Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250

[don3194]

I installed the bushings in the original baseplate and reinstalled the carburetor with the original baseplate. It still seems to be leaking air from somewhere, its a lot better though. I actually have the idle just around where I want it, I'd probably want it about 50 rpms lower but the idle screw is backed all the way out. I can live with that but still makes me wonder what I'm missing.

I haven't rechecked timing, but couldn't too much initial advance artificially raise the idle?

[Kenth]

Check for too short throttle wire/rod holding the throttle blades open some.

[Cliff Ruggles]

Yes, if the initial timing is pretty high on a "stock" or very mild application you can loose control of the idle speed.

EVERYONE doing this sort of thing will Google up the subject and switch or use manifold vacuum to the advance putting the initial timing WAY off the scale or at least 25-30 degrees or so.  That's all fine if it works for you but some engines make a LOT of vacuum without much timing in them and adding a bunch more will have them idling too high even with the speed screw backed clear out.......Cliff

[bry593]

I'd probably want it about 50 rpms lower but the idle screw is backed all the way out. I can live with that but still makes me wonder what I'm missing.

To further lower your idle, I recommend plugging your idle bypass air (IBPA).  Just drill and tap the throttle plate and install some 1/4-20 setscrews.  There are pics of this mod on the internet.  This trick allows using carbs from larger displacement 350 engines on smaller motors like your 4.3 V6 or a 305 V8.

Once IBPA is plugged, you will have control of your idle screw.  This will allow you to open the thottle blades just enough that these are not sticking in the primary bores.  I'd bet with the idle screw backed all the way out, you currently have a bit of stick/slip when first applying throttle.

By the way, the IBPA is probably your mysterious vacuum leak.  That's what it is, a controlled vac leak used by Rochester to raise or lower the idle so that the idle screw works within a small window.  The range of usable idle screw is from blades fully closed to thottle angle which begins to draw fuel from the transition slots.  So, fairly narrow range, hence the IBPA.

[don3194]

Okay, I verified timing, verified my throttle cable isn’t restricting. The throttle plates are all completely closed. I blocked the idle bypass port you suggested to block bry, and that made 0 change whatsoever. I’m baffled that it didn’t change anything. The idle will not go below 800 rpms, I can live with it being 50 higher than where I’d like it but I mean it shouldnt run with the blades sealed.

One thing still to note is that I can hear a nice whoosh/sucking sound through the primaries at idle.

[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.

[don3194]

Well, when I get chance I'll check the gaskets and vacuum acessories. The only things that are hooked up directly to the carburetor are the distributor and the the vacuum cannister. I will recheck it, but those two items I did not have hooked up and still had the problem. I know my brake booster is going bad but I do not believe it is the problem. I will not eliminate it as I have not rechecked it since I noticed the high idle.

I also did verify my throttle blades do completely seal.

[bry593]

Another thing to look for is intake manifold gaskets.  If the gasket is leaking to the outside of the engine, it is much easier to find.  Usually, you can start the engine and let it idle.  Then pour some water on the intake manifold, along the valve cover and wait to see if the idle slows.  If it doesn't, try the other side.  I like this method because it is not flammable, nor does it dissolve your engine paint.

Internal vacuum leaks are more tricky to flush out.  I've actually plugged the valve covers and put a vacuum gauge on the dipstick tube.  Using this method, you should see pressure build as piston ring blow-by slowly pressurizes the crankcase.  If you see vacuum, you definitely got a problem.  If you see no change, then your engine is not well sealed.

[Cliff Ruggles]

Another way to find a leaking intake gasket is to let the engine idle and one by one pull a spark plug wire from the cap.  If it's leaking on one runner, which happens on occasion you will not get nearly as much engine miss-fire/RPM drop when that plug wire is removed at idle speed.

Until I outfitted my current 455 with the large port Felpro "blue" gaskets (steel core) for the Super Duty 455's I had 3 different sets of aftermarket large port gaskets fail.  Each time they started leaking at one intake runner.  It took a few years for each failure, but it was difficult to diagnose other than a slight miss-fire at idle speed. 

I found the problem by gently removing one plug wire at a time from the distributor cap and listening for a miss-fire or pretty decent engine speed change.  When I got to the cylinder with the leaky gasket the change in RPM was very minimal.........Cliff

[don3194]

Well, I sure hope its not my intake gasket. I did just do those about 4 months ago. I haven't had a chance to get out and do any more diagnostic work, but I think tomorrow I can eliminate more possibilities.

[don3194]

Idle still seems high after removing every vacuum operated item and capping the ports.
I do have a question though, when performing the tip-in procedure, should the power piston be seating as far down as the APT screw lets it?
I found that when I revved the engine up to 2000 rpm's, the power piston still seemed to be in an  upper position. 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.
If it also helps I've driven it around a decent amount and found I get 11.5 mpg city and maybe 12 on the Highway. Those two numbers being so close is suspicious to me.