No fire in cylinder?

[1BAD68]

I noticed that my engine (318) is idling rough, so I started pulling wires and found that #8 and #6 had no change in idle when pulled.
I did a compression test and have between 160 and 150 psi on all 8 cylinders, I checked for spark and it has good solid spark.
I switched the spark plug and it didn't help.
I can actually hold my hand on the #8 exhaust header tube while its running but #6 is very hot like the rest of them, any idea's?

[mikepmcs]

Wire, dