As the title says, does anyone here really know how to setup a Q-jet? I just bought a brand new Edelbrock Performer Q-jet and it runs pretty good right out of the box, but I am looking for more. It has a flat spot at the top end that I need to get rid of. So if anyone knows how to tune a Q-jet and doesn't mind helping me out I would be very appreciative. Thanks

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Mat Ellison
Lincoln, Ne
70 SS 396 Chevelle
62 Impala Sport Coupe
2001 Grand Prix GT
Aces #2424
http://www.geocities.com/elliboom/Mats_70_Chevelle.html?994781720927

Before you do anything, make sure your ignition timing and the condition of the ignition system itself is squared away. Make sure the chhoke is functioning properly, and make sure the idle mixture is set correctly.

The flat spot at the top end could be the result of a secondary air valve spring that's a little too tight. The factory probably sets them a little tight so that the car doesn't bog out on the low end. Look at the secondary air valve and look off the passenger's side of the carb. You should see what looks like a small screw on the side of the carb in that area. Directly beneath it, viewed from the bottom up, is a small Allen head screw. That's the secondary air valve adjustment screw, and the Allen head screw is the lock screw. While HOLDING the flat blade screw with a small screwdriver, loosen the Allen screw. There is spring pressure on the screw. Allow it to back off about a 1/4 turn and tighten the Allen screw just enough to lock the spring adjustment screw. Take it out and drive it. If it starts bogging on the low end, you'll have to tighten the screw back towards your original starting point. If it doesn't bog and the problem persists, loosen it a bit more. If the top end power feels beter and it's not bogging off the line, then you have it adjusted properly.

[This message has been edited by Randy Mosier (edited 08-19-2002).]

Well Randy, I guess your answer would be [i]yes[i] then. http://www.chevelles.com/forum/wink.gif

Great job, I'm thinking of getting myself one of those and this info really helps http://www.chevelles.com/forum/smile.gif

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Tommy B.
71-72 Malibu (http://www.geocities.com/beldarr/Chevelle.htm)
"I'm high all right, but on the real stuff
High octane gasoline
A clean windshield
And a shoe shine"

And as Randy indicates, 1/4 turn is a pretty BIG adjustment on that thing. If that doesnt help you want to make sure your fuel system is OK at the higher rpms. QJ's dont have huge fuel bowl capacity, so your system has to run well up there. Lastly, you might have to get a smaller secondary rod. ( just guessing that your too rich, it might be lean as well, that would mean a bigger rod ). The secondary rod governs the amount of fuel delivered to the secondary curcuits.

It's an absolute must to have at least 3/8" fuel line from the tank to fuel pump with a quadrajet application.
I tried a quadrajet on my '64 El Camino which has 5/16" line from tank to pump. Due to the small fuel bowl on the quadrajet, it would run out of fuel before I could get it wound up tight in 1st gear.
Not wanting to change the fuel line from tank to pump, I stuck a Holley on and the problem disappeared.

Sounds like this thread could be a "FAQ" on how to tune your QJ.

I honestly don't know anything about them, but put one on my Chevelle.

How about a mixture adjustment setup?

I do have the 3/8 fuel line. I had the forsight to know I would need lots of fuel when I was putting in my lines. Stainless, too!

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Mark
Member #605
Better to BURN OUT, then to fade away

I will try the secondary air valve adjustment tonight. Every part on the car is new. The entire ignition system is a new HEI unit, the fuel lines, tank and pump are all new. I have a 3/8" fuel line, and the rubber part of the line is not pinched down. I was wondering about either float level or a timing issue. Where is a good place to set the timing? The sticker and an old Motors book say either 0 or 2 degrees, but it does not like to start there, plus I have made some mods to the engine. Another thought I had was mechanical advance, is there a way to check the advance without putting the distributor on a dyno?

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Mat Ellison
Lincoln, Ne
70 SS 396 Chevelle
62 Impala Sport Coupe
2001 Grand Prix GT
Aces #2424
http://www.geocities.com/elliboom/Mats_70_Chevelle.html?994781720927

And Gene brought up an important point. Check the fuel pump pressure! 4 - 7 is acceptable, but if it's long of tooth, you may want to install a high volume pump. Be careful with that Allen screw. They can be broken off fairly easily. Ask me how I know! http://www.chevelles.com/forum/mad.gif

[This message has been edited by Randy Mosier (edited 08-20-2002).]

Do I need to check the fuel pressure while I am driving to see if the pump can keep up, or is sitting in the garage enough? I would think I would need to be driving the car. Let me know so I can build up a test gauge at work tomorrow. As far as fuel pumps go, what is everyone running. The engine is a .030 over 402 with no really big mods except for a slightly bigger cam.

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Mat Ellison
Lincoln, Ne
70 SS 396 Chevelle
62 Impala Sport Coupe
2001 Grand Prix GT
Aces #2424
http://www.geocities.com/elliboom/Mats_70_Chevelle.html?994781720927

I don't know about Edelbrock Qjets, but most factory ones have a diaphram connected to the secondary air valve that slows the opening of it. Sometimes you can back the adjustment screw off all the way and it still opens too slow. I just remove the linkage rod from the diaphram to the air valve. That way the opening rate is controlled entirely by the resistance spring, whose adjustment was previously described. It becomes very sensitive to the spring adjustment this way though. There is a way to drill the restrictor opening in the diaphram housing but I've never tried it.

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von '69 300 Dlx SS TC #15 ACES #1575
My '69 SS (http://mywebpage.netscape.com/jerryacheson/vons69.html)

Elli,
Go to a local book store like Borders or Barnes and Noble and buy the book "Rochester Carburetors" by Doug Roe. It has very good step by step tuning instructions for the Q-jet in all performance stages...from stock to full race. Also, like Randy said...make sure that the problem really is carb related. it could be ignition. The book I mentioned is a "must have" for anyone with a Q-jet carb. It is also available at amazon.com

Hope this helps,

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

About the secondary rods: smaller for richer, larger for leaner. Watch out and make sure you have the proper length rods for you carb as there are two lengths which look very similar.