFR gauge in the car so you can see what it's doing and how it works out the next day you drive the car after a complete heat cycle.

If you've tuned carburator idle mixtures or lawn mower carburator needle valves by ear in the past it's the same except you turn the CIS idle mixture screw the opposite direction to make it richer. Turn the CIS CO adjustment screw left for lean and right for rich.
If you turn it a little just remember how far so you can turn it back the same amount if you decide to. The final adjustment should always be clock wise to keep your adjustment stable and consistant.
It does change air fuel mixture at upper RPMs a little as you adust it for idle mixture but not as much as at idle. There's a curved slope to the AFR change if you could make a graph showing it.

It's funny how some people here try to make such a simple thing so dramatic... lol
Old 10-08-2014, 01:34 PM
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Quote:
Originally Posted by JFairman View Post

It's funny how some people here try to make such a simple thing so dramatic... lol
Well Jim, we certainly are a dramatic bunch aren't we? The key, as you've mentioned, is to remember where you started so a person doesn't get things completely out of whack. The other important key from Miguel is to "go easy" with the adjustment. Quite frankly, even 1/8 of a turn can be way too much unless the mixture is way off to start with.

Interestingly, while on this topic, I attempted the other day to lean my "new" ride down a bit. Depressing the spring loaded screw until it almost touches the control arm, then turning, I felt no resistance. Seems that spring loaded "post" should engage after you push it down enough and turn, but again....no resistance when turning it. My other 930 had a distinct feel when turning the screw....quite a bit of resistance actually.
I've never taken the fuel head apart so can't visualize what's going on in there with the linkage, but I don't think my turning it was having any affect. Thoughts????
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Old 10-08-2014, 03:52 PM
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I got my understanding of how the mixture screw works by a generic CIS document. Had a good illustration and, as with most things on the car. Once you understand how it works, it's quite simple.
Old 10-08-2014, 04:27 PM
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Mark, you just confirmed what I have suspect for several years now; you have a screw loose, my friend (and not just the mixture screw, either)!

But on a serious note, when you say you pushed it down, I assume you mean you had the 3mm hex in there while doing so, right?

Last edited by Ronnie's.930; 10-08-2014 at 04:46 PM..
Old 10-08-2014, 04:43 PM
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Yeah gotta push down with the 3mm hex and turn a little until it seats. There is definitely resistance when actually turning it.
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Old 10-08-2014, 05:34 PM
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Yup, pushing down on the spring-loaded shaft and turning can fool you into thinking it you're making an adjustment. Without the allen-head wrench in place though, you're not actually turning anything that matters.
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Old 10-08-2014, 07:05 PM
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Mark's been doing the ole' CIS tuning for quite some time, so I doubt the was trying to make adjustments without the 3mm hex - I was just yankin' his chain about that.
Old 10-08-2014, 07:32 PM
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Hey friends, it's ok to ask but duh....yeah....can't very well adjust the thing without the proper allen. I swear I had that thing pushed all the way down and just a pub hair short of contacting the control arm but still, no resistance when turning. Maybe I'll give it another go. Can hardly wait to have my garage (and house) back so I can start with some serious tinkering.

And yes Ronnie, if only I had but one screw loose.....
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Old 10-09-2014, 04:16 AM
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Quote:
Originally Posted by JFairman View Post

It's funny how some people here try to make such a simple thing so dramatic... lol
Sorry, but it is a matter of lessons learned, trying to help a fellow pelicanee, more than being dramatic about it...
Old 10-09-2014, 04:22 AM
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Quote:
Originally Posted by mark houghton View Post
Interestingly, while on this topic, I attempted the other day to lean my "new" ride down a bit. Depressing the spring loaded screw until it almost touches the control arm, then turning, I felt no resistance. Seems that spring loaded "post" should engage after you push it down enough and turn, but again....no resistance when turning it. My other 930 had a distinct feel when turning the screw....quite a bit of resistance actually.
I've never taken the fuel head apart so can't visualize what's going on in there with the linkage, but I don't think my turning it was having any affect. Thoughts????
Hi Mark, In the past I can remember you telling people to use a 4mm allen head wrench to adjust the CO screw. I posted a couple times after you back then that it's a 3mm allen head you use. All the CIS fuel heads I've adjusted on early eightee's BMW's were also 3mm.
Maybe you're still trying to use a 4mm allen head and it isn't engaging and that's why you're having this problem.
Old 10-09-2014, 06:37 AM
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This is really little help to the original poster as the early air meter assembly has no external spring loaded adjustment arm. The actual adjustment screws on all models are internal, you have to engage the screw with the spring arm. He will need to remove the cover and use the T tool to reach the adjustment screw.
Old 10-09-2014, 08:46 AM
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Thanks for every bodies input.

Silly me put a 4mm allen key into a screw that had been cranked up. Then assumed something was wrong.

After looking at a generic CIS document I got to thinking there is no way an adjustment screw designed like that could possibly have it's adjustment thread firmly fixed to the upper housing of the meter.

So I took that 4mm SHCS all the way out.... low and behold. A 3mm allen key engages at the bottom of the vacant hole, and floats with the metering plate. Very tough to adjust, but a result never the less. Waiting on an AFR monitor before I do too much else

Am I correct in thinking that this hole will leak vacuum if I was to run the engine with this 4mm "plug" screw removed, giving false readings?

Everybody's input is valued here. Thanks again. I will no doubt have more questions in the next few weeks.

Last edited by wildturkey; 10-09-2014 at 01:26 PM..
Old 10-09-2014, 01:22 PM
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Making progress...

Do your thing a put the plug back on - everytime.
Old 10-09-2014, 01:27 PM
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Old School [I was told by my mechanic], is to have the car idling and warmed up. Then, depress the Air Metering Plate ever slightly. Take note as to the Engine RPM.

If RPM immediately momentarily goes up, then your car's idle mixture is a bit lean. If the rpm momentarily goes down, then it could have been right on, or a bit rich. Interpolate from there...

Like Jim said, I needed Wideband to be sure. My advice [to me and others] is to start with a set of CIS gauges and Wideband AFR. I know those have been the keys for me to be able to "dance the CIS dance..."

Good Luck!
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Old 10-09-2014, 02:10 PM
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Quote:
Originally Posted by JFairman View Post
Hi Mark, In the past I can remember you telling people to use a 4mm allen head wrench to adjust the CO screw. I posted a couple times after you back then that it's a 3mm allen head you use. All the CIS fuel heads I've adjusted on early eightee's BMW's were also 3mm.
Maybe you're still trying to use a 4mm allen head and it isn't engaging and that's why you're having this problem.
No Jim, I use a 3mm and it fits nice and snug. Yes, I did once post using a 4mm from a faulty memory. Thanks for remembering.

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Old 10-10-2014, 07:20 AM
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