# Mixture Screw Question



## michaelnahodil (Apr 27, 2010)

Where is the fuel mixture screw on a 1.7l motor? Any help much appreciated! thanks!


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## WaterWheels (Aug 14, 2005)

The mixture screw is inside the air sensor housing. If you look down from above the air mixture unit (air sensor and fuel distributor) you should see either a small plug or hole between the rim of the air sensor and the body of the fuel distributor. In that hole (with plug removed) is an Allen type set screw. It requires a long 3mm wrench to get down and turn it, but don' just twist it at random or at will. It is somewhat touchy and small turns can cause big changes.


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## ABA Scirocco (May 30, 2001)

I'm not sure why you're asking but you should know that the mixture setting on CIS does not tend to drift under normal circumstances you never need to touch it so if you're experiencing some sort of problem and think adjusting the mixture screw will fix it, it probably won't.

Don't mess the mixture screw unless: A: you know what you're doing AND 
B: you've eliminated all other possible causes for your problem. ​I'm a little touchy on this subject right now because I just spent a few hours working on a CIS Cabby undoing "repairs" done by the previous owner, who obviously had no idea what he was doing.


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## antichristonwheels (Jun 14, 2001)

After 30 years they tend to want a bit more fuel from how they were set at the factory...


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## ABA Scirocco (May 30, 2001)

antichristonwheels said:


> After 30 years they tend to want a bit more fuel from how they were set at the factory...


True enough but, not so much that you'd be having any type of significant driveability issues because of it especially if the O2 sensor circuit is operating properly.


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## michaelnahodil (Apr 27, 2010)

:thumbup:I'm just gathering information everyone. i one day plan on doing some motorwork to a 1.8l 8v and am just gathering all the information i need.. and sorry for not responding right away, i found an online version of the bosch cis manual and read it front to back!

thanks for the info though!


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## GTIspirit (Dec 13, 2002)

antichristonwheels said:


> After 30 years they tend to want a bit more fuel from how they were set at the factory...


All the mixture screw really does is adjust the mixture at idle. The mixture screw has a negligible effect at full load. So I totally agree with the above comments about leaving it alone, unless something in the fuel system, such as injectors, fuel distributor or pump were changed. Set it to the stock specifications, there is nothing to be gained here and I would question who claims otherwise.


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## WaterWheels (Aug 14, 2005)

GTIspirit said:


> All the mixture screw really does is adjust the mixture at idle. The mixture screw has a negligible effect at full load.


That's not really true. The screw is in direct contact with the air sensor plate arm with it's own shorter arm that is in contact with the mixture plunger. As the air sensor moves so does the rest, shorter arm and plunger. So making any adjustment to the mixture screw is linear in that it has an effect throughout the entire movement of the air sensor. That said, I'm not advising any tinkering, just stating that it does have an effect at all engine speeds. In a basic CIS set-up this can be used as a performance tuning aid, but if you increase the upper engine speed mixture, the mixture is also rich at idle and can/does cause "an un-happy" idle and starting issues. So in a race only car it can be set to a rich situation for better power at the risk of a bad idle. The 83' has a Lambda system, O2 sensor and the control, so no real benefit is seen as the system will always try to return the mixture to where it should be. It can still be used in a very slightly richer setting as long as the ability of the Lambda control is not over-powered. The Lambda control will return the mixture at idle but when the WOT switch is activated and the Lambda system is cut-out, the mixture will be slightly richer than if it were set to factory. This also has side effects as it can take a toll on the Lambda control and over stress it to failure.


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## GTIspirit (Dec 13, 2002)

WaterWheels said:


> That's not really true. The screw is in direct contact with the air sensor plate arm with it's own shorter arm that is in contact with the mixture plunger. As the air sensor moves so does the rest, shorter arm and plunger. So making any adjustment to the mixture screw is linear in that it has an effect throughout the entire movement of the air sensor.


The mixture screw adjustment is an offset, an adder. This has a profound effect on the mixture at idle airflow but a negligible effect on the mixture at full load flow. It's not a multiplier, not a factor, does not have a proportional effect across the entire range. It only adds an offset, so at low airflow this offset has much greater influence than full load airflow. I have dyno test data to prove this, that the mixture screw imperceptibly affects fueling at WOT.

