I have a 350 with an Edelbrock 1405, and HEI. It is running rough at idle. The number 2 and 7 cylinders don't seem to be firing all the time. The header pipes for these two cylinders do not get as hot as the other six do. When I give it some more RPM, around 2000-3000, it smooths out and the 2 and 7 pipes get hot. So, I've pulled the new plugs after running for a bit and the 2 and 7 plugs are black with carbon. So far I have checked firing order which is correct, replaced plugs and wires, checked for vacuum leaks, cleaned and rebuilt the carb, switched out the cap and rotor with a known good set. I have also pulled the valve covers and checked that the valves are adjusted properly and are opening and closing the same as the rest.

Does anyone have an idea as to what is going on?

My next guess is to pull and re-install the intake manifold incase there is an internal vacuum leak.

 

Did you put an intake on it?
How did you confirm valves were adjusted properly?
Try starting the car at night in a dark garage and see if the ignition has any fireworks going on..(electrical crossfire).

Jebby

 

As far as the Intake Gasket leaking, usually that is two cylinders side-by-side. Or at least two cylinders on the same bank of cylinders and THAT gasket was not sealed properly. The odds of your two cylinders, 2 & 7 make it less likely, but maybe.

Plugs will blacken when they are not firing hot enough to burn off deposits. It's a keen eye that figures out if its oil, gas or coolant fouled. (sometimes ATF)

Intake gaskets that may leak are almost impossible to detect. Some people spray an assortment of aerosols around the Intake gaskets listening for a change in RPMs.
However, if the bottom edge of the gasket is leaking no amount of spray will ever indicate that issue. The only test for that is to use a vacuum gauge. A gauge needle that bounces is usually a sign of an invisible Intake gasket leak. Every time your cyl 2 & 7 are on the Intake Stroke the gauge needle will plumet IF they are drawing air from the lifter valley instead of thru the carb.

Anytime I read that someone just changed all the plug wires is a red flag to me. 9 out of 10 people will install cyl #5 & #7 wires incorrectly.
Due to the firing order, those two cyl fire very close together, plus share a common plug wire loom. Make sure those two are as far apart as possible.

I wouldn't get to jumpy about pulling the Intake yet. Keep looking.

 

Did you replace the spark plugs? You did the wires, cap, and rotor so if you do the plugs you'll have eliminated virtually all electrical possibilities. I would definitely try spraying carb cleaner around your manifold. Yeah, if the leak is on the bottom side of the gasket it won't work but it might not be. Did you test compression? How about a leak down test to see if it's valves. It may be worth paying to have it scoped.

 

Could be as simple as a defective cap and rotor. Start with stupid simple and work from there.

 

Update:
Just to clarify... These symptoms existed before I touched anything. Nothing I have done has change the situation as of yet. I have not done a compression test. I did put a vacuum gauge on the baseplate of the carb this morning. The driver side port has good vacuum 45-55 in. hg. The passenger side port is only reading 20-25 in. Hg and doesn't seem to respond at all to adjustment.

 

The vacuum numbers are wrong.....where are you getting the numbers from? Outer Space is 30hg vacuum.....it cannot be more than that, a bone stock V8 with a tiny cam will pull 20hg....what "port" are you hooking this to? Vacuum readings are taken at one port in the intake manifold......generally in the back, or the port off the carb in the back.....what are you adjusting?

Jebby

 

This is the gauge I'm using and the ports I'm connecting to I've marked with arrows. the adjustment I'm talking about is the idle mixture adjustment screws.

 

The ports you're using aren't "left & right vacuum ports." The one port is manifold vacuum - the other port is ported vacuum. You can't use ported vacuum to check manifold vacuum. And you're reading the metric scale on your gauge- in America we use inches...

Lars

 

My bad so the passenger side port (ported) is reading in the 7-10 In Hg and the driver side port (manifold) is reading is reading around 18-19 In Hg. Do those seem to be where they should be?

 

Actually...there should be zero on the ported vacuum at idle...which means you are not at idle but slightly higher and the transfer slot is not exposed.....only way the ported vacuum can have vacuum of any kind at idle is if the throttle plates are open too far.....
The 18-19 number is what you want....when you hook your vacuum gauge up, does the needle bounce around a lot?

What is your timing set at? Take a picture of the engine without the air cleaner installed....

Jebby

 

I have the idle set high because two of the cylinders don't seem to be firing below 2000 rpm and the only way to keep it running somewhat smoothly is to have it idle around 1000rpm. The needle is very stable when the gauge is hooked up. I'm not sure what the timing is set to. I did loosen the distributor to see if see if I could get it to smooth out, but no luck.

My thoughts are it could be leaky intake ports causing the AFR in those two cylinders to be off, but I'd also think that would make those two cylinders run lean which would mean hotter. Which doesn't explain the cooler exhaust temps or the carbon on the plugs. Or could there be such an intake leak that almost no fuel is getting there? Or low compression in those two cylinders. Am I on the right track?

 

First thing I would do is either do a leak down or compression test of the motor. You could have a lot of things going on here. Let’s get some numbers and go from there.

 

No you are not....you do not have enough information yet to make any clear diagnosis....
First....loosening the distributor and moving it around to see if it smooths out is the worst thing you can do....you need to set timing....with the vacuum advance hose unplugged using a dial back or digital timing light....you should have 36 degrees total and around 15 at idle....but you can't check idle because your car won't idle.....you set total timing by revving it around or past 3000 rpm....
The reason the idle screws do nothing is because you have the idle set too high....so you can't set it right now either....
So you have air/fuel, ignition, and compression to check.....first thing to do is check compression on all 8 and record.....
Get an inline "window" style spark tester and check spark on all cylinder....they should all be the same strength of spark.....
Last thing to do is run an unlit propane torch and fog gas around the intake flange....see if the idle changes.....

Until you perform these procedures....you are 100% guessing....

Jebby

 

I'll let you know what I come up with. Thanks for your advice so far.