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Old 07-13-2005, 05:11 PM
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Q-jet experiences - Tuning suggestions?

Hi Folks,
I just installed a 750/800 cfm Q-jet 4MV on my '72 442. It was originally from a '73 Chevy 350 truck application, and was in decent shape. I completely disassembled it, and gave it a proper rebuild with new gaskets, needle and seat, float, etc.

Anyways, it works AWESOME!!!!! It replaced a 600 cfm Edelbrock, what a POS. Low-end throttle response is superb and when those secondaries kick in, WOW. The idle screws are set at about ~3 turns out on both sides, and it has the stock metering rods all around. Accelerator pump is set on outer hole for largest shot. It idles about 850 rpm on my mild 455.

The motor will finally rev above 4500 rpm now. I can also pull 3rd gear rubber on a WOT throttle shift.

My big question is tuning for max performance. The metering systems are rather a bit more complex on this carb that your average holley and I would appreciate some tips. Tell me some of your own experiences -

Cheers,
Andy

PS I am sure I will hear lots of "quadrajunk" stories as well.....

Edit - Forgive any stupid questions as this is my first carb rebuild. My brother said he thought I would never get it back together!

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Old 07-13-2005, 06:41 PM
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This article in the mag had some good q-jet info. I couldn't find it in the online version Maybe you know someone with a subscription because it was only a few months ago.
http://www.carcraft.com/techarticles...une/index.html
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Old 07-13-2005, 07:50 PM
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A lot of people slam the qjet but truth be told, it can be a great carb. Sounds like you did good on re-doing it.

Personally, I'd leave the primary rods and jets alone if it cruises smooth. The secondaries can be tuned for WOT performance. The secondary jets are actually a fixed hole, but the rods are easily replaceable from the top of the carb.

The "full open" angle of the top secondary butterfly can be re-set too with some easy filing on the limiting tab. GM limited the opening angle according to each engine and combo. Its not uncommon to find one limited to barely 50 degrees of opening on a low hp engine. Optimal opening is 85 degrees.

Also, the tension on that butterfly is important too. If you don't have any bog when you nail it, leave the tension adjustment alone.

Mark
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Old 07-13-2005, 08:01 PM
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you have the mix screws out 3 turns??? i've never really messed with a Q-jet before but i know on holleys i usually keep the screws 1 1/8th - 1 1/2 turns out. are Q-jets different in that respect?
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Old 07-13-2005, 08:07 PM
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Depending on idle vacuum, the mix screws can run from 1 1/2 to 5 turns out to get a good idle mixture. The later model qjets can be a tad harder to tune on a strong engine due to the even leaner primary and idle circuits but still a very good carb and are still extremely affordable and easily worked on.
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Old 07-13-2005, 08:56 PM
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I am still a little perplexed on the idle mixture issue. It seems to idle quite well around 3 turns out, the 455 BBO has been been cammed a bit, has some head/intake/header mods and so probably doesn't have the full manifold vacuum of a stock one.

For the secondaries, they had no tension when I first got the carb. So, I set them to open at light-moderate pressure, using the ever accurate "finger push" gauge They seem to come in a little late so I will loosen the tension very slightly till I get quick opening but no bog.

I don't have enough open road to tune the secondary metering. With this current setup it hits 100 mph at the end of an on ramp and I don't feel like pulling over there to get a plug reading.

This is a 4MV so it is devoid of the later carb's emissions junk. I don't even have a choke on it now!

I was initially concerned that I would have to bush the primary throttle shafts to get good idle quality, but for all of you unbelievers out there even a Ford man can get a GM product to work! Another thing that I believe was important was getting a "virgin" carb that hadn't been hacked or "creatively" tuned.

Cheers,
Andy
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