Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250

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Cliff Ruggles:
The Power Piston should be all the way down when the engine is running and you are adjusting it.

The vacuum at idle, off idle and even up at 2000rpm's would be high.  The only time you can drop vacuum low enough would be hitting the throttle really quick and vacuum will only drop momentarily allowing the spring under the piston to raise the PP for just an instant.

IF the PP is up at idle, off idle, or even at 2000rpm's light load you have another issue someplace.

Start by checking the hole in the baseplate to make sure it's drilled all the way thru and not plugged or blocked off.

Check the power piston to make sure it isn't tight or sticking in the bore.  It should move up/down easily without any resistance.  The retainer can also interfere with it's travel, so make sure it's correctly installed on the PP and not impeding it's movement.......Cliff

bry593:

--- Quote from: don3194 on March 19, 2021, 09:01:00 PM ---Idle still seems high after removing every vacuum operated item and capping the ports.
should the power piston be seating as far down as the APT screw lets it?
To adjust the APT I had to push it down and seat it through the vent so the APT adjuster tool didn't get caught on the pin sticking out of the power piston.
--- End quote ---

1. If your idle is still high, and you closed off the IABP, then yes... you probably have a vacuum leak.  Put a vac gauge on it.  The needle movement will help diagnose the problem.  There are tutorials on the net to teach what the needle is saying.
2. The power piston should be sucked down against the top of the screw, regardless of how far down the screw is.  If the piston feels springy at idle, then the spring is too stiff for your cam vacuum (it should be down).  You need to switch to a weaker spring.
3. Did you really have to push it down?  You can feel it go up and down with the engine off?  Did it go up and down with the engine running?  If so, see 2.  But yes, the APT tool will tend to get hung up on the piston "pin" unless you hold it a little high so as not to get the pin caught in the tool.

Kenth:
Yours 17059222 have the 8-4 in/hg power piston spring from factory, which means it starts rising the power piston at approximately 8 in/hg and is fully up at 4 in/hg.

And, with APT screw bottomed  it will take 2 full turns of the screw to reach the lift pin on the bottomed out power piston before it rises the power piston at all.

With engine off, check movement of power piston thru the vent hole. Does it move down when pushed on, does it returns upward when you release it?
With engine running at operating temperature, check intake vacuum, rpm´s in and out of gear (A/T).

don3194:
Hey all, I found the hole was NOT drilled in the baseplate. You may or may not agree with what I did, but it worked. I compared how the holes were on the other baseplates and drilled this one out. And it made a huge difference while driving. The power piston is being sucked down now.

As for my idle problem, it still exists, I have attempted a vac gauge on it, but it honestly looks like a perfectly healthy engine (according to the gauge). Nice solid steady needle, no erratic movement.

Another thing I just want to inquire about is, whenever I lay into the throttle, enough to open the secondaries, I hear a weird rattling type sound. I'm not too experienced when it comes to sounds like knocking/pinging or other engine sounds, but it honestly sounds almost like metal against plastic. Its not very loud, but noticable. It sort also sounds like something in the carburetor is rattling.

tayto:
pcv valve? what brand are you using here?

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