Quadrajet Air Fuel Ratio

[Old60Driver]

So, I just recently replaced a few parts on my '83 K20. I put in:
Summit 1103 Cam
Summit Vortec Intake (still running my Qjet with an adapter plate, about 1" thick) Thoughts on this??
Reworked 906 heads

She runs pretty well, but I feel like there's a bit left on the table. After break in, I adjusted idle AFR to best vacuum. I installed an AEM Wideband sensor to check myself and monitor under WOT. I'm getting an average of 12.3 at idle, so I need to lean that a bit, but I'm also going lean under heavy load. Like 15.5 or so? She runs well, but I'm now concerned about going lean when I feed her the onions. It was only a quick drive in the neighborhood to see if things were working, so I don't have cruise info, or 'true' WOT numbers.

Am I over thinking this, or do I need to tinker with the Q-jet to dial in my WOT numbers?

Thanks y'all!

 

[fast 99]

If the carb is an 83 there is a small plug on top that will allow access to metering rod adjustment screw. I usually take those out when rebuilding then reinstall after carb is set up. If carb is together using a small [1/8th inch] drill into it far enough to get a sheet metal screw started them pry it out with a side cutter. DO NOT drill all the way through it. You will need a double D tool or make one out of a 3/16th brake line, crush it somewhat in a vice turns into a double D. Can then raise and lower rods to get the mixture you want. This only adjusts the primary venturis.

Idle mixture screws should change that 12.3 to what you desire.

 

[Old60Driver]

That's a slick idea. I've got a double D tool, so I'll tackle that next!

 

[Ricko1966]

May need to change secondary metering rods as well

 

[fast 99]

May need to change secondary metering rods as well

Yes, [alternate option] if lean a thin piece wire under the rod holder to raise the rods. Be sure the secondary spring is wound up the correct amount. Probably in the area or 3/4 - 7/8 turn on adjuster. Should be on the spec sheet that came in the kit.

 

[Geoduck]

Those vortec 906 heads the intake and SUM-1103 cam is a smart move. Great all around HP improvement using reliable components for not a lot of money.
The one inch riser is something you will have to play around with. It would be my guess with a medium cam it could help?
Once it's dialed in please let us know your thoughts?

 

[Bextreme04]

If the carb is an 83 there is a small plug on top that will allow access to metering rod adjustment screw. I usually take those out when rebuilding then reinstall after carb is set up. If carb is together using a small [1/8th inch] drill into it far enough to get a sheet metal screw started them pry it out with a side cutter. DO NOT drill all the way through it. You will need a double D tool or make one out of a 3/16th brake line, crush it somewhat in a vice turns into a double D. Can then raise and lower rods to get the mixture you want. This only adjusts the primary venturis.

Idle mixture screws should change that 12.3 to what you desire.

No... absolutely not. The APT adjustment is to adjust how much the metering rods come out under partial throttle cruise conditions. It will do absolutely nothing to adjust idle AFR.

So, I just recently replaced a few parts on my '83 K20. I put in:
Summit 1103 Cam
Summit Vortec Intake (still running my Qjet with an adapter plate, about 1" thick) Thoughts on this??
Reworked 906 heads

She runs pretty well, but I feel like there's a bit left on the table. After break in, I adjusted idle AFR to best vacuum. I installed an AEM Wideband sensor to check myself and monitor under WOT. I'm getting an average of 12.3 at idle, so I need to lean that a bit, but I'm also going lean under heavy load. Like 15.5 or so? She runs well, but I'm now concerned about going lean when I feed her the onions. It was only a quick drive in the neighborhood to see if things were working, so I don't have cruise info, or 'true' WOT numbers.

Am I over thinking this, or do I need to tinker with the Q-jet to dial in my WOT numbers?

Thanks y'all!

You don't want to adjust a quadrajet to vacuum. You should adjust it using the lean drop method. Basically you adjust the idle mixture screws and idle set screw until you get peak RPM and then set that RPM to what you want. With an 1003 cam it will probably need to be higher than usual.. probably 750-800RPM or so. Then you lean out the idle by screwing in the idle mixture screws evenly, 1/4 turn at a time until the idle has dropped 50rpm. That will give you a slightly lean of peak idle.

For a quadrajet going that lean at WOT, you will likely need to reset the secondary hangar height and then make adjustments. Are you sure that the choke is coming all the way off and the secondaries are even opening?

Since you have an AFR gauge, I would start with a good base setting of 41/64" from the top of the airhorn to the hole on the secondary hanger when you have the secondaries held all the way open.

 

[Matt69olds]

Give the engine what it wants, not what you think it needs. You will probably find the ideal cruise AFR is far leaner than you ever thought possible, or would be willing to go on your own, and the engine will be perfectly happy. I’m

Set the idle mixture using the lean drop method. The engine needs To be fully warmed up (like after a 20 minute drive) to be accurate.

The APT is best set on an average weather day. Modify the carb to allow easy access to the adjustment screw. Take the truck on a drive with a long smooth stretch of road. Keep leaning out the APT screw until you get a lean miss. A lean miss feels kinda like a fish nibbling at the bait, but hasn’t latched on yet. Once you have found the lean miss point, richen the mixture just enough to eliminate the miss, and go just a tiny bit further.

Once all that is done, THEN it’s time to play around with the secondary metering rods. Cliffs Q-Jet has a selection of rods, the hangers are kinda hard to come by. I acquired my assortment by visiting a lot of old junkyards!!! Cliff might have a few. The hangers are best thought of as fine tuning, the rods will have a much bigger influence than the hangers.

Once all that is set, then you can play around with the secondary air door opening rate.

It’s time consuming, but worth it as far as performance and economy. You can have both.

 

[AuroraGirl]

to be clear, does OP have the Original Qjet?

 

[fast 99]

No... absolutely not. The APT adjustment is to adjust how much the metering rods come out under partial throttle cruise conditions. It will do absolutely nothing to adjust idle AFR.

Never said they adjust idle mixture.

 

[Old60Driver]

Hi there y'all! I got called away for work, and am just now getting some down time.

Quick update:
I found a vacuum leak. It was the modulator on the tranny. Makes sense, as the tranny seemed to be holding gears longer than normal. Anyway, once I got that taken care of, I went out for another evaluation. The AFR is much better now. I'm still fat, but I'm fat everywhere, for the most part. At cruise, I'm about 14.2 plus or minus. I do get a momentary blip of lean (15.1) when I nail it.

Thoughts?

Also, to answer Aurora Girl, it's the original Q-jet, I rebuilt with a Ruggles kit about 2000 miles or so ago?

 

[DoubleDingo]

Hi there y'all! I got called away for work, and am just now getting some down time.

Quick update:
I found a vacuum leak. It was the modulator on the tranny. Makes sense, as the tranny seemed to be holding gears longer than normal. Anyway, once I got that taken care of, I went out for another evaluation. The AFR is much better now. I'm still fat, but I'm fat everywhere, for the most part. At cruise, I'm about 14.2 plus or minus. I do get a momentary blip of lean (15.1) when I nail it.

Thoughts?

Also, to answer Aurora Girl, it's the original Q-jet, I rebuilt with a Ruggles kit about 2000 miles or so ago?

You need to do more sit ups, russian twists, dead lifts, etc if you're fat everywhere

 

[DoubleDingo]

I'd say you're good on the carb, though

 

[Old60Driver]

You need to do more sit ups, russian twists, dead lifts, etc if you're fat everywhere

Haha! Needed that today. Thank you!