Could this be the slipping clutch at speed?

 

No, it’s definitely not clutch slippage. Thanks for all of the suggestions though. Having owners (more experienced than me), is a huge help as I sometimes overlook the obvious.
KC

 

If he wants vacuum advance all the time, he'll have to bypass the CEC Solenoid. If the solenoid is working it only has vacuum advance when the transmission is in 3rd or 4th gear and also for 15 seconds at startup.

"Combined Emissions Control System:
The heart of the CEC system is a unique solenoid that was mounted on a bracket attached to the carburetor. The CEC
solenoid (GM 1114444) was unique because it provided two functions. First, the solenoid had a plunger shaft that
would extend or contract based on electrical input. In its energized state, the solenoid’s shaft extended to contact the carburetor’s throttle and INCREASE idle RPM. This gave engines two curb idle settings dictated by the CEC solenoid. Second, the solenoid had an internal vacuum passage. When the solenoid was energized, the solenoid’s two vacuum nipples were joined causing vacuum into the solenoid to be routed out of the solenoid. When the solenoid was not energized, the passageway between the two vacuum nipples was blocked to deny vacuum to advance spark timing."

 

That is a very specific RPM. Sounds like a lean bog to me.
That is about where the cam overlap kicks in and changes the carb tuning,
It could be the Qjet transitioning to main jets and having a hard time.
Do you have any AFRs from the dyno?
Especially in that zone?
Lars tests it on a running motor, but it is not a running 500HP BB, and that could make a difference.
He would know.
And the AFRs will tell you.
Or somebody touched and "adjusted" the air valve after Lars set it correctly.
That is also about where the air valve starts working, under load, but only on a dyno, or in car, and under load, and WOT.
It is very important to tell us if it is doing this "bog" under part throttle or wide open throttle.

 

Thanks for all of the suggestions, I need the outside thoughts to get on the correct path. I could take this to a shop to sort out but need to learn these systems and develop the skills anyway. Thanks for mentioning the CEC “solenoid”, this was something I was going to sort later not even knowing it ties in with vac advance, I read the article on CEC from the NCRS and have an overview of how it works. If I’m correct in my understanding, there is no advance until third and fourth gear? Taking off in first and second with no advance? I didn’t even think you would have decent performance? More reading on this for sure but I may have to leave this in place and bypass.

Concerning my original QJet, I had it rebuilt by Lars about seven years ago and ran it on the dyno with the fresh engine. It had great throttle response and the AFRs were spot on. The car sat some so I sent it back to Lars about six months ago to be rebuilt again. The carb hasn’t been touched since it left his shop and I wouldn’t know what the AFRs are for sure now. Lars had all of the original documents on this carb so I’m guessing it was setup the same as before.

I’m going to do a road test with the CEC by passed and see what that does. It is noteworthy that when I set the timing (car warmed up) that the timing advances correctly (35* in by 2800rpm). It’s my understanding that this shouldn’t be advancing after 15 seconds if the CEC is installed and functioning correct?
KC

 

This last highlighted part makes me wonder if your timing is set as expected.

At best the CEC can delay or eliminate your vacuum advance. All-in, 36 (or so) degrees of MECHANICAL advance by 2800-3000 rpm should be correct. Is your vacuum advance can disconnected during this test? Do you have a Mity-Vac to confirm that your vacuum advance is working properly, and independently?

 

The distributor was sent off about seven years ago to a reputable shop for overhaul. It is advancing as described but I haven’t checked it with a vac pump and not sure how to.

 

It is possible your distributor is not installed correctly.
The procedure is to UNPLUG the vac advance and set the timing to 35* That is the mechanical advance.
Then plug the vac adv back in.
Depending on the "can" it will add another 10-16* when it comes in.
That is on top of the 35*.
The CE only turns the vac adv on or off, it has no affect on the mechanical adv in the distributor.
When they are added together you could have somewhere around 50*, but only at 3000rpm, with almost no throttle, ie: "cruise".

I think I have a paper on the CEC somewhere, I'll go look for it.

 

Once you have the mechanical advance nailed down, you can use a Mity-Vac to manually pull the vacuum can. This can tell you the max vac advance available, and confirm that it is working at all. But the mechanical advance must be done first, with the vacuum can disconnected (and the vacuum leak plugged, though that won't affect timing).

