'74 Ventura 2bbl lean condition
 

The 2bbl (7043062) on our '74 Ventura 350 has an off idle stumble that I've been trying to resolve. It's really pronounced when the engine is cold, so much that it will backfire or stall sometimes, but it's always there even when at operating temp. I figured it was going lean, so I welded in an O2 bung this afternoon and used my wideband to see what was going on.

The mixture is spiking to 15.5-17ish when I step on the pedal from idle (~14.0 AFR). It will cruise ok once you get past that initial spike and otherwise the mixture seems ok throughout the range. I've gone through several accelerator pumps trying to diagnose this. I can see that it's getting a good squirt of fuel immediately so I don't think that's the problem. I'm wondering if I need bigger jets. It's got 54s in it now which I believe are from the factory.

I know most folks have 4bbls, but figured I'd ask since these are similar to the center carb of a tri power.

Does this carb have a power piston for enrichment like a QJ? If so, is it possible you have a metering rod sitting too low in the jet or could benefit from a lighter spring?

Any chance richening the idle mixture could help?

It has a power valve in the bottom of the fuel bowl between the jets that's spring loaded. No metering rods like a QJ.

I had the idle mixture richer (13.4) before I plugged in the 02 sensor and leaned it out.

Assuming the power valve is vacuum-actuated, is there any chance your vacuum has changed? For example, did your brake work introduce a vacuum leak?

It crossed my mind, but I haven’t found any vacuum leaks yet. I supposed one way to test is to disconnect and cap all the vacuum ports on the carb and see if it runs better.

If it's like a QJ, the vacuum holds off the enrichment, correct?

If so, a vac leak wouldn't explain a lack of enrichment, would it? Although if the vac was so low that the valve never closed and you compensated by enriching the idle, then maybe?? I'd say get a vacuum gauge before assuming??

Or maybe it's all about your vacuum advance and the legendary "carb problem is an electrical issue" has snuck under your hood?

Might be worth verifying your vacuum advance can with a mighty vac and a timing light?

It pulls a steady 19 inches of vacuum at idle, but I haven't left a gauge plugged in while we're driving to see what's going on. I picked up a new vacuum advance anticipating this potentially being a distributor issue. Will dig in more tomorrow.

What if anything else was done to the carb or the motor before this problem started up.

Was the carb sitting for months?

If so then the power valve in the bottom of the fuel bowl might be hanging up due to crap getting in the fuel bowl vent or simply from failed broken down jelled up gas.

I would:
1. Check inlet valve for correct style and size.
2. Check float settings.
3. Open idle tubes to .034".
4. Open idle needle holes to .075".

HTH

Does the power valve rod move freely?
I've had to un-stake some to polish the bore and piston.

It’s hard to say when this started. The car sat for 6 years before we got it. We did a bunch of things to get it roadworthy.

I went through and thoroughly cleaned the carb including removing the power valve and cleaning all passages. It moves free. I ensured the float is within spec. I tried two different needle and seats. Getting fuel to the bowl doesn’t seem to be an issue. It just doesn’t get enough to the engine when transitioning from idle.

What will opening the idle tubes and idle needle holes do?

It will give better control of the idle needles, strong idle and more even transition to low-speed.

I forgot to mention what post 10 did.
That spring loaded rod must move without hesitation also, not just the valve in the fuel bowl.
That passage to each booster under the valve could be gummed up also .

And also that whole booster pump discharge cluster could have been put together with the wrong gaskets because as I recall the rebuild kits supply two different ones.

I did some checking and and your 2GC carburetor 7043062 was used in both '73 and '74 and had 7002654 jets (54) like you said and is for A, F, X automatic transmission cars Non-California.

The vacuum plumbing is rather complicated and there are many possible points for vacuum leaks. In addition, hot and cold temp switches can fail or their wiring breaks.

With my daughter's '71 LeMans with the 350 2GC and A/T, it had an off idle stumble from several causes.
The vacuum motor in the air cleaner had torn it's diaphragm and needed replacing.
The vacuum modulator hose at the transmission came off.
The distributor vacuum solenoid plastic nipples had cracks and the plunger did a poor job of sealing when activated. It got replaced.

