Trying to adjust idle mixture on my 1980 Quadrajet after rebuild.

My carburetor has number 17080504 on it. It is California first year computer controlled Quadrajet, originally installed on 305 engine.
It has three electric connections: electric choke, Throttle Position Sensor output and Mixture Control Solenoid input.

Adjustments:
There are two fuel screws on the front of bottom later.
There is Mixture Control Solenoid adjustment screw on the top (please see picture and correct new if I am wrong).
There is some small screw on the top next to MCS adjustment screw.
There is idle throttle adjustment screw on the left side of the carb next to throttle wire linkage.

Here is what I did:
– Warmed up engine by driving for 10 minutes.
– Disconnected all accessory vacuum lines and plugged ports.
– Verified that choke is completely opened and that fast idle is not engaged.
– On fully closed throttle set TPS output to 0.56 volts by turning adjustment screw next to accelerator pump lever.
– Set RPM to 600, set base timing to 10 (spec for my engine).
– Turned fuel screw all the way in, then turned them out 3.5 turns both.
– Set RPM to 800
– Turned MCS screw all the way in, then out until engine started working smoothly (about 3 turns out).
– Reset RPM to 800, verified that it drops to about 600 in gear, repeated previous step

Turning MCS adjustment screw has same effect as turning fuel adjustment screws.

The engine runs smoothly now, except slight hesitation on hard acceleration, and slight dieseling. I am now getting one to five engine cycles of dieseling and then backfire through the carb after turning ignition off. This was not happening before carburetor rebuild.

During rebuild I did:
– Set float level to spec 0.375"
– Replaced accelerator pump
– Resealed drain holes on the bottom of middle layer with epoxy
– Replaced throttle position sensor
– Replaced and adjusted secondaries spring
– Adjusted vacuum brakes rods
– Checked saddle and needle, both in good condition
– Checked all gaskets for cracks, all good.
– Checked for fuel and vacuum leaks, all good.
– Checked all fuel and air lines, all clean.

I need help figuring out:
– How to correctly set MCS adjustment screw?
– What are factory settings for fuel adjustment screws?
– What is that small screw on the top near to MCS screw?

 

the small screw is your "lean mixture" stop screw, it regulates just how lean the engine gets via the computer. the larger screw you have marked as MCS is the "rich mixture" stop screw, it regulates how rich the computer can make it.

the computer controls the mixture (within the range allowed by these two screws) using what is called "dancing needles", the computer bounces the needles up and down at varying rates to control the mixture. if it needs more fuel it slows down the "dance" and speeds it up to lean out the mixture. the needles move full range everytime the computer pulses them.

there is a book available on how to set up this carb, I know there are some special measuring tools to do it, basically you set the rich limit first to a certain distance, then you set the height of the lean stop using a different measuring tool.

kind of hard to explain but hopefully you get the idea.

 

Jig A Low, thank you, I got the idea. I did not know that. It makes perfect sense now, square wave signal is used everywhere in this car electronics.

A book would be great, do you remember who is the author?

Here are several that I found:
does not seem to have anything about MCS adjustment.
,
– this one seems to have some MCS adjustment instructions.
Which one should I get?

 

A2,, it's been a long time since I messed with one of those carbs. I honestly don't remember what the book was. it's not difficult to do IF you have the measurements and the tools to do it with. I'm not sure they are even still available but I know it's possible to make your own version of them. just do an internet search about the CCC setup and how to do it and you'll find out how to make them up.

good luck

 

a few months ago there was an excellent article on fixing and adjusting those carbs in hemmings muscle machines. maybe someone can post it as a sticky as a lot of people have issues with them. i know everyone will take exception with this but if it were mine id change the distributor and carb and be done with it .

 

After reading several books on adjusting computer controlled Quadrajet I was able to get rid of dieseling. But I got some new problems...

I have got a dwell meter for reading computer control signal, multimeter for measuring TPS signal, vacuum gauge and timing light RPM meter.

