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Why can't I get a vacuum adjustment on my 800cfm Quadrajet carburetor?

1966 Pontiac 421ci engine, KRE heads, mild camshaft, Doug headers, electronic distributor module, 12 degrees initial advance, vacuum advance.

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  • George Fisher
    George Fisher Nov 10, 2014

    Thank you. I am not able to get any response from the air mixture screws on the carb at idle of 650rpm with 10 inches of vacuum.



1 Answer

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  • Cars & Trucks Master
  • 44,293 Answers

Check that you have the vacuum line connected to the right pipe at the base of the carby. The pipe should be at the point where the throttle butterfly touches when closed. Check that the ignition module is on the advance plate so that it can move when there is vacuum s it needs to be able to move with the vacuum action.

Posted on Nov 10, 2014

  • Bill Boyd Nov 10, 2014

    The vacuum reading is too low for idle. Check that you do not have a vacuum leak in the manifold gaskets or hoses connected to it. Mixture screws adjust the fuel/air mixture and the screw that you should be adjusting is the throttle screw to close off the butterfly. Check the cam timing as I suspect that you are a tooth out. Also the carby you have stated is too big for a mild cam and all you will be doing is pouring fuel down the throat and putting the fire out for no gain in power. A 650 cmf would be plenty big enough for the cam. 850 cmf is for a extra big cam with a heavily reworked motor and special flow manifold.



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SOURCE: engine sluggish

By all means tweak the distributor and adjust the idle to get your vacuum up to where it should be. At the same time have you checked the timing with a timing lite? If you have and it is bang on you may want to consider taking a look at your timing chain in the near future as it may be stretched a little. But before you do that, check your compression. hope that this helps and that you take the time to rate this information.

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SOURCE: how to rebuild an autolite 1100 carburetor

your in luck because all repair kits come with instructions
but dont get caried away taking out choke and throttle plates it isnt nessesary just take off the top and bottom and choke assemly clean up parts.blow out passages with air hose were safty glasses in case a check ball you missed or some dirt comes flying out

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SOURCE: I want to clean my honda crv engine and the

Here is a website that a free manual can be down loaded for your vehicle

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SOURCE: 1990 Chevy Blazer K5 5.7 engine: has engine ping

The temperature sensor is where I would start. If the sensor tell the computer that the engine is cold it will initiate the warm up mode which includes advancing the timing along with a rich fuel mixture. Unfortunately, there are many sensors that assist in controlling timing and fuel mixtures that it is difficult for me to give you a guaranteed solution.

Posted on May 27, 2010

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SOURCE: Emissions sticker under hood is

if this car has a 307 with a carb it did not come out with it the 1989 fleetwood came out with a 4.5L cadilliac V8 with DFI fuel system, so i have no idea whet to tell you on the vacuum lines.

the last year cadilliac used the 5.0L (307) was in 1986, and they used 2 fuel systems so you need to know your vin# the 8th vin# will be either a Y or a 9

vin# Y

vin# 9

now for adjusting the carb. if it has never been adjusted before you will need to remove the carb and turn it over and remove the factory plugs over the adjusment screws then reinstall the carb and using a double D adjustment tool turn the screws out 2 turns then start turning the screws in one at a time untill the rpm drops then back back out just untill the rpm leves back out then 1/4 more this is called a lean drop carb adjustment also this carb has a mixture control unit inside the carb and it can also be adjusted but you need a dwell meter adjust the mixture control unit to 30 degress dwell on the V8 scell.

Posted on Aug 07, 2011

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1 Answer

How do I adjust a quadrajet carburetor?

You can only adjust the two idle screws,
unless your rebuilding it,then the instructions
will show you lots of adjustments

I would install a new float,regardless of how it
work now,as they get saturated & sink

The adjustment is the same as with all carbs

You start at about one turn out on the screws
& end up at about 1 3/4 turns

You try for the highest vacuum reading & then go
in about 1/8th turn

Have the vacuum advance hose of the dist & plugged

After the carb-- recheck your initial spark with the hose
off the dist

Vacuum gauge has to be from a manifold source,not

(below the carb bottom throttle plates)

Oct 07, 2014 | Cars & Trucks

1 Answer

Car had been sputtering and stalling while stopped in "drive". Mechanic mentioned to change the thermostat so I did it myself. The problem still exists and now the blower for the heat and A/c doesn't

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Where is the camshaft sensor on the 05 pontiac g6?

The camshaft position sensors of your Pontiac G6 are on drivers side of engine mounted sideways in head with plug extended out towards driver's side.Please see the attached figure.

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What kind of carburter do I need for a 1984 firebird

If you have the original engine, likely it's a Quadrajet if it's 4bbl, or another possibility is a Rochester 2gc. The Quadrajet is a modified version with a purge valve solenoid and a throttle position sensor built into it. (funny to see your post because I just took one off a Montecarlo this morning....did a vorteck head swap, cam, high perf. intake, headers and a way larger Holley carb)

Apr 12, 2011 | 1984 Pontiac Firebird

1 Answer

Fast idle

The specification calls for 900 rpms for the idle speed when warm for the 1987 Dodge Shadow.

One can adjust this with a screw on the carburetor, if it has one.

If it's fuel injected the idle speed is automatically controlled.

