I run the CCC system on a mild Olds 350. I hadn't much time for serious tuning, but I've been playing with it here and there.

I've been trying to smooth out the rough idle. No matter what I seem to do, the idle is rough when I'm at or near the correct dwell so I decided to tune for best idle regardless of dwell to see what happens. Result? Smooth idle = VERY high dwell reading ( 94.7% ), and the dwell goes down ( to about 68% ) when I give it a quick rev, then goes back to 94.7%.

High dwell indicated ECM is responding to an overly rich condition. OK, so back off on the IAB right? If I do, it idles rough again. Hmmm.

Unplugging the MC solenoid ( full rich ) provides smooth idle. Obviously, my engine appears to like 'rich'.

Tried adjusting mixture screws but haven't tried anything lower than about 3 1/2+ turns out. I figure that turning the IAB out accomplishes the same as turning mixture screws in though. Will try 3 turns out next.

Adjusting rich stop ( and to a lesser extent lean stop ) effect idle quality too. As I said though, I adjusted for smooth idle and not good dwell. I don't know if my current setting achieve the 3mm plunger travel.

It just seems like when I try to adjust things by the book, it doesn't work. I want smooth idle AND proper dwell darn it!

The carbs rebuilt and I understand what all the adjustments do and generally how the system works. Only carb mod is 75 jets were replaced by 76 jets.

 

Many be that one of the cyinders is running at a different Air fuel ratio than the others.
Vacuum leak?

 

Engine is newly rebuilt ( professionally ), so I don't think there's an issue there. I checked about 100 times for vaccum leaks and everything is good with that too.

Engine pulls about 20" of vaccum at idle too.

 

Read the voltage of the 02 sensor with a VO A meter as you tune. A heat gun will show exhaust manifold port temps variations. Indicating cylinders that are running different than all the others, or miss fireing.

Engine is newly rebuilt ( professionally ), so I don't think there's an issue there

you are assuming a lot. Everything mentioned leads to 1 or more cylinders wanting for more fuel.
A intake manifold vacuum leak is highly suspect. Reguardless of how much you paid for the rebuild.

 

I have a scanner that reads volts and crosscounts, but I don't know what's considered to be good numbers or not. What are good crosscount numbers and voltage readings?

O2 sensor is a brand new GM AC Delco unit.

What do you think would be the most likey reason(s) for having 1 or more cylinders wanting more fuel than the others on a fresh engine with proper break-in? I haven't checked the plugs since last year, but when I did, they all seemed equally normal. A tad on the lean side perhaps.

Intake manifold leak was the first thing I checked for. Sprayed starting fluid all around ends and carb base. Nothing. Retorqued intake bolts ( they were all good ). Even asked the builder if he might have made a mistake sealing the intake and he said 'no way'. Of course that doesn't mean it's true...

Wouldn't any major vaccum leaks/sealing problems show up on a vaccum gauge?

 

Anyone?