New Quadrajet, idling problems - Chevelle Tech
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post #1 of 13 (permalink) Old Aug 5th, 13, 5:13 PM Thread Starter
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Adam
 
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New Quadrajet, idling problems

Just installed a Shawn Murphy rebuilt Quadrajet on my brother in laws í70 GTO (I know, not a Chevelle). Itís the original 400 motor, original ignition system, (which means points) and the motor has a mild cam in it Ė thatís it, no other upgrades.

I canít seem to get the motor to idle smoothly. Iíve played with the idle mixture screws and seem to have them at the point which the engine idles the highest (getting a vacuum gauge tonight to check manifold vacuum), but the motor still idles rough.

Possible causes?:

- Could the vacuum advance canister not be getting enough vacuum? It currently connects to a vacuum ĎTí fitting rather than getting its own direct vacuum fitting on the carb.
- Could the timing just be off given the new carb.?
- Could the motor possibly just need a tune-up (plugs, cap, rotor, set the points, etc.) given the new carb installed.

The motor ran smooth at idle with the old Quadrajet, but someone over the course of the GTOís life had installed a Chevy carb. and my brother in law wanted the correct front-in fuel line Quadrajet used on all Pontiacs.
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post #2 of 13 (permalink) Old Aug 5th, 13, 8:58 PM
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leo
 
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Re: New Quadrajet, idling problems

the vacuum line should not T with anything. Do you have the correct PCV valve?

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post #3 of 13 (permalink) Old Aug 5th, 13, 11:50 PM
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Matt
 
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Re: New Quadrajet, idling problems

Please be sure to post your final solution. I installed an SMI qjet carb on my rebuilt 350 and it never ran right. I sent it back to him and it came back exactly the same. I had him build me a Holley (which he charged me a 75% restocking fee lol) and the carb base came to me warped. The Holley was supposed to be setup for my engine but an 8.5 PV when I only make 8" vacuum at idle is way off (amongst other things).

Several mechanics in Huntington Beach stopped going there because of the quality of work. Nothing against Sean, but maybe some of his mechanics are less than experienced.


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post #4 of 13 (permalink) Old Aug 6th, 13, 8:46 AM Thread Starter
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Adam
 
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Re: New Quadrajet, idling problems

The motor has the correct PCV valve, but I believe the problem is probably with the vacuum advance line not getting it's own vacuum fitting on the carb. Odd thing is, I'm rather sure thats how it was setup with the old carb., so I'm not sure why the problem now.
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post #5 of 13 (permalink) Old Aug 6th, 13, 3:05 PM
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Mike
 
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Re: New Quadrajet, idling problems

Adam,

I'd eliminate the ignition system first as the possible cause of your idle issue. If that fails to correct it, then I'd verify that the float is set at the proper setting. Verify also that it doesn't rub against the fuel bowl wall, and that the needle and set are installed correctly, are working smoothly, and are clean.

Finally, if you happen to see the nozzles dripping at idle I'd verify that the pcv system is working fine and perhaps try running manifold vacuum to the distributor. It's possible that the cam is too big for the carb (as currently set up).

Mike
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post #6 of 13 (permalink) Old Aug 6th, 13, 3:31 PM Thread Starter
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Adam
 
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Re: New Quadrajet, idling problems

Funny thing is Mike, after installing the new carb, the car sounds like it's got a bigger cam. Weird, for sure. But probably due primarily to the roughness at idle.

The ignition system will be first on the list of items to try bright and early this Saturday am.
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post #7 of 13 (permalink) Old Aug 6th, 13, 8:51 PM
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Eric
 
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Re: New Quadrajet, idling problems

What is the ID number on the carb?

Have you verified the base gasket is sealing?

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post #8 of 13 (permalink) Old Aug 6th, 13, 10:35 PM Thread Starter
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Adam
 
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Re: New Quadrajet, idling problems

Not sure of the ID numbers, but it's one of Sean Murphy's stage one quadrajets I believe.

