Q-Jet Problem - Team Camaro Tech
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post #1 of 8 (permalink) Old Apr 19th, 06, 12:53 PM Thread Starter
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Mark
 
Join Date: Feb 2005
Location: Georgetown, TX
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Q-Jet Problem

Iím having a carburetor problem with my 69 RS that I hope someone can help me with. My car has a nearly stock 350 engine with an Edelbrock dual-plain spread-bore aluminum intake, a 350 THM and 3.08 gears. Originality is important to me, so Iíve been gradually replacing aftermarket engine components with original equipment where possible, or in some cases with items that look original.

I recently purchased a slightly used Edelbrock 1904 Q-jet, rebuilt it, and installed it on my car along with a new stock 3/8 inch fuel line between the fuel pump and the carburetor. I used to rebuild lots of Rochester Quadrajets years ago, so Iím pretty familiar with them. The float is set correctly, Iíve verified the jets and metering rods are correct, and the fuel filter is new.

The car starts, idles, accelerates and cruises fine; and the spark plug color looks normal. The problem occurs at WOT when the secondaries open. As the secondaries open, the car begins to act like itís running out of fuel (lean), and doesnít develop power like it should.

The stock mechanical fuel pump is nearly new, and the 600 cfm carburetor I removed from the car worked fine, so Iím inclined to rule out fuel delivery as a possible cause. I removed the check valve from the in-line fuel filter, which seemed to help a little. Iím thinking about installing some smaller secondary metering rods in order to fatten up the mixture a tad at WOT, however I would think this carburetor, set at the factory settings, would work just fine on a nearly stock 350 engine. Iím concerned that I may have overlooked something, and could be treating a symptom instead of the disease.

Iíd appreciate any suggestions you may have to help me diagnose this problem. I donít want my neighborís kid to outrun me with his Neon!!

Thanks as always!!
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post #2 of 8 (permalink) Old Apr 19th, 06, 01:17 PM
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Steven
 
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Re: Q-Jet Problem

Jump over to this sight...You will find a LOT of good q-jet info

http://www.florida4x4.com/tech/quadrajet/index.php
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post #3 of 8 (permalink) Old Apr 19th, 06, 01:27 PM
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Re: Q-Jet Problem

I would take a serious look at the Power Piston and Spring assembly.
Make sure it's not sticking in bore.
Check the spring type and make sure it's 'cut in point' (given in In-Hg in catalog numbers) is matched to the vacuum available depending on your cam and set-up.

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post #4 of 8 (permalink) Old Apr 19th, 06, 01:31 PM
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Re: Q-Jet Problem

Sounds like the secondary air valve needs a tweak.

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post #5 of 8 (permalink) Old Apr 19th, 06, 08:59 PM
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Wes
 
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Re: Q-Jet Problem

Agree with ZZ430 above,..you'll need to tune the secondary air valve (big butterflies on top the secondaries) opening rate to your engine. If they open to fast, she'll bog. Too slow and you'll get weak performance. Just right will put a smile on your face.

On the passenger's side of the shaft for the air valve, you'll see where it's adjusted with a small allen head wrech (1/8"?) and a small screw-driver. The adjustment screw is locked into place with with the allen head screw. Don't let go of the screw while it's 'unlocked' or the spring will unwind. Adjust the screw a 1/4 a turn at time,..then test.

Good luck!
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post #6 of 8 (permalink) Old Apr 20th, 06, 08:49 AM Thread Starter
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Mark
 
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Re: Q-Jet Problem

Thanks guys!! I'll tinker with the secondary air valve this weekend and post the results. I bet that'll correct it...
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post #7 of 8 (permalink) Old Apr 20th, 06, 06:18 PM
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Rob.
 
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Re: Q-Jet Problem

Are you sure it's the carb, if you're using a dwell meter, you said it was original, so you should have points in the distributor, if thats the case, forget about the dwell meter and set the points close to right then run it and readjust, use new points.

One more thing, see if you have any pressure at your gas cap after you run it.

You could also have some electrical problems like plug wires or boots, find a dark area, let it idle and look real close for any tiny flashes around the wires and cap, don't reach your hand in there in the dark, kind of a dangerous situation.

I use to have a chevelle in 1970 that would do what you are saying your car does, I put new points set the gap right on the money, checked it with a dwell meter, everything set perfect but didn't have it all at wot. Adjusted the points a couple of times, and that brought her to life.

Just throwing a few things at you.

Good luck

Rob

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post #8 of 8 (permalink) Old Apr 22nd, 06, 10:43 AM Thread Starter
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Mark
 
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Smile Re: Q-Jet Problem

Well, I had a chance to adjust the secondary air valve today.

I first tried adjusting the air valve based on the instructions in the rebuild kit. That helped a little, but not much.

I then backed off the spring tension about 1/4 turn. Oh baby!! Runs darn near perfect now!! I'll probably try backing off another 1/8 turn or so, but it's clear the air valve adjustment was the problem. Now its just a matter of zeroing in on the ideal setting.

Thanks everyone for your suggestions, and the link to the q-jet site. Lots of good info there!!
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