Still Trying,

Your compression test data is excellent, within 10%. So that rules out valve-lash.

I hate to see you chase new parts chasing an issue. You have run some tests but you could try one more.

Your issue & complaint is: Low Temperature On Two Cyls Header. First, sometimes those thermo guns can be off. I have seen a differential of 200+ degrees on my header pipes depending on where you aim the pointer laser dot.

You also have oily plugs. Is that because they are not getting a fuel wash on every cycle? IDK.
Or is it because there is not 50,000 volts at the plug? IDK

However, I think I would hook up a Timing Light at the alternator lugs. Then with engine idling, hook the timing light up to #8 plug wire. (You can do this near the distributor cap) Pull the trigger and monitor the strobe for a half a minute. Then do cyl #5.

Normal flash? Erratic?

 

Are you running the oem distributor and ignition system or an aftermarket set up?

 

This probably has nothing to do with your problem, but I remember this thread from 2021 that had 2 cylinders not firing, 5 & 8.

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

 

that would do it....

 

THat would be the leak I couldnt believe would exist

 

I don't see how the vacuum port on the number 8 Intake runner is going to make number 8 & 5 cylinders header run cold.
If there was a leak at that vacuum source, doesn't a lean condition run hotter?

When you think about it, the entire Intake is under a negative pressure or vacuum regardless. If you took a vac reading at the #8 runner or a port at the carb base or at a port off the front of the carb or even at the distributor vac can, it should all be the same amount of vacuum.

 

Yes but the vacuum at the front cylinders could be strong enough to draw fuel in that direction while the rear is sucking on the vacuum port. If you have an unknown issue and you tune your motor to hide it is possible it will only show up in a specific area.
Anything how ever improbable, is possible. I still lean towards a dizzy issue but this is easy enough to debunk

 

do you have one of these

 

Did you OHM out the plug wires and check resistance?

 

Generally speaking - I try not to say "can't be". Murphy (of Murphy's law fame) is just way too good at fooling us...

So - back to Combustion 101 - we need three things to make an internal combustion engine work.

Fuel (at something approximately the right ratio)
Spark (at about the right time)
Compression

Your compression test numbers are good - so we'll put that on the back burner for now...

So - let's start with spark. You said you switched the two plug wires for the cylinders that appear dead. How about switching wires from a cylinder that is performing properly to the dead cylinders. And while you're at it throw in a new spark plug. Check the cap and rotor - and look for ANYTHING abnormal. Check terminals for corrosion, Have someone else crank it while you watch the spark at the plug - does it look nice and robust ? (It could be something like a coil going bad - and cylinders 5 & 8 need just a bit more voltage to light off - and the coil just cant deliver quite what Cylinder 5 & 8 need.)

Now let's move on to fuel. If I were you - I would hook up a vacuum gauge and see what you get while idling. Then check it at about a steady 2,500 RPM ??? If it's more than a little low at idle compared to mid speed (especially if the needle is moving rapidly) - i would think vacuum leak - would also repeat the spray around the various areas - looking for vacuum leaks - but use something a bit more flammable than WD-40 (like Flammable Brake Cleaner). But - you could have an intake manifold gasket leak at one of the lower points of the intake - and there is no way that you'll be able to get spray in there - so low vacuum at idle that you can't find a reason for (meaning no leaky hoses, probably means it's time for an intake manifold gasket change. While it wouldn't be my first area to look at - it certainly isn't impossible for it to be carb related. If you look at dyno data - Air fuel ratios vary among the cylinders. That is especially true at both low load low speed and at high load high speeds. Your carb could be a bit lean - and due to the way the manifold flow works - cylinders 5 & 8 are just a bit too lean to fire. Do you have access to a carb that is known to be running good - if so - swap it, and see if the problem changes. If not - it might not be a bad idea to pull the carb, and give it a good spraying out with carb cleaner - particularly the idle circuit - including the mix screws.


Finally - even though you compression numbers are good - there is still a possibility there is an issue. Compression is always tested at cranking speed - but the slow rotational speed mean there is no real airflow requirement (you can verify this by doing a compression test on a cylinder with the throttle closed - then repeating the test with the throttle wide open - when I've done that, the throttle open compression is typically not much different than the throttle closed compression because even though flow is dramatically restricted - there is relatively lots of time to et the air to flow into the cylinder before the intake valve closes. You already pulled the valve covers and checked to see that all the rockers are moving approximately the same amount - so the cam is not "wiped", but if you hadn't taken that extra step - the compression test alone wouldn't have shown that.

