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Old 07-16-2012, 11:37 AM
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Quadrajet high idle problem - rod/jet sizing?

I just restarted my sbc 350 with a rebuilt quadrajet. I have 74 jets, and 46 primary rods. Before the carb rebuild, it ran fine. I damaged one of the origianl rods, so had to order two new idlebrock aftermarket rods (1946). Its a 75 era' MV quadrajet. It it now idling high, and have jets backed out as far as they can go to get my vac reading at max (19.5 inches). If I screw the idle screws in to lower rpm, it runs worse and vac drops to 15 ish. Prior to rebuild I was at 22 inches, and around 900 or so. Its now idling at 1300 and wont go any lower (cold and warm). Im thinking I have a lean condition. I placed hand over carb after startup and the rpms increased. Readings are taken in neutral with AT.

Thanks for any ideas.

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Old 07-16-2012, 11:46 AM
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Quote:
Originally Posted by highgrade View Post
I just restarted my sbc 350 with a rebuilt quadrajet. I have 74 jets, and 46 primary rods. Before the carb rebuild, it ran fine. I damaged one of the origianl rods, so had to order two new idlebrock aftermarket rods (1946). Its a 75 era' MV quadrajet. It it now idling high, and have jets backed out as far as they can go to get my vac reading at max (19.5 inches). If I screw the idle screws in to lower rpm, it runs worse and vac drops to 15 ish. Prior to rebuild I was at 22 inches, and around 900 or so. Its now idling at 1300 and wont go any lower (cold and warm). Im thinking I have a lean condition. I placed hand over carb after startup and the rpms increased. Readings are taken in neutral with AT.

Thanks for any ideas.
To begin with be sure the fast idle cam isn't raising the idle. Does the curb idle screw (not idle mixture screws) change the idle speed- as in, can you turn the curb idle screw out and cause the engine to stall (or nearly so)?

Did you check the metering rod length to be 100% sure they're the same length? The 1946 rod is for '74-older Q-jets, although the design was used later than that on some applications- but in any event, the rod lengths have to match what came in the carb originally.

Did you change the APT setting? What about the float level? Vacuum leaks?
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Old 07-16-2012, 11:53 AM
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Forgot to add- the rods and jets have little to no effect on the idle unless the power piston were to be hung "up" and the curb idle screw turned inward so far that the carb was idling on the transition circuit.
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Old 07-16-2012, 11:58 AM
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Thanks Cobalt. I checked the fast idle setting and it was working correctly. The rods I have are for the older quadrajet because my model number 7045583 (74-77)was for a truck application that they used a bit later than the 75 changeover so still used the older rods.

I dont have APT setting on my carb.
Checked float level several times before I sealed it back up. Vac. readings looks pretty stable, and looked around for potential vac. leaks but will do the carb cleaner check just to be sure.

I never adjusted the timing beacuse it was dialed in perfectly before the rebuild so I didint want to introduce any more variables by adjusting timing. Its around 32-36 degrees if I recall.
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Old 07-16-2012, 12:02 PM
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In response to your additional thread Cobalt, I did forget to mention I did replace the power piston spring with a med range spring. The original spring was on the soft side, and with my original 22 inches of idle vacuum I thought maybe a stiffer spring would improve performance. Could this be a potential cause of the high idle if my power piston spring was too stiff?
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Old 07-16-2012, 12:08 PM
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You have APT, just not the traditional type. Most 1975 Q-jets have an aneroid, or a spool that replaces the aneroid.

No chance the small rubber coated steel gasket that goes under the needle seat was left and a second one installed over the old one? Happens a lot. Was the needle hung correctly from the cross piece of the float and not hooked through one of the holes (if you have that style float)?

From quadrajetparts.com:

Quote:
Testing a power valve-
You can easily test your Rochester Quadrajet power valve spring to see if it is too strong. At idle, the engine vacuum needs to keep the power valve fully seated (down postion, lean position). Take a normal plastic drinking straw and place it in the top vent of the airhorn, by leaning the straw on a slant you should be able to feel the power piston as you press the piston down and up. With the engine shut off, place a mark with a marker on the straw showing to top height of the vent. When you start the engine, the straw should pull down and stay down. If you see the straw move up and down, you know that the power piston valve spring is too strong for the vacuum produced by the engine at idle and needs to be replaced with a lighter (weaker) spring. Also if the power pistonis moving up and down at idle, the engine rpm will also be unstable and will rise and fall.
This will also tell you if the PP is hung in the up position.
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Old 07-16-2012, 12:27 PM
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I have the kind that has the APT valve sealed and no "APT" tower. Nor do I have the spool. The cavity is empty. I found a straw and will give the PP a test and report back.

Thanks!
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Old 07-16-2012, 12:35 PM
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Quote:
Originally Posted by highgrade View Post
I have the kind that has the APT valve sealed and no "APT" tower. Nor do I have the spool. The cavity is empty. I found a straw and will give the PP a test and report back.

Thanks!
No APT in the baseplate between the idle screws under that small cup plug? You should have something...
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Old 07-16-2012, 12:43 PM
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Yes, My APT is the sealed kind in the baseplate between idle screws.

BTW. Did the straw test and my staw is definately getting sucked down about 3/16 of an inch. So now what? Put the softer spring back in I guess? Would that then be the cause of the high idle?

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Old 07-16-2012, 12:48 PM
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Cobalt, never mind, I read your reply again. The down straw is the "normal" mode for PP spring. So that must not be a contributing factor. Back to square one.
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Old 07-16-2012, 05:10 PM
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Quote:
Originally Posted by highgrade View Post
Cobalt, never mind, I read your reply again. The down straw is the "normal" mode for PP spring. So that must not be a contributing factor. Back to square one.
Correct- down if the engine is idling, and up if the engine is off.

What happens when you turn the curb idle screw in to lower the idle speed? Does the engine slow down? Is the throttle linkage contacting the curb idle screw at idle?

If the distributor worked loose, it might have advanced (this increases the idle speed and can cause it to run rough if advanced far enough) although you'd expect it to retard more than advance- but anything's possible.

The fuel filter might be clogged causing a lean condition or the gas tank may be too low, not enough fuel pressure.
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Old 07-16-2012, 07:47 PM
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not quite stoteometric

THE 1300 idle sounds like the motor is getting air from somewhere, and the cranked
open idle screws are feeding it with a weak fuel signal, that is a strong indication the air
is coming from somewhere other than the primary throttle bores, sometimes the
secoundarys are hanging open, check that first, and it sure sounds like air is getting in
from somewhere, try the unlit propane touch trick. on every possible connection and
gasket, include the intake manifold gaskets.
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