Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250
Cliff Ruggles:
Did you block the idle bypass air on both sides?
If you don't have idle bypass air and all the throttle plates are completely closed at idle you should have full control of the idle speed and it will go below 800rpm's. The engine has to be getting air someplace?
The "whooshing" sound thru the primaries sounds like it still has bypass air and is "whistling" past the entry slots in the airhorn where they lead to the voids in the main casting......
bry593:
Double check the thick gasket between your intake manifold and carb base. I once used the wrong gasket and a bit of the front edge of the carb overhung the gasket causing a substantial vacuum leak. Don't recall if I used a 170 series on a 70 series, but it was something like that.
The IABP ports are shown on your baseplate pic. These round holes are to the left and right of your circled as cast ports. I think you'll find those as cast ports deliver vacuum to different ports on the exterior of the carb.
Double check all your exterior ports to make sure these are plugged. For non-emissions, all you need are four ports:
* Choke Pulloff
* PCV Valve
* Full-time Vacuum (for "manifold" vacuum advance)
* Ported Vacuum (for vacuum advance except at idle)You might also need the NPT port at the back to run your power brake booster.....
I ended up pulling the pressed in nipples (vice grips and a torch), and then resizing the holes to fit aluminum rod (3/16, 1/4 and 3/8). I cut the bar stock to appropriate lengths, coated the OD with red LocTite and tapped into place. This really cleaned up the looks of my 170 series carb and will never leak (unlike rubber or vinyl caps that will deteriorate).
don3194:
I did block the Idle Air Bypass on both sides, following this picture http://www.wallaceracing.com/Adjustable_bypass_with_wrench_small.jpg from this thread https://cliffshighperformance.com/simplemachinesforum/index.php?topic=2587.0 I know all of my external ports that I'm not using are plugged for now. I did verify my vacuum advance is on a ported vacuum source. The only thing you mentioned that I didn't check was the intake to basplate gasket.
The gasket is one that Cliff sent so I've gotta assume its right, but I suppose I can check when I get the chance too.
I suppose I also could have blocked the wrong holes too.
Cliff Ruggles:
If you blocked the bypass air and all four throttle plates are tightly sealed when closed (idle speed screw backed off) then the only place it's getting air should be at the PCV valve.
If you have a leak someplace, power brake booster, intake gasket, carb to intake gasket (unlikely if you used a pretty thick one) then the engine can get enough air to idle with the throttle plates fully closed.
Most of the carbs I get in here for rebuilding do NOT close the primary plates all the way in the bores and require some "tweaking" with the linkage to make that happen......
bry593:
The PCV is a vacuum leak by the very nature of its design. I seriously doubt if you have plugged the IABP and the idle is still high, that your PCV is the culprit unless the body is cracked or something odd like that.
As for the brake booster, I worked on a friend's car that had a high idle. He had removed the quad and put on an AFB to try to fix. I simply plugged hoses until the idle changed with the brake booster hose. I put the quad back on and fixed the booster. By the way, if the booster is leaking, you will have to push abnormally hard on the brake pedal.
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