I'd also check if your EGR diaphragm isn't leaking and it's got a good gasket seal and the vapor canister valving is in good shape. Check your radio fuse since it's used to power the TCS.

Check the throttle shaft for excessive play. The 2GC base is cast iron and usually doesn't experience wear like the quadrajets so I doubt it needs throttle bushings; however, I have seen cracks in the cast iron along the throttle shaft, perhaps from uneven tightening onto the manifold.

On my 455 with a '71 quadrajet, it had an off-idle stumble. Replacing the accelerator pump didn't cure it and no vacuum leaks were discovered. Crazy as it sounds, the cure was to adjust the mixture screws using the shop manual's lean drop procedure. I included those section's from the '74 shop manual for you.

Your 17:1 AFR lean measurement just off idle is a clue that the accelerator pump shot isn't enough to compensate. (Too much oxygen, not enough fuel combining with it in the burn). The idle and off-idle carb slots need to provide more fuel. I'd try the lean drop adjustment to find the spot where there do. Back in the '70's, inspection stations allowed as much as 4% CO and 400ppm HC for '74. Using your wideband sensor, set the mixture at idle to a hair above 13 at the factory idle speed and see if that improves the off idle stumble.

Forgot to mention that I removed all the TCS stuff on the car and have disconnected the EGR.

It’s my understanding that if the EGR does not function then the motor will have a lean condition unless steps are taken to correct for having the EGR disconnected.

If the EGR does not function the inert exhaust gas it would normally let into the combustion chamber won't enter, so the car will be more likely to ping. Although with a '73-'74, with low compression, doubtful. EGR only has vacuum on the carb side of the diaphragm, so if the diaphragm is torn, that would cause a ported vacuum leak if all the thermo switches still work.
An EGR not working in itself will not cause a lean condition unless the diaphragm is torn, which is not likely even on a 50 year old valve. Much more likely that in this case the carb is too lean for whatever reason.

Sounds like the EGR situation may not be helping. Th valve is still installed, I just unhooked the vacuum to it. The thermostat is switch is long gone and all the vacuum lines were rotted. With all the TCS stuff gone, the vacuum advance is now hooked up directly to a ported source on the carb.

I just tested the EGR and it holds vacuum. The vacuum advance does not so there’s a likely culprit.

New vacuum advance didn't fully fix it, but it's better than it was. I am getting a full 36 degrees of advance now, too. I ran it with and without the EGR connected to vacuum. It actually seems to run a little better/richer with the EGR disconnected.

I'll take the carb apart again, remove the power valve and make sure everything's clean under there, then I'll try the idle adjustment procedure Ben mentioned above.

14:1 AFR is kinda lean for a carbureted idle. Connect a vacuum gauge to manifold vacuum, set the idle mixtures to lean best vacuum--achieve best vacuum. Best vacuum can be had with a range of AFR, you want the leanest AFR that allows best vacuum. Then lean-drop the vacuum about 1/4 inch on each of the two screws--about 1/2 inch, total. So a very slight lean-drop from lean best vacuum. A slight lean-drop eliminates a fair amount of tailpipe emissions without really affecting idle quality.

Spiking even higher at tip-in really sounds like an accelerator pump issue--but I am biased. I had perpetual problems with the accelerator pump on Mom's '77 Nova 305 with 2GV. I eventually installed a Cliff Ruggles-supplied accelerator pump which seems to have cured that problem.

If the pump were truly working, tip-in should show a momentary rich condition. We used to watch for a momentary spike in HC emissions when a tailpipe-sniffer (Exhaust-gas analyzer) was hooked-up.

The carb would have been set up for E-0; and I bet you're running E10. So going slightly richer may be warranted...but I don't think main jets are the problem here.

ANY misfire for ANY reason will cause an O2 sensor to report a false-lean condition.

Plugged passage on the power valve? Failed vacuum advance; advance connected to ported vacuum? Weak ignition coil? Failed pickup coil wires on an HEI distributor?

Low CO and high HC indicates a too lean idle/lowspeed circuit.
Disconnected EGR requires richer lowspeed circuit.
No EGR also requires less ignition vacuum advance. Aim for 16°-20° instead of 25°-30°

Did i mention opening the idle tubes?

