lean and variable A/F ratio

[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?

[blarson]

The vacuum gauge APT explanation makes a lot of sense.

Part throttle up a hill is bucking so hard that I couldn't make it to the top unless I went WOT.  The gauge climbs lean and off the gauge at any throttle opening past the 650 rpm idle.  Definitely seems to be a problem on the primary side there if float and vacuum leaks can be ruled out.

Unless.... my fuel pump is borked.  I'll double check fuel pressure before I go to the float.  I ran the carb I pulled off the truck before putting this one on with the gauge installed and it was also lean everywhere.  It just never ran so lean as to cause driveability to suffer if you don't count being down on power and poor fuel economy.  Perhaps I've been experiencing a problem there as well.  With that carb I had a vacuum leak around the throttle shafts that I could use carb cleaner on and make the RPM rise so I just assumed that was the problem and didn't consider a fuel pump problem.  I'll throw a gauge on it before I do anything else.  Maybe I'll make a discovery.

The float itself was set to spec for the carb number.  17/32nds if I remember correctly but I'll have to look at the spec sheet when I get home tonight to know for sure.  I don't own bigger jets yet and Cliff seemed to think it was float related so I'll look at those things first and report back.  Main jets sound mighty plausible tho too.

[Shark Racer]

17/32 is a pretty lean float setting. It doesn't hurt to bring it up, and it will move the readings in the right direction.

I think you will need to upjet from your description of driving up a hill.

A fuel pump shouldn't have an impact at idle/off-idle unless it's REALLY on its last legs, then, however, I'd imagine you have an idle issue.

So yes, the float adjustment is free and one you should make anyways. After that, give it a quick go and go with primaries. FWIW, just off idle will not be impacted by the primaries as you'll be on the transition slots and this is determined by the tube/DCR relationship. The hill climb is 100% jet.

[TommyK]

Adjust the float height and richen the idle circuit. It may be necessary to resize the idle tubes by a couple of thousandths.

14.7:1 is meaningless in the context of carb tuning. I don't know where people get the idea that their idle A/F ratio should be stoich. Your idle setting is way too lean and as a result your tslot has no fuel when you open the butterflies, thus the bucking. If you are still having part throttle driveability issues after you get the idle circuit sorted, I would try backing out the APT 1/4 turn at a time before I went to a jet change. If Cliff supplied the jet/rod combo, chances are it is pretty close provided the air bleeds haven't been molested.

You may also want to check for a warped airhorn.

[Shark Racer]

Adjust the float height and richen the idle circuit. It may be necessary to resize the idle tubes by a couple of thousandths.

14.7:1 is meaningless in the context of carb tuning. I don't know where people get the idea that their idle A/F ratio should be stoich. Your idle setting is way too lean and as a result your tslot has no fuel when you open the butterflies, thus the bucking. If you are still having part throttle driveability issues after you get the idle circuit sorted, I would try backing out the APT 1/4 turn at a time before I went to a jet change. If Cliff supplied the jet/rod combo, chances are it is pretty close provided the air bleeds haven't been molested.

You may also want to check for a warped airhorn.

Taken out of context, the 14.7 line is a bit off. Yes, it's meaningless in idle tuning but it's important at areas where signal is improved, especially if you have to deal with emissions. 14.7 is the "sweet spot" for CO and HC emissions, and EGR can control NOx.

If emissions aren't a concern you can tune cruise into the 15s to eke out some MPG, being mindful of what the engine wants, of course...

I have found that my engine will not idle any leaner than mid 13s before it starts to have the occasional miss. Off idle and cruise it will tolerate a lot more.

[blarson]

I spent some more time with it and discovered a few things over the weekend.  Fuel pressure was 7 psi at idle but dropped all the way to 2 psi at part throttle.  I think I might need another fuel pump based on part throttle but I can't say that for sure because I've never seen specs for anything off idle.  Can someone answer that one? 

It appears that I was right about 17/32nds float height.  I adjusted it to 1/4.  I'm sure that will help considerably.

Idle not returning to... idle.... appears to be the work of sticking throttle shafts.  I was able to remove the accelerator cable and after snapping the throttle rpm would stay high.  If I pulled the throttle arm closed with my hand it went back to normal.  I bought a bushing kit so I'm tearing it down to the baseplate to install that.  I'm a bit surprised because it really did feel like the throttles moved completely free when I had the carb apart.

I'm going to guess that the variable A/F after throttle snap is down to a combination of leaky throttle shafts and potentially the power piston being hung up.  The piston itself felt like it moved free to me but since I'm already there I might as well polish up the power piston's home. 

I'll report what I discover once those problems are resolved.  I hope someone can answer the fuel pump pressure off idle.  Thanks again.

[blarson]

I pulled the base plate apart and did the primary throttle shafts last night.  They weren't binding at all like I found before but I did them anyway which is probably for the best because I did find some wear in the shaft once I had it all the way apart.  I lubed the shafts up with some 3in1 oil and put it back together.  The secondaries were quite lose but I didn't worry about them per suggestion in Cliff's book.  If they bind again on a throttle snap I'll just go with a heavier return spring because they are completely free to move.   Cliff's self tapping drill bit really made that job a piece of cake.

I double checked the base plate, air horn, and main body while I was doing the shafts to make sure they were straight with my machinists straight edge and they were.  It also appeared that the gasket between the base plate and the main body was slightly compressed equally all the way around leading me to believe that area to be fine. 

My intent is to go to 15:1 for fuel economy through as much range of the throttle as I can get.  I will have to do emissions when I move in a few months so I can't be too far from stoich to pass.

[Shark Racer]

Yes, stick with ~13.5 at idle but 15:1 is a good target for off-idle, cruise.

I need to play with my off-idle a bit because it's in the high 13s and that's making me high on CO.

[blarson]

I'm still lean with a new pump, raised float, 3.5 turns APT, and bushed throttle shafts so bigger mains it is.  A heavier return spring solved the problem with not returning to idle.

[Cliff Ruggles]

Did you raise the APT more than 3.5 turns?  It may like more fuel at part throttle, nothing wrong with going up to 4 to 4.5 turns or so and seeing if it responds favorably....Cliff

[blarson]

Up goes the APT.  Is there such a thing as a max APT height?