Fast Idle Speed HOLLEY 5220/6520 Fig. 5: Turn the adjusting screw with a screwdriver to raise or lower the fast idle speed — make sure that the adjusting screw is resting against the lowest speed step 86735g04.gif
NOTE: This procedure is performed while the carburetor is installed on the engine.
  1. On 1981–82 cars, disengage the two-way electrical connector at the carburetor (red and tan wires).
  2. On all years, disconnect the jumper wire at the radiator fan and install a jumper wire so that the fan runs continuously.
  3. On 1983–86 models, pull the PCV valve out of the valve cover and allow it to draw underhood air.
  4. Disengage the oxygen sensor system connector located on the left fender shield near the shock tower.
  5. Ground the carburetor switch with a jumper wire.
  6. Open the throttle slightly and place the adjustment screw on the slowest speed step of the fast idle cam.
  7. With the choke fully open, adjust the fast idle speed to comply with the value indicated on the underhood sticker.
  8. Return the vehicle to curb idle, then reposition the adjusting screw on the slowest speed step of the fast idle cam to verify fast idle speed. Re-adjust as necessary.
  9. Turn the engine OFF, remove the jumper wire and reconnect the fan.
  10. Reinstall the PCV valve and remove the tachometer. On 1983–86 models, reattach the oxygen sensor system connector, and remove the jumper wire from the carburetor.
Fast Idle Opening MIKUNI
  1. Before adjustment, leave the carburetor alone for approximately one hour at 73°F (23°C).
  2. Adjust the fast idle opening by turning the fast idle adjusting screw to the following specified values (drill diameter):
    • 2.6L engines equipped with manual transaxles — 0.028 in.
    • 2.6L engines equipped with automatic transaxles — 0.031 in.
Choke Valve Setting MIKUNI Fig. 6: After tightening the choke cover lockscrews, cut the heads off of lockscrews A (total of 3 screws) and stake the heads of lockscrews B (total of 2 screws) with a blunt punch 86735g56.gif
NOTE: This procedure need only be performed if the carburetor is disassembled.
  1. Fit the strangler spring to the choke lever.
  2. Assemble the choke valve, aligning the inscribed line or black painted line on the tooth of the choke pinion with the inscribed line on the cam lever.
  3. Temporarily tighten the new lockscrews.
  4. Set the choke valve by moving the pinion arm up or down, align a punched mark on the float chamber cover at the center of the three inscribed lines, and secure the pinion arm with the lockscrews.
  5. Install the choke cover and tighten the lockscrews.
  6. Cut off the heads off of lockscrews A.
  7. Stake the heads of lockscrews B using a blunt punch.
Vacuum Kick HOLLEY 5220/6520 NOTE: This procedure is performed while the carburetor is installed on the engine. If the vacuum kick is adjusted to open the choke too far, the engine may stall or idle very roughly just after cold start. If it is adjusted so that the choke does not open enough, there may be black smoke in the exhaust. NOTE: To perform this procedure, you will need a hand held vacuum pump capable of producing at least 15 in. Hg (50.6 kPa) of vacuum. The vacuum kick diaphragm may be damaged if you attempt to retract it manually. You will also need a drill or dowel which has a diameter equivalent to the specification for Vacuum Kick in the Carburetor Specifications Chart.
  1. Remove the air cleaner. Open the throttle, close the choke and hold it in the closed position, and then release the throttle to trap the fast idle cam in the choke-closed position.
  2. Disconnect the vacuum hose at the choke vacuum kick diaphragm. Connect a vacuum pump and apply 15 in. Hg (50.6 kPa) or more of vacuum.
  3. Gently move the choke blade toward the closed position just until play is eliminated from the vacuum kick linkage (so that the vacuum kick is determining choke blade position).
  4. Insert the drill or dowel into the gap between the upper edge of the choke blade and the air horn wall, toward the center of the gap. The dowel or drill should just fit into the gap. If necessary, rotate the Allen head screw in the center of the diaphragm housing to create the proper gap and then recheck with the measuring device.
  5. Restore all vacuum connections and reinstall the air cleaner.

Aug 05, 2010 | 1987 Dodge Shadow

1 Answer

Hi i need to knoe what kind of camshaft i sould get to get a choppy idle on a 1969 350 v8 small block

Any cam grind that has over 220 degrees duration will give that kind of idle. However, most with that kind of duration also have a lot more lift than a stock cam. That will give you very low engine vacuum that will change how your automatic transmission shifts (not good) and can break valve springs, pull rocker studs out of the heads and cause valve/piston clearance problems. (read: bent valves, holes in piston tops)
Overall it isn't anything you want to do to your engine, especially just for the rumpy idle which though it sounds "cool" is actually a byproduct that just happens with those cams by design, but has no benefit.
Engines you hear that kind of idle also have extensive modifications to the carburetor, intake, exhaust, pistons and lower end (crankshaft) and if automatic, they have different torque converters and trans valve body mods as well.
If you want to go fast, get a good aftermarket intake and carb, a mild cam that works in mid-range rpm's, put a set of headers on it and a better exhaust system. Don't ever buy a used camshaft and always replace the lifters with the cam (once they are used, each lifter always needs to be on the same cam lobe forever. Used ones most often have not been kept in order and will quickly round off any different lobe they are used on)
Do a lot of reading and studying before buying anything or you will spend a lot of $ and wind up with junk or less than you had before. Everything needs to work together.It's not about just putting in parts. There is a ton of science and math there that can't be ignored.
good luck!!

Mar 18, 2010 | 1988 Chevrolet Nova

1 Answer

1988 Oldsmobile Regency 98 will idle but won't quits on the road

Mixture solenoid Quadrajet carburetor? I might start in that neighborhood if it has that carb. Quite cantankerous if not in freshly rebuilt and operating mode. Can cause more than mild symptoms. All your electrical connections and engine/body grounds in good shape? Trying to rule out ignition breakdown so you can focus on fuel delivery, which is my suspicion in this case. Let me know what you find. Thanks, TT

Jan 28, 2009 | 1988 Oldsmobile Regency

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