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1986 GM truck with Quadrajet computer carb, Customer wants a cam that has a nice little performance idle to it. But still wants it to pass smog. Any suggestions? I'm going to research some cam companies and see if anything off the shelf exists. Thank you, Ken
 

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Put in a 220 @ .050" cam or similar in it and add around 20% E85 or isopropyl alcohol (91% by volume) to the tank come smog time. It will run like sh!t, but it will help it pass the HC and NOx portions of the test. Just be sure to drain the tank and flush the lines immediately after smog.


Cole
 

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Put in a 220 @ .050" cam or similar in it and add around 20% E85 or isopropyl alcohol (91% by volume) to the tank come smog time. It will run like sh!t, but it will help it pass the HC and NOx portions of the test. Just be sure to drain the tank and flush the lines immediately after smog.


Cole
That's what I wanted, the .050 number, Comp has a cam with that duration. Thank You
 

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That's what I wanted, the .050 number, Comp has a cam with that duration. Thank You
I'm not necessarily saying to use that duration only. There are a number of things that will effect idle quality. Two cams with 220 @ .050", one has 108 lobe separation and the other at 114 lobe separation will idle very differently. I just gave the 220 as a reference point.

Also, by just putting a cam in it to make it "sound cool" is not going to cut it. You will also need to make tuning adjustments and be prepared to spend some time with it. I can't say that I'm a fan of the Quadrajet, but I'm sure there are a few people that can tune it to suit the changes.
 

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Discussion Starter #5
I'm not necessarily saying to use that duration only. There are a number of things that will effect idle quality. Two cams with 220 @ .050", one has 108 lobe separation and the other at 114 lobe separation will idle very differently. I just gave the 220 as a reference point.

Also, by just putting a cam in it to make it "sound cool" is not going to cut it. You will also need to make tuning adjustments and be prepared to spend some time with it. I can't say that I'm a fan of the Quadrajet, but I'm sure there are a few people that can tune it to suit the changes.

Agree, The comp cam is 114 and is designed for computer controlled, just wanted another opinion on duration. I think the 220 will give him the idle he wants without changing the converter or gears.
 

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Agree, The comp cam is 114 and is designed for computer controlled, just wanted another opinion on duration. I think the 220 will give him the idle he wants without changing the converter or gears.
with a 114 sep, i bet it still sounds stock
 

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Discussion Starter #8
This has to pass smog, any recommendations on lobe center and duration and .050. Doesn't have to sound like a top full car.
 

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220 @ .050 on 114 LSA will be barely noticeable idle.
I had the GM Hot Cam Kit in my '94 LT1 and it's 218/228 @ .050" with a 112 LSA, and it sounded pretty good and passed smog. This was a hydraulic roller though.

I'm pretty sure the '86 truck 350 is going to be a flat tappet, so the 220/220 should have similar or slightly less overlap to the hyd. roller, which will affect the idle quality/sound.


Cole
 

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I'd cam it to make good power, and pass smog, then mess with the idle mixture on the carb, to get it to lope.
I've never messed with that carb, but I'm sure someone knows how to make it produce a rough idle.
 

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Because that QJet is electronically controlled by the ECU system with feedback from the one-wire narrow band O2 you can use an Innovate MTX-L wide band O2 gauge and connect the analog output channels to the ECU’s NBO2 input and have laptop tuning ability to adjust the A/F to whatever you want within the range of the carb calibration.

You set the MTX-L to output the same voltage oscillations of the original NBO2 sensor but at whatever A/F you want between 7/1 and 22/1. This will allow you to set a lean cruise mixture, 17 or 18/1, or however lean the engine will run with acceptable performance. You adjust the AFR by programming the MTX-L with your laptop. This way you can control the carb and easily adjust it to whatever provides DEQ compliance.

The MTX-L has two output channels so you can set each different, one to pass DEQ and one to get mileage, or one to give a richer idle for the cam and one to cruise lean. Switch between the two with a Bosch cube type relay or a toggle switch. I’ve done TBI’s and feedback QJets this way and it works slicker than sliced bread.

Because the QJet has the double booster primary main nozzle they vaporize fuel much better than a single booster. The result is improved part-throttle distribution and engines will run smooth with much leaner A/F than is possible with other carbs. In comparison to a Holley, the QJet can make the same engine run well with 18/1 in part-throttle acceleration when the Holley may begin to surge at 16/1. QJets are awesome. Too bad Holley spent so much money on negative advertising instead of improving their own product.

A pal has a 350 TBI engine in a Toyota 4WD truck with 34” tires, two spares, winch, big dumbass light bar, tool box with chainsaw winch, handyman jacks, etc, etc, he’s ready for the apocalypse and zombie attacks. Anyhow, this trick with the MTX-L bumped his mileage from 12 to 17 towing his aluminum jet sled (boat).

The situation with those early systems using NBO2 feedback is the A/F is oscillating rich/lean/rich/lean such that half the engine combustion cycles are richer than stoichiometric and waste fuel.

Any of you who have scanned the O2 sensor voltage know it oscillates from near 1V to less than .1V. Near 1V it is 13/1 or richer and near .1V it is 17/1 or leaner, the average is supposed to be stoichiometric 14.7/1 (1λ).

With the MTX-L trick you set the output so 16.8 is 1V and 17.2 is .1V and the ECU will average it so the engine runs at 17/1. Because the feedback voltage is oscillating in the normal range the ECU is happy and does not set a malfunction code.
 

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I do not think the truck has a feedback carb. It has wires going to it like a feedback carb but it is a cold/hot accelerater bypass system. Key on engine off if you do not hear a clicking it is not feedback. I have never in Oklahoma seen a truck stock with a feedback carb. Also look at the front if there are not three wires coming out directly under the accelarater pump arm it is not a feedback carb. Rod
 
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