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Old 10-15-2018, 04:12 PM   #1
demian5
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Quadrajet Question

Hi -

Im pretty familiar with rebuilding Q-Jets, but I have ran into a couple that have me stumped. I have rebuilt two in a row and both act like they have a vacuum leak afterwards. The mixture screws do nothing. Pulled them, apart, everything looks great! No clogged passages. And of course the shaft bushings leak a tiny bit but not the amount that im expereincing.

Timing is set to 10 BTDC. No Vacuum leaks on the truck verified by disconnecting all vacuum coming off the intake and carb and plugging off (including booster). Intake manifold gaskets are recent and not leaking.

AND I put on a brand new demon carb, and no problems. But both of these Q-Jets, same thing.

Anyone run into this? Maybe I missed something inside?
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Old 10-15-2018, 04:23 PM   #2
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Re: Quadrajet Question

Its actually an original 1973 carb on a 1973 blazer BTW...

This is pretty good. Found through the search feature:

http://67-72chevytrucks.com/vboard/s...d.php?t=483753

Needle Retaining Clip Incorrectly Installed to Float Arm
Results in: Flooding, poor idle, poor hot-start, fuel puddling in bottom of intake after shut-down.
Comments: This is one of the most common assembly problems I see on Q-Jets. GM actually issued a Service Bulletin on this subject back in the late 60s because GM technicians were doing the same mistake. There is a small retaining wire on the needle. The float arm has two holes in it where the needle interfaces. People think the retainer goes through the float arm holes. Fact is, the retainer slips over the rear edge of the float arm, and must not be installed through the holes. Installing the retainer through the float arm holes results in the needle and/or float jamming.

Incorrectly Set Power Piston Stop Height
Results in: Poor idle, rich idle, ineffective idle mixture screws, hesitations and stumbles.
Comments: Post-75 Q-Jets have an easily adjustable power piston stop that determines the lowest (leanest) position of the piston. Raising or lowering this stop outside its limits will adversely affect all performance parameters of the power enrichment circuit. Pre-75 Q-Jets also have an adjustable stop hidden under a steel cap in the forward side of the lower throttle plate.

Wrong throttle plate gasket
Results in: Poor idle, erratic idle, ineffective idle mixture screws, symptoms of a vacuum leak.
Comments: There are several different designs for the throttle plate gasket, and they do not interchange. Two common problems occur: The first problem was addressed in a GM Service Bulletin around 1971. There is a difference in the open area in the gaskets just forward of the centerline of the primary throttle holes. Some later carbs use a gasket with a larger open hole in this area. If this gasket is used on an earlier carb, you will end up with a massive, undetectable vacuum leak. The other problems with these gaskets occur due to the idle fuel and vacuum bleed holes not lining up from one design to the next. Use of the incorrect gasket can result in blocked idle fuel and blocked vacuum signals. Always lay the gasket onto the float bowl and onto the throttle plate to check the hole alignments.
Another odd twist to these gasket designs occurred in the late 80s. We are seeing more and more late 80s truck Q-Jets (non-ECM carbs) being used on musclecars, so the problems are becoming more frequent: In the late 80s, there is a fuel discharge hole drilled from the secondary fuel well to a small hole located between the secondary throttle holes in the base of the float bowl. Carbs with the fuel hole require use of a gasket that has NO HOLE at this location. If a standard late 70s or early 80s gasket is used with this carb, manifold vacuum will siphon fuel right out of the secondary side of the carb, discharging the fuel right out of the power brake vacuum hole location in the bottom of the carb throttle plate. You will have a massive rich-running condition, even with the idle screws completely closed (blocking all primary fuel flow) and with no evidence of fuel being discharged on the primary side.
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Old 10-15-2018, 05:38 PM   #3
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Re: Quadrajet Question

Gasket was my first guess, but I suppose you've looked at that.

Have you tried spraying carb cleaner around the base of the carb to see if it changes the idle?
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Old 10-15-2018, 05:44 PM   #4
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Re: Quadrajet Question

You likely have the transfer slot uncovered due to the need to increase your curb idle to get it to idle.
Cure is more timing either from initial increase or hook the vac pot to manifold vacuum.
The demon likely feeds a little air and fuel to the idle and allows the primary to close enough to cover the transfer slot.
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Old 10-15-2018, 08:31 PM   #5
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Re: Quadrajet Question

How about throttle shaft bushings check to see how much play is in them just a suggestion
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Old 10-15-2018, 09:10 PM   #6
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Re: Quadrajet Question

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Originally Posted by anew4961 View Post
How about throttle shaft bushings check to see how much play is in them just a suggestion
Yes, even though the OP says they seem OK. Sometimes the primary throttle shaft leaks worse on the opposite side. OP, check both sides!
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Old 10-15-2018, 09:38 PM   #7
Greasey Harley
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Re: Quadrajet Question

Did you remove the idle tubes and clean them out?

I made a video
https://www.youtube.com/watch?v=_UsWCvs7mO4&t=2s

Last edited by Greasey Harley; 10-15-2018 at 11:57 PM.
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Old 10-16-2018, 07:20 AM   #8
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Re: Quadrajet Question

A common and seldom addressed problem with Quadrajet's is a "warped air horn" Over tightening the breather cover wing nut will warp the air horn especially on a motor that has been overheated for whatever reason...Google "warped air horn" and watch some YouTube videos for trouble shooting and fixes for "warped air horn's"
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Old 10-16-2018, 11:32 AM   #9
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Re: Quadrajet Question

A float level too low can cause this problem.
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Old 10-16-2018, 12:07 PM   #10
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Re: Quadrajet Question

All great responses! Ill check it out, but another problem arose after I changed the oil. Old oil was sludge and the plugs that came out were caked with oil so bad that two of them were closed up. And its a 50/50 shot when you change the oil that it causes more problems that it solves. Well the motor didnt survive the oil change to say the least
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Old 10-16-2018, 12:35 PM   #11
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Re: Quadrajet Question

Quote:
Originally Posted by demian5 View Post
All great responses! Ill check it out, but another problem arose after I changed the oil. Old oil was sludge and the plugs that came out were caked with oil so bad that two of them were closed up. And its a 50/50 shot when you change the oil that it causes more problems that it solves. Well the motor didnt survive the oil change to say the least
OK, now I'm curious. I've never heard of a motor not surviving an oil change...
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Old 10-16-2018, 02:27 PM   #12
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Re: Quadrajet Question

Is it a stock motor or modified for mild performance. Needles and jets are set for certain performance specifications. It could be lean with the a leaky shaft and stock set-up.
** I put a newer set of heads on one of my SBC 350's. I had to go up a number on the jets to get the Air/Fuel mixture right.

Let me know what you the numbers are on your jets and needles and I can tell you if they have been changed from stock.

If interested: http://67-72chevytrucks.com/vboard/s...d.php?t=751483
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