Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

lean and variable A/F ratio

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blarson:
I believe that I may have some issues with my throttle shafts causing an internal air leak but before I come to that conclusion I was hoping to get a second opinion.  I'm using a AFM UEGO gauge to help tune.

The idle right around 14.5-15.2 and is pretty smooth.  As soon as the throttle moves off idle the gauge climbs over 18:1 and pegs out.  If I try driving the truck at part throttle it starts bucking and tries to die until I slam it WFO.  At WFO the A/F ratio momentarily richens up with the pump shot then stabilizes lean around 17.8:1.  Abruptly lifting the throttle results in some intake popping before returning to the 14.5-15.2 range at idle most of the time.  Sometimes after lifting the idle speed returns to around 600rpm and some times 1200rpm seemingly at random.  Sometimes A/F ratio richens way up at idle to around 10:1 and other times it goes to around 12:1.  The random nature of the A/F ratio and idle speed after lifting is what leads me to believe vacuum leak but I wonder if it's possible that I have a air bleed blocked somewhere....

The crazy thing is that there doesn't appear to be any play in the throttle shafts when the throttle cable is hooked up and only a small amount when it isn't hooked to the throttle cable.  The cable itself has a smooth action while disconnected from the carb.  I also checked the throttle shaft for binding and the butterflies for proper operation while it was apart and everything appears to be fine.  The carb itself is a 17085003 with APT.  The primary needles were corroded so I bought new primaries, jets, and secondary needles from Cliff.  That result being that I'm quite sure the calibration itself is fine.  I also installed the new idle tubes that came with my rebuild kit.  The APT is adjusted 3 turns out.  I also confirmed the choke as being all the way open and the float height as being at the stock setting.

Any ideas?  Thanks.

Shark Racer:
I'm assuming WFO = Wide F"reaking" Open... if so, 17.8 is WAAAAAAAY too lean for high load conditions like that and you need to discontinue until you solve that condition.

First off, I'd try richening the idle up just a bit. It's really hard to maintain a stoich idle, aim for mid-high 13s and it should stabilize.

I'd start by figuring out your AFRs in medium throttle condition - if you can find a hill, go to it at steady throttle and then as the slop begins give some medium throttle. This should get you into the mid-high 12s. If not, you need to go up a jet size.

Once you've done this, you can set your APT setting, mid 14s to mid 15s should be a good target.

Finally, you can start tuning WOT. The primary circuit will be spot on and you can tune secondaries via the rods...

Judging from your description of the issue, it sounds like you're either way too lean on the primary jetting, or there is a vac leak somewhere.

To help Cliff out, you should post sizes of your bleeds, jets, etc.

blarson:
Correct, WOT = WFO,  I think I might have picked that up from the motorcycle world but I can't really remember.  I never remember who uses what term to describe the same thing these days.  Don't worry, I promptly parked it following that little experience.

Jets: 71
Primary rods: 44
Secondary rods: DA

The application is 77 K20 Chevy for towing and very light 4 wheeling.  The engine is somewhere close to stock 350 but I've never had it apart since I bought the truck to confirm.  It doesn't sound like it has a hot cam.

Does APT control mixture at all throttle openings or only during light throttle application?  I could conceivably see a situation where the vacuum signal isn't strong enough to pull the piston against the APT screw past some measurable throttle opening before WOT.  Thus leading to a rod/jet issue if vacuum leaks can be ruled out perhaps?

blarson:
Just got off the phone with Cliff.  He says raise the float to 1/4 and try again.  I'll post back what I discover.

Shark Racer:
If you had a vacuum gauge hooked up, APT would follow peak vacuum. In other words, max lean occurs when the vacuum gauge is at its peak reading. As you increase throttle (load), the vacuum decreases and the mixture trends towards mixture. There is a taper on the rod so it will gradually travel towards power enrichment.

Depending on your APT adjustment, your max lean could actually cover a larger range of vacuum if you've backed it out quite a bit - because it's basically vacuum vs piston spring, with maximum lean limited by the pin on the piston against the APT screw.

Pulling a hill with medium throttle will likely have the same effect as going WOT on flat ground, but should only engage the primary circuits. This is an easy way to test your primary jetting, the alternative is to lock the secondaries or disconnect them from the primaries (the former is doable but you can accidentally lock the secondaries OPEN and that's a lot of excitement, the latter would require disassembling the throttle body).

Once you've tuned the primary you can then go into the secondaries. I suspect you'll find you're lean on both. My guess is that you're QUITE lean on the primary side and a little lean on the secondary sides, but the secondaries are getting enough fuel to get you back into the readable part of the gauge.

I might try going up two jet sizes. You will probably need to tune idle fuel as well.

What does your WB tell you when you hold the motor at roughly 2K in neutral? 1500? I'm assuming idle is ~700.

*edit* Raising the float will richen you up everywhere, which will def. help. Where was it at?

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