So did replacing that failed vacuum advance get you to maybe 50% better?

Don’t forget that same issue can take place with the vacuum operated stuff in the dash.

My '74 Firebird (2GC) was very lean on the idle circuits when I bought it. At 50-55 mph cruise, it was running 16:1 or leaner AFRs, and I could feel a light surge (I do have the EGR blocked off). As Kenth mentioned, I used pin drills to open up the idle tubes and the idle feed restrictors (both are located in the venturi cluster) a little at a time until I was satisfied with it. Every car's different, and you can't go back once you start drilling, but I wonder if the '74 carbs were set up super lean for emissions. They did some interesting stuff in '74.

Be careful if you go this route, and check everything else first, of course.

You can solder closed brass tubes that might have made too big and start over again, but it takes careful use of a propane torch.
Just be sure to write down what size you started with .

The new vacuum advance made the spike go down to around 15:1, so it helped, but I'm looking for that momentarily rich dip that Schurkey mentioned. I did adjust the idle down to 13.5.

I'm out of town the next couple days and my son is using the car as is daily so I'll jump back in later this week. According to him, it's running much better today but IMO the bar was low. I'm hesitant to modify the carb by drilling out passages until I've tried everything else.

Got the carb parts today and installed a fresh kit along with a new power valve, power piston and accelerator pump with associated springs. I made sure all the passages were clean and went up two jet sizes to 56s. The old power piston and valve moved, but not as free as the new parts. I’m sure that was a bulk of the issue.


After a quick test drive, it seems like a big improvement! There’s still a very brief lean spike to 14.5 if you stomp on it, but you can’t feel it like before and the mixture quickly adjusts to 12.5 or so.

The big test will be tomorrow morning after sitting overnight and completely cooling off, but I’m optimistic we’re on the right track.

Increasing main jets size does nothing for idle and off-idle, it will only increase fuel consumtion.

It is the lowspeed jets that needs to be opened to cure idle/off-idle issues.

And, it is the lowspeed circuit you´re running on up to 30-40 mph, after that the main system starts to deliver A/F mixture.

FWIW

In terms of that brief leanness make sure that the accelerator pump linage is set so that when you even breath on the gas pedal a shot of fuel is produced.

I didn't expect the jets to change anything at idle. While it's not overly lean during regular driving, since I had it apart I did see some room to richen it up a little.

This morning it still stumbled and sputtered when cold. l may need to adjust the accelerator pump rod to tune out that lean spot. We may need to look at the choke adjustment again, too.

With a stock motor once the motor starts the choke pull off should not pull the choke open by more then 1/8”
At least for a starting point, it may require a larger opening but the 1/8” should keep it from just cutting off from being too lean.

Try this:

I think we pretty much have this solved now, thanks for all the help. The car runs much better when cold after some choke adjustments. The choke was opening way too early when the engine was cold, so I adjusted it a couple notches to the rich side. I also adjusted the choke rod.

I think the jets I put in have the car running a little too rich now, so I'll probably dial those back.

In summary, I had a few problems causing the lean spot:
- Bad vacuum advance
- Sticky power piston
- Choke opening too soon

Where did you get your parts?

Good news to hear!

Mike's carburetor parts. He has a lot of 2bbl videos on YouTube that were really helpful. The kit was good quality, but not much different than what I got from O'Reilly other than it being more curated for my specific carb.

Part of the challenge is getting my son acclimated to driving a worn out 50 year old car as a daily driver. He's flooded it a couple times trying to get it started and has had to get used to giving it time to warm up in the morning.

If that carb is adjusted properly, you wouldn't need more than twenty seconds of "warm up".

Start the engine, fast-idle stabilizes. Tickle the gas pedal to drop the idle cam to the second-highest step. Drop it in gear, drive away cautiously.

I finally had some time to fire it up and give it a drive from cold today. Fired on the first crank - easier than my GTO! (Been following the fuel evaporation thread.)

No need to warm it up for an extended period anymore since the choke is adjusted, and the hesitation is gone when driving. I’m calling it good.

Now if I could just figure out where the trans is leaking from…