Maybe some numbers for the sake of argument will help you better understand. Adjust the screw by some amount, maybe 1/8 to 1/4 turn to make the mixture 10% richer at idle. Let's say that idle airflow is 1 on a scale of 0-8. So adjusting the screw has added 0.1 to the fueling, 10% richer at idle. Now at a full load airflow of 7, that 0.1 amounts to 1.4% richer. I would consider that insignificant, imperceptible.


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## WaterWheels (Aug 14, 2005)

I really do try to think of myself as being understanding and tolerant when it comes to discussing matters with other people, really I do. But “…mixture screw adjustment is an offset, an adder.”? Try and try as I may, I just can’t understand the basis for nor the meaning behind your stating that. What exactly are you saying it “offsets” or is adding to? It’s not like some fine adjustment being made to the base setting due to air temperature or engine temperature like on say the L-Jetronic systems, it is itself the base setting. With this system, CIS W/Lambda, the Lambda control does the adding on or off to the base setting (fine tuning if you will) via the frequency valve, not the mixture screw. Plus the air funnel (cone) is designed to alter the fuel delivery based on in-coming air, again to the base setting. I also fail to see the how, where or why of your comment to the mixture screw, I assume, not being a “multiplier”. I find no mention above to anything doing any kind of multiplying in the text preceding your comment. 

As you have quoted my post I will assume you are directing things towards me, so to that, I have never said it was some kind of “Super Fuel Enrichment” deal now did I? I believe the words I used were slightly richer, yes? Here is a quote from a BMW tuner in his writings about tuning the CIS injection system which sums it up well, “It is NOT an idle mixture screw. While it DOES have the greatest effect over the idle mixture, it will have an effect on the mixture throughout the entire rev range. Though at higher engine loads the bowl shape has a greater effect on mixture than the mixture screw does” It just can not work any other way, they are in direct contact with one another, if one moves it moves the other. Yes, the air cone effects things as it determines how much air is deflected or used to lift the sensor plate which adjusts the amount of fuel delivered, but if the screw is adjusted to lift the plunger at idle, it has also been adjusted to lift it during higher air sensor plate heights. Bosch also mentions this in one of their manuals, and I quote “There is a linear relationship between the sensorplate travel and the slit section in the barrel which is opened for fuel flow” . And in a Bentley manual on injection systems written by Charles Probst he states in talking about performance tuning in the old SCCA classes,“… used to carefully adjust the idle mixture to a very rich setting – … The result was that the large enrichment in fuel mixture at idle brought with it a smaller but meaningful enrichment at high rpm, where it would give a little more power.” I’m sure I could thumb through all the manuals and such I have to find more quotes, but why? I know and understand how the system works and I’m content and secure with what I said. Save the dyno sheets or graphs as posting them by themselves with nothing but your description of what they represent does not really mean squat, right? Heck, I can cut and paste dyno sheets and tell people whatever I like, who can argue anything different, the sheets prove it right, NOT. Anyhow, this is not the reason for my posting again, it is the “Fuzzy Math”.

This is a form of mathamatics used, or at least said to be used by the opposing force, in politics or economics. It was brought to the main stream media I believe back in the Bush days about some of his plans. What I mean by fuzzy math here is, it just does not add up or make sense. I read your above example to help me better understand, and I understand less (at least less of what you posted). The beginning about turning the screw and how much is just usless, but it makes the mixture 10% richer, than what, on what scale? Really does not matter at this point I guess so OK fine. We are to measure the incomming air flow on a scale of 0 – 8, zero I assume is none, and our air flow is at 1, so a tiny bit. OK fine so far, mixture is 10% richer (increase) at idle with an air flow of 1 on a scale of 0 – 8. And you then say the screw has added “0.1 to the fueling”, zero point one what? I don’t see the 0.1 coming from the above in any way. I understand the 10% richer, got it, simple, but the rest, guess it went over my head. OK, fine, now at full load our air flow is at 7 (on the same scale above) and you calculate that this amounts to a 1.4% richer mixture, but how was the calculation done? I can’t seem to make any of the numbers above come out to your calculation, which you also claim to be insignificant. Sorry, this is all just to un-connected and cloudy at best to mean anything. And if the bottom line is that a 1.4% increase in a fuel mixture is worthless, first it has to have something base line to be 1.4% higher than to make that determination, no? Is this by weight, or air/fuel ratio or something else?