To verify no vacuum leaks at the gasket, or anywhere else for that matter, I did the old hand covering the airhorn method. When doing that, the car immediately stalled out.
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post #9 of 13 (permalink) Old Aug 6th, 13, 10:42 PM
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Re: New Quadrajet, idling problems

Is it a older "70" carb, or a newer "170"? Look at the carb number starting off with either one of those. If it is "170", what is the Adjustable Part Throttle screw set at? Nozzle drip can be too lean a mixture, esp with those carbs.
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post #10 of 13 (permalink) Old Aug 7th, 13, 1:02 AM
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Re: New Quadrajet, idling problems

SMI delivered a 750 DP Holley to me with no jets (at all!) in either metering block, and the blocks reversed (primary on sec side, sec on primary side). Garbage.

Check the top casting for mismatch with the main body. These should normally be paired for life, since Q-Jets take a "set" over time when the front bolts are tightened. They often look as though the top casting droops when viewed from the front and not attached to the main body yet. Castings that are alien to each other have a great chance to leak fuel, vacuum, etc., even when "pillowed" by the thick gasket. It will run, but otherwise perform strangely.

Is your Q-Jet a Carter-built unit? It will say so right on the side of the carb: "Quadrajet by Carter". These do not stand up to time very well. The problem is inferior metal chemistry compared to Rochester versions, and internal corrosion affecting the drilled passages. Again, it will generally run, but exhibit strange tendencies.

I have built hundreds of Quadrajets over the years and found them to be generally excellent street carburetors, and among these are the lessons I've learned about the particular foibles possible with the type.

Hope this helps.

Eric

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post #11 of 13 (permalink) Old Aug 7th, 13, 1:12 AM
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Gary
 
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Re: New Quadrajet, idling problems

with the aircleaner off start the car and warm it up a bit to stabilize the idle speed and gradually choke the airhorn by hand.if the engine speed goes up its lean.eliminate vacuum leaks as a cause .if thats ok the carb idle circuit needs to be checked out.this is after you have made sure the igniyion and timing are ok
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post #12 of 13 (permalink) Old Sep 23rd, 13, 9:44 AM Thread Starter
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Adam
 
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Re: New Quadrajet, idling problems

Finally got to the root cause of my brother in laws new quadrajet rough idling issue on his í70 GTO 400. After replacing all the ignition components, which did nothing to fix the problem, and discussing with Sean Murphy on the phone, (heck of a nice guy, very helpful), itís clear that the idle circuits need to be opened up to allow more fuel to flow at idle. With the aftermarket cam the car has (unfortunately, we donít know itís specs), the engine just does not make sufficient vacuum at idle Ė made roughly 10 inches at best.

Talking with Sean, he recommended, with the engine running and vacuum gauge connected, slowly close the choke blade to observe the engine idle increase and vacuum number go up. Then, let him know approximately how close to being nearly closed the choke blade was to achieve the highest vacuum reading. This will tell him roughly the cam specs. And how large to open the idle circuits to correct the problem.

It was amazing to see how the cars idle immediately began to smooth out while blocking off approximately 60% of the opening of the air horn.

Canít wait to get the carb. back from him and bolt it back on that 400. Should purr like a kitten.
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post #13 of 13 (permalink) Old Sep 23rd, 13, 2:12 PM
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Tom Terrific II
 
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Re: New Quadrajet, idling problems

Quote:
Talking with Sean, he recommended, with the engine running and vacuum gauge connected, slowly close the choke blade to observe the engine idle increase and vacuum number go up. Then, let him know approximately how close to being nearly closed the choke blade was to achieve the highest vacuum reading. This will tell him roughly the cam specs. And how large to open the idle circuits to correct the problem.
yeppers, that's a good way to find out what's really going on.

regardless of the back-biting blind-sider post here SMI has a long term good reputation, I'd be comfortable with sending the carb back to them for re-calibration.

Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife.
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