[Kenth]

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?

[steve25]

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.

[Aaron65]

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.

[steve25]

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 .

[Verdoro 68]

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.

[Verdoro 68]

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.

[Kenth]

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

[steve25]

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.

[Verdoro 68]

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.

[steve25]

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.

[Kenth]

Try this:

[Verdoro 68]

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

[77 TRASHCAN]

Where did you get your parts?

[steve25]

Good news to hear!

[Verdoro 68]

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.

[Schurkey]

Originally Posted by Verdoro 68

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.

[Verdoro 68]

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…