As I stated up front, I really do try to understand and I really have read this many times to see where I fail to make the connections, but I just can’t find anything. For the sake of education, please lay this out again in a maybe 6th grade level or in more detail as to how it all links together. Or if someone else has a clue as to how this all works out, please jump in.


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## antichristonwheels (Jun 14, 2001)

Regardless, I would adjust it on any CIS car I worked on. I use the O2 sensor or by ear if it's an early car. Unless it still has an undrilled plug it's hard to beleive it has not been touched in the past or is factory perfect. A little bit of tuning and you can make it start, idle and run better.


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## scirockinit (Oct 16, 2008)

so which way do u turn to richen and lean it out


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## jonnyp60 (Jul 30, 2005)

http://www.cabby-info.com/cis.htm
http://merlyyn.com/tech/


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## John Milner (Jul 20, 2005)

scirockinit said:


> so which way do u turn to richen and lean it out


Right turn/clockwise = rich
Left turn/anti-clockwise = lean


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## Glegor (Mar 31, 2008)

antichristonwheels said:


> After 30 years they tend to want a bit more fuel from how they were set at the factory...


its cause of all the CRAP in the gasoline now days. and all the ethanol. thats why they like being richened up a bit.. it takes ALOT more ethanol/alcohol to get a stoich mixture, than it does with gas.


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## ABA Scirocco (May 30, 2001)

Glegor said:


> its cause of all the CRAP in the gasoline now days. and all the ethanol. thats why they like being richened up a bit.. it takes ALOT more ethanol/alcohol to get a stoich mixture, than it does with gas.


I'd never given that any thought before and I've not sure if it's correct but it definitely seems plausible considering that ethanol has a stoichiometric air fuel ratio of 9:1 compared to 14.7:1 for "normal" gasoline.


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## Glegor (Mar 31, 2008)

im almost positive that all the ethanol, and junk in the gasoline now days has an effect on a/f ratios..

i bought some non-ethanol premium a while back, and my GTI LOVED THAT STUFF..

kinda surprised it didnt foul my o2 sensor, but it may have been un-leaded non ethanol.. didnt pay much attention.


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## ABA Scirocco (May 30, 2001)

It should only be the CIS-basic systems that might have a problem with ethanol laden fuels. CIS-lambda and CIS-E system would be able to adjust for the ethanol rich fuel provided of course the O2 sensor and related parts are working properly. 

BTW, that premium gas you bought would most definitely have been non-leaded. The only leaded fuels you can get in North America these days are racing fuels and Avgas. And in Canada at least it's becoming increasingly difficult to find leaded racing fuel, my normal fuel supplier is now unable to import leaded racing fuels.


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## Glegor (Mar 31, 2008)

it was Sunoco fuel, and it was over $4 a gallon.. but i doubt it was leaded, because they were selling it to street driven cars..


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## dkfackler (Feb 8, 2010)

scirockinit said:


> so which way do u turn to richen and lean it out


To adapt on old maxim:

Righty-richey
Lefty-leanie!


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## EuroKid83 (Jan 26, 2005)

ABA Scirocco said:


> I'd never given that any thought before and I've not sure if it's correct but it definitely seems plausible considering that ethanol has a stoichiometric air fuel ratio of 9:1 compared to 14.7:1 for "normal" gasoline.


Gasoline in WA contains up to 10% ethanol and in the winter time they change the formulation to help with cold starting/running and I always have to adjust my idle mixture on "winter gas" Then when they change back to the normal crap I have to adjust it again. My car has CIS basic so I have to make adjustments to compensate for different formulations of fuel at different times of the year. Just one example of a situation where you would have to adjust idle mixture.


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