Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
lean and variable A/F ratio
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:
--- Quote from: TommyK on March 02, 2013, 06:58:57 AM ---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.
--- End quote ---
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.
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