 

For what it’s worth a few years ago I went through this with 3 cylinders not firing.
what I found was 3 small magnets missing on the petronics unit I had.
a new piece fixed the problem.

 

How about some pictures with the air cleaner assembly removed?

There are a lot of smart folks paying attention to this thread, perhaps, they'll spot something obvious.

The manifold vacuum port is tapped into the 5 8 plenum run, isn't it? Is there a vacuum leak there? That would be neither a carb problem, nor ignition.

 

From the link I posted in post #23:

 

Hopefully the OP reads that thread! That could be exactly his problem.

Not sure he wants to drive around without brakes, but it would be easy to plug those ports and see if the engine goes away, at least in the driveway.

 

I am glad valve train and compression are good.
the vacuum leak idea seems possible and the thread about it is good info.
Hope you find the issue, dist or vac leak.

 

Not a fuel/runner issue.....if a vacuum port was open in the back, 2 and 3 would show different temps as well....maybe not to the point of the rear two....but they will be different than 1, 4, 6 and 7.......
Look at ignition......

Jebby

 

When I removed each lead from the distributor cap there was a large spark, however, we’re they different fro cylinders 5 & 8 I couldn’t really say.
The laser temperature gauge is not the most accurate but I pointed to an area approximately one inch from the head itself on all of them, also, even after thirty seconds or more on idle I could still touch number 5 header pipe quickly and number 8 header pipe easily, whereas the others were too hot. (I have a large blister where I tried as evidence). I will give your suggestion of using a strobe light once it is back together.

Running standard ignition throughout.

Thanks for the link, I will give that a go when it is back up and running.

I will attached the vacuum gauge when it is up and running again.

No I don’t have one of those, maybe time to go get one.

No I didn’t do a lead resistance test as for starters they are new but also I swapped the leads from various cylinders just in case and no other cylinder altered, just the same two.
Thanks for the in-depth reply, I have ordered a new rotor arm, distributor cap and coil, which if nothing else will eliminate them from the equation. If nothing changes then I will remove the intake and do a reseal, would do that now while it is down but want to know what the actual cure is rather than do loads of stuff in one go and it work fine afterwards but I didn’t get to the bottom of what was causing it.

All standard ignition but will check everything under the rotor arm to see if everything is in place and working correctly.

Have read that link also happy to drive without a booster around our lanes to check.

I would like to find the actual issue, so hopefully someone else in a similar situation can read this and get an answer.

I am looking at every option, ignition should be sorted whenever the parts I have ordered arrive from the states.

 

I’d wait until your new cap and rotor come in before chasing anything else. I have a feeling #5 & #8 have some issue on your existing one.

I bought a brand new HEI dizzy a couple years back, and the cap to rotor gap, or lack thereof, was lousy. 20 min of tweaking the rotor contact so it just cleared them all was time well spent.

 

The new parts arrived and so have been fitted one at a time, firstly the distributor cap, no change, then the rotor, no change, then the coil, still no change. So that basically eliminated the electrical side of things.

I then clamped off the brake booster hose, still no change, at this point I decided to remove and plug any vacuum hoses, still no change, the only thing left was the hose to the PCV on the rocker cover, WELL WE HAVE A WINNER!!!!!!!!!!!

Clamp the PCV hose and it runs smoother, not 100% as the temperatures on the exhaust headers still differ but now all within about 20% of each other, however, I need to recheck the carb settings so that may well help. The idle is smoother and it is definitely firing on all 8 cylinders.

I may have done a school boy error in that I connected the PCV to the port on the number 8 runner along with the vacuum for the lights etc. so I will see if I can get some hose and connect it to the base of the carb at the front, if that makes it run rough again then I need a new PCV although this one is brand new, for the moment I will just let the engine breath and bypass the valve.

Hope this thread will be helpful to someone else with a similar issue, so to recap: Not the carb, as LARS stated at the beginning but sort of related, the carb still needs a bit of tweaking to get it running a little better but it’s not far off as it is. MAPMAN’s manifold diagram highlighted a common point for the two cylinders which helped to explain roughly what could be the issue and indeed was the connection between the two cylinders and the link posted by 71GREEN454 was very useful.

I really appreciated EVERYONE’s input as without people giving me ideas as to where to look, no matter right or wrong it helps to eliminate things, so a big THANK YOU to you all.

 

In your first post you stated you have an Edelbrock Torquer intake dual plane with a Quadrajet pattern. The Torquer is a single plane with a square bore pattern.
Exactly what intake do you have?