Here's a post where I was bench-testing an adjustable vacuum can. You can do the same on the car with a dial-back timing light, or timing tape.

 

Leigh1322, it’s possible that the distributor is installed incorrect but there is very little room for error with the tach drive. I’m not really sure what error I could have made. I’ll try to run the vacuum test with the Mighty Vac. Yes, it’s looks the weights are moving back to the original position and I can see the timing come back to “initial” timing with the light. I think some of this info is going to get this resolved but I’m still studying the CEC and related components. That’s a bit to take in and then I’ve got to check all of the related wiring, temp probes and transmission switches related to the CEC. From my initial study I’m wondering if I may end up having to leave the CEC in place and bypass it? More study.
Thanks to the member that brought up the CEC but it has raised a bunch of new questions and decisions.
KC

 

whats your total with the vacuum?

 

For now I would bypass the cec and get the car running well. That you can fix later.
On timing just confirm that you set it to 35* with the vac can unplugged. Then you're good.

 

Found the cec paper. Attached
There is also a good thread titled 1971 CEC and AC research by 71 green454 but I do not know how to do a thread link while on my phone.

https://www.corvetteforum.com/forums...-research.html

 

It’s currently at 35* all in by 2800 RPM with the vacuum unplugged.

 

I’m still waiting on the new fuel pressure gauge and fittings to show up. I got a few minutes today and did a few checks on the CEC solenoid. The solenoid is clicking at start, third and fourth gear but not moving. I did bypass the CEC vacuum lines for now, more work on that later.

I mentioned this earlier in the post but it seems like my engine is not burning fuel on one cyclinde( I smell like fuel after a test drive). I did remove the plug wires one at a time but didn’t notice any difference between the cylinders. I removed the valve covers and everything is in place and all valves are moving properly. I have also tried three different plug wire sets with two of them being new. I’m going to remove the plugs at the next opportunity and see if any of them is fouled.

This Vette doesn’t ever smell like fuel when sitting in the garage, only when driving. Never any fuel leaks noted.
KC

 

" but it seems like my engine is not burning fuel on one cylinder"

You should use a cheap infra-red temp gun on the exhaust manifold ports just outside of the head.
That will very quickly tell you if you have a cool / under-performing cylinder, and which one.

These things have gotten so inexpensive everyone should have one!

Link

Link

 

The heat guns are awesome! I bought when I was trying to get my temp gauge calibrated. I did check the cylinders right after start up and they did seem pretty close to each other.

Huge update though, I noticed the vacuum on top of the #8 cylinder and decided to plug that off and do a quick road test. This made a really big improvement on the low end (off throttle) and mid range. There is still something going on as you can feel moments that the engine really wants to “turn on”. My next move is to remove the plugs and see how they look.

Not sure if the vacuum leak caused a rich or lean area over the # 8 cylinder or several cylinders possible? I’m not super strong in that arena (air fuel mixing). It does make sense that it ran so strong on the dyno though.

 

I got my new fuel pressure gauge in and did a test. The gauge is showing 3.5 psi at idle and 6 to 6.5 psi at 3000 rpm. It’s my understanding that this is a bit high for the Quadrajet carb. Could this be causing the loss of acceleration at 2500 to 3000 rpm?

I would like to see what it’s doing (pressure) on a road test but I don’t see a great way to do that. I welcome your input.
KC

 

Update:
I set the fuel gauge up where I could view it while driving, the pressure is about 4 psi at idle and 4 to 4.5 psi while driving. There isn’t any large fluctuations under hard acceleration.
Please read the thread so we don’t have to go over every item that has been checked out to this point. For those that have followed, I’m thinking about sending the distributor back out to be looked at. I can’t find the receipt but think it went to Lars about seven years ago with only a few miles on it. Everything appears to be working on the distributor but I don’t know what else to check at this point.
I welcome your input!
KC

 

Perhaps you are getting ignition wire cross-talk? Looks like your 3 and 7 wires are twisted together for much of their length.

Here's a summary you posted of what had already been done. I'd try replacing the coil with the old one, if you kept it, just to eliminate that.