Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250

[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]

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.

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?

[bry593]

I have attempted a vac gauge on it. Nice solid steady needle, no erratic movement.

Since you have blocked IABP, rebushed the throttle plate and still have high idle here are a few other suggestions:

Make sure your throttle stop tang is bent properly to allow full closure of the primary blades.

How many turns out are your idle screws?
What is your float measurement?  Maximum 3/8 if no fuel regulator.
What size needle seat are you using?  Maximum .135" for trouble-free daily driver.

Whenever I lay into the throttle, enough to open the secondaries, I hear a weird rattling type sound. sounds like something in the carburetor is rattling.

Detonation usually has a tiny, pingy sound.  Like bb's bouncing around in your cylinders.  Heavy detonation can sound like a rattle.  If you are using a Chinese distributor, most have no limit on the mechanical advance curve and advance well past 32 total.  This will cause serious detonation.  The solution is to limit the mechanical advance with heavier springs or a mechanical stop.  Better yet, hit a swap meet and pick up a used GM distributor.  The old HEI were well designed and usually only need lubricated, new weight bushings and lighter springs to wake 'em up.

[bry593]

The float level listed above should be a minimum of 3/8.  It will work fine between 3/8 and 7/16.

I bet it does drive a lot better if the power piston was never rising.  That will cause a lean hesitation whenever leaving a stop light or during moderate acceleration.  This assumes your throttle angle results in sub 8 in Hg.  I noticed my truck leaves a light at about 10-6 in Hg.  And when my spring was not lifting the piston, it hesitated and I had to flutter the throttle (add fuel with accelerator pump) to get it going. 

[Cliff Ruggles]

Hey all, I found the hole was NOT drilled in the baseplate.

Then it was from a turbo carb most likely 

CCC carbs aren't drilled either, but they are slightly different in other areas like using a much thicker rod up to the accl pump arm, etc.

Your PP spring should NOT be a player for "normal" driving at light load.  Heavy load or quick throttle movements yes.  Any heavy or load applied quick enough to DROP vacuum below about 10" or so will also retard the timing as the VA will also come out of play.

I've logged many thousands of miles custom tuning with a vacuum gauge rigged up where I could see it and it's rare to see vacuum fall off much at all at vary light engine load.  Most will actually be higher than they were at idle speed, unless you push down the throttle pretty quick and heavy.  When this happens the accl pump is there to cover a momentary lean condition until the carb increases fuel delivery accordingly.......Cliff

[don3194]

Just to put this part to rest right now, my PCV is not hooked up at all right now.

Its been a while but want to say I set my float level at 13/16" but again its been a month or so and I just cant remember for sure. I do have a fuel pressure regulator set to about 5-6 psi.

I dont know what size needle seat it is, Cliff sent it in a kit for a 17059222.

Where is the tang located, I mean having the carburetor off I can see that the blades close but I just want to check everything here. I can back the idle screw out until it is not touching.

I think my idle mixture screw are out in the range of 3-4 turns but I cant remember that either.

I also just want to say again that I remember when I first put this carburetor on, it had a good low idle and then something changed, a sucking sound started and the idle went up.

[bry593]

If the idle screw can be backed out until not touching, that part of the equation is okay.

Idle mix screws at 3-4 is about right. So, nothing unusual there.

You are not blowing the needle off the seat and adding too much fuel if regulated to 6psi.  So no problems in this area.

A sucking sound could be a problem.  Another way to hunt down a vacuum leak is to take a length of vacuum tubing and hold one end to your ear while moving the other end around the carburetor base, manifold edges, etc.

If you had good low idle and then something changed, that had to be something you did.  What did you change?  Was this before the rebuild? 

[don3194]

Well see, the sucking sound started while it was running. I had just got the engine warmed up after the rebuild and was about ready to start adjusting the throttle screw. Quite literally, I touched the tool to one mixture screw and I heard the sound start and the idle went up.

[bry593]

If it wasn't doing it and then all of a sudden, with the engine idling, a sucking sound started and the rpms increased....  it sounds like a gasket was barely sealing, and then the idle vacuum was enough to unseal it.  Again, carb to intake gasket would be suspect as that is an easy one to hear.  Another possibility is the airhorn gasket since you can hear those leaks as well.  If your carb body is smashed down at the front two mounting bolt holes, then possibly the airhorn gasket is leaking slightly.  Obviously, both of these would make your idle lean, and would require additional turns out on the idle mix screws to add more fuel to make up for the extra air.  More air + more fuel = faster idle.

You'd be surprised how loud a vac leak in the carb is.  Simply opening idle air bypass (IABP) from .052 to .100 makes a very obvious sucking/whistling noise.

By the way, to tell if the airhorn gasket is leaking, you have to remove the airhorn and check the gasket for witness marks caused by the raised ridges on the top of the carb body.  The ridges trace around anything that must by 100% sealed.  So, if any part of the gasket is not showing these witness marks, that would be a suspect area.

Another thing that might make a sudden "sucking sound" is a ruptured diaphragm in your choke pull off.   Put a hose on the port, suck on it and block the end of the hose with your tongue.  If the vacuum bleeds off, then you have found your problem.

[don3194]

Well I'm kinda feeling like an idiot, but I guess I'll just say make sure you use the right timing mark on your harmonic balancer.
As far as I could tell there was only one, but come to find out there are a couple on there. I'm not quite sure what the purpose of the one I was using is, but I retimed it to the right one and now I have the idle screw turned in about a 1/2-3/4 turn, there may still be a problem because the sucking is there and blocking the IABP didn't do anything.
Anyways it seems to drive well, the only thing is that if I go and floor it, it bogs down quite a bit at lower rpms once the air doors open.