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Old 09-24-2006, 05:01 PM
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adjusting the secondary air valve? (Q-Jet)

Adjusting the spring tension is the only thing I can't figure out. How do you adjust the secondary air valve? Will the carb. need to be removed? Would not enough tension cause a bog?

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Old 09-24-2006, 06:00 PM
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Yes, not enough tension lets the doors slam open causing a lean condition. The adjustment can be done on the car. Look at the passenger side of the carb. Notice the shaft for the secondary doors disappears into a little triangular tab in the air horn casting. Inside that tower where the shaft turns is a spring the provides resistance. On the outside of that tower is a set screw. Loosen it. Then (using an allen wrench I think) tighten the adjustment screw which is up underneath that tower. Go in half-turn increments until the bog disapears and then re-tighten the set screw.
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Old 09-24-2006, 09:53 PM
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The flat-head screw visible from the right side is the adjusting screw. Underneath, not visible without a small mirror, is an allen-head (3/32", I believe) set screw. Be sure you have control of the flat-head screw BEFORE you loosen the set screw. Otherwise, it will loosen by itself, and you won't know where you started.

If the air valve is set too tight, it can act like a choke plate and cause a rich condition, with black smoke coming out of the tail pipe. Too loose and, as stated above, you'll get a lean bog.

If you don't have a mirror, press the tip of your finger upward real hard underneath the triangular projection, and then look for the impression of the set screw in your skin.
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Old 09-25-2006, 05:22 PM
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Thank you. I'll have time to work out the bog this weekend.
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Old 09-25-2006, 07:43 PM
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Here is a pic of the secondary butterfly layout. Never mind about the "grind this tab, that was there for something else.

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Old 09-26-2006, 08:20 AM
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I think half-turn adjustment increments are too much at one time. You aren't supposed to tighten it more than 1.25 turns at most from "loose", or no tension.

Most articles I've seen say try 1/8 turn increments. Mine was originally at 7/8 turn from no tension and bogged badly when I went to only 1/4 turn. I ended up at 5/8 turn from no tension.

John
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Old 09-26-2006, 08:56 AM
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John, you are correct. Half turns is way too much! My experience and info I've gathered in all my reading ends up being between 5/8 turn to 7/8 turn from "no tension" for proper operation.

If you still can't get the bog out, its possible that the choke pull-off is releasing too SOON, remember it acts as a "break", so to speak, so the top butterflies don't whip open too soon. Street engines just don't need ANY carb to be fully open on all 4 barrels at 800rpm.

This is by far THE best book on the Qjet. Well worth the money.

Qjets by Doug Roe
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Old 09-26-2006, 11:28 AM
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A excellent new book on the QJet came out this summer, "How to Rebuild and Modify Rochester Quadrajet Carburetors", by Cliff Ruggles, $24.94 list, $16.41 at Amazon.

I have seen it at Barnes and Nobles, but bought mine through Amazon. Shows with many color pictures and a well organized approach how to put to practice the theory in the Doug Roe book. Also, contains specific recipes for setting up a QJet for 3 typical engines (high vacuum mild 350, mid vacuum more agressive engine, low vacuum radical engine).

John
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Old 09-27-2006, 04:51 PM
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Quote:
Originally Posted by Jmark
John, you are correct. Half turns is way too much! My experience and info I've gathered in all my reading ends up being between 5/8 turn to 7/8 turn from "no tension" for proper operation.

If you still can't get the bog out, its possible that the choke pull-off is releasing too SOON, remember it acts as a "break", so to speak, so the top butterflies don't whip open too soon. Street engines just don't need ANY carb to be fully open on all 4 barrels at 800rpm.

This is by far THE best book on the Qjet. Well worth the money.

Qjets by Doug Roe
How can I check if the choke pull off is releasing too soon?
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Old 09-27-2006, 05:35 PM
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The choke pull off has a linkage coming out of the back of it. With the engine off, push the linkage fully in (when the engine is running, engine vacuum pulls this linkage in which holds the secondary air valve closed until a low vacuum condition occurs) then release it and see how long it takes to come back out. Two seconds is typical.

You can also check that it holds vacuum by pulling the vacuum hose off the front, push in the linkage, put a finger over the front hole where the vacuum hose was hooked up, and then release the linkage. If the pull off is good, the linkage should not move out as long as you have your finger over the hole in the front. If the linkage comes out, the pull off is not holding vacuum and needs to be replaced.

John
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