I did:
- Set initial timing to 10 degrees in neutral at 800 RPM.
- Set RPM in drive to 600.
- Turn off engine.
- Set idle fuel screws to 4 turns from seated each.
- Set idle air bleed screw to 4 turns from seated.
- Measured TPS voltage, it was 0.56 volts
- Started the engine, it halted after a few seconds. Repeated with same result.
- Turned idle air bleed screw 2 turns in (so it's now 4 turns from seated)
- Started the engine, it continues running a bit roughly.
- Turned idle fuel screws out in 1/4 turn steps, idle air bleed screw in in 1/8 turn steps, idle speed screw to keep RPM at 600 in drive, till dwell 30 degree value reached and RPM was stable.

Idle fuel screws are now at 5 1/4 turns out from seated, idle air bleed screw is 1 6/8 turns from seated. Solenoid plunger travel is 3/32 inch, which is what specs say is normal. Pushing solenoid plunger down makes engine halt on idle. Mixture control solenoid dwell is 30 degrees in drive, throttle position sensor signal is at 0.51 volts.

Now engine idles very good at 850 RPM in neutral, 600 RPM in park, makes 18 inches of vacuum in park and 15.5 in drive. This looks very close to specs.

There is just one problem left now: poor acceleration on part throttle.
When I push the pedal between 1/4 and 3/4 the engine seems to run rough, acceleration is much slower than when I press pedal 1/4. If I just keep pedal pressed at the same position car slowly accelerates for a few seconds and then returns back to normal giving me the acceleration I expect. This happens only on low speeds (<30 MPH).
When I push pedal less than 1/4 everything is good and car accelerates normally.
When I push pedal past 3/4 everything is also good, secondaries open and car accelerates making a really pleasant sound.

What could cause such problem? Could the TPS voltage be too high? (specs say 0.41 volts for idle, but people everywhere on the internet recommend setting it to higher value)
Is this a symptom of lean or of rich mixture?

Also engine became hard to start, I have to crunk it for a few seconds, then it idles roughly for a few seconds with check engine light on, but then stabilizes. I am not sure if this is normal or not.

How can I read and interpret exhaust oxygen sensor signal?

 

A2k1
I have a 81 CCC with the E4ME carb. I went on eBay and bought the "OTC Monitor 85" scan tool which has the 5 and 12 ALDL adapters and is specific for our CCC vettes. You can use it to read real time output of the CCC components, the O2 sensor being one of them.

 

These tools:
http://www.montecarloss.com/communit...&Number=890550
A dwell meter is also needed. Set the upper and lower mixture control stops, and then adjust the idle air bleed with the gauge for that.
The GM service manual is really kind of a necessary tool for that carb too.

 

The large screw that is labeled MCS is NOT the MCS, it is the idle air bleed adjustment screw. The one that is circled with the ???....that is the MCS adjustment. Do you have the plug that covers the MCS?

The MCS travel is to be 2-4/32". If the carb is not taken apart, then a scale is inserted into the "D" vent hole and the solenoid depressed with the travel distance measured by this scale. If the carb is taken apart, then further refinements can be made with respect to the primary metering rods, i.e. fuel mixture can be made more lean for the solenoid travel, or richer over the solenoid travel distance.

The idle air bleed screw is adjusted with respect to the MCS full rich position and is adjusted with the engine running and a dwell meter on the test lead, reading the 6 cylinder scale...yes 6 cylinder scale is correct. The dwell is to vary from 10 to 50 degrees with a midpoint of 25. There should also be a cover the idle air bleed screw.

The idle mixture needles should start out at 3 1/3 turns from full seat. If dwell is not varying or average value is below 25, turn both mixture needles OUT one turn. If dwell is above 35, then turn both needles IN one turn.

Then readjust idle air bleed screw to obtain a varying dwell with an average of 25.


First time making these adjustments on the E4ME is difficult, but becomes quite easy. There are only three adjustments that can be done on the MCS.

Good luck.

 

I have the OTC Monitor 85 and it is fantastic. It tells me just about everything I need to know. Should have bought one when I got my 81 6 years ago.

 

Thanks guys!

I have managed to adjust it correctly.

I have probably put too much effort into this... but... I have made a circuit and microcontroller program to read sensors data and control the carburetor. After setting carburetor control signal duty cycle to 50% I simply adjusted fuel screws till I got exactly 0.45 volts from exhaust oxygen sensor. This solved my problem completely.