Q-Jet Tuning

[mudbog42]

I am going to be getting a new Quadrajet for my truck and want to try and adjust it myself because the shops around here just charge way to much to turn a couple adjustment screws. Is there a good way to tell if it is running too rich or lean and when it is just right? Also its an electronic Q-jet

 

[Hogback Fabrication]

I don't know very much about adjusting electronic Q-jets other than part of their tuning is an adjustable part throttle screw.

The way to tune cruise speed: make sure the engine is warm and drive it at a consistent speed (highway) for at least a few minutes. Then stop and pull a couple plugs (do not accelerate, just come to a stop after the highway run). The plugs should be very light tan. White is too lean, brown is too rich.

If the carb seems to bog on acceleration you need to change the spring under the power piston.

If you can tell us a little about what the truck drives like now we may be able to help you with more specifics. Putting a cheap O2 sensor into your exhaust and putting a voltmeter on it can tell you alot about how the engine is running. The O2 sensor will put out between 0 and 1 volt based on how rich/lean the engine is running. More towards 0 mV is lean, more towards 1000 mV is rich. The ideal ratio of 14.7:1 will show 450 mV. Shoot for 400 mV at cruise, 5-600 mV under acceleration and 7-800 at WOT.

 

[dyeager535]

This is a former CA truck then? (True CCC, not one of the non-CCC carbs with electrical plug?)

Not much you can do to tune the thing other than idle. Mix screws are only for the idle, other than that part throttle is all computer (mix control solenoid) and acceleration is old school secondaries, full rich on the primaries.

There has been a fair amount of research on the CCC carbs, if you REALLY want to know about primary jets and what not, there is a guy that has a spreadsheet up that shows the differences.

 

[mudbog42]

Right I believe all I need to adjust is the mixture on idle and the idle speed, thats all I ever see the shops do but they have it hooked up to a computer to read what the carbs putting out. And those readers are expensive for OBD1 It is a california truck too and a true CCC. Where would I hook the voltmeter at on the 02 sensor to get a good reading and are voltmeters expensive

 

[dyeager535]

I wouldn't waste time trying to voltmeter the O2 sensor, but I've got a scanner or winALDL I could hook up and watch it.

Autozone apparently has scanners you can use for free, or so I've heard. O2 sensor is pretty pointless to determine carb idle mix, it's not accurate at ANYTHING other than 14.7:1, and I don't see idle being stable enough to tune that way anyways.

 

[mudbog42]

Autozone only has scanners for OBD2, they have nothing for OBD1 at least thats what they told me

 

[Hogback Fabrication]

I wouldn't waste time trying to voltmeter the O2 sensor, but I've got a scanner or winALDL I could hook up and watch it.

Autozone apparently has scanners you can use for free, or so I've heard. O2 sensor is pretty pointless to determine carb idle mix, it's not accurate at ANYTHING other than 14.7:1, and I don't see idle being stable enough to tune that way anyways.

I didn't realize you could only adjust the idle on the carb we are talking about. Using an O2 sensor is not perfect but will give you a good idea of the mixture at various throttle positions. I have tuned a few carbs this way and it works really well for everything except WOT (the voltmeter doesn't update fast enough to get an real reading). Since you can't really tune this carb I obviously would not bother putting an O2 sensor in just to tune the idle.

 

[mudbog42]

What about those air/fuel ratio monitors? Has anyone used of those, they plug directly into where your o2 sensor goes and edelbrock has one for $129

 

[Hogback Fabrication]

What about those air/fuel ratio monitors? Has anyone used of those, they plug directly into where your o2 sensor goes and edelbrock has one for $129

They do exactly what I was describing except instead of your voltmeter displaying millivolts the gauge would display rich/lean. I would save yourself the money and use a voltmeter instead, just my .02

 

[dyeager535]

You might as well figure out if you can datalog your system if you've already got a laptop, I think starting around 1985 (at least in cars) "standard" datalogging software (maybe even winALDL) will log the info, including the O2 sensor.

I really don't know how those carbs do idle...yes, they have the same idle mix screws as any other Q-jet, BUT the mix control solenoid operates at the same time, which leads me to suspect that MAYBE changes are made via the O2 sensor even at idle with the MCS, which means watching the O2 is pointless, except to see if it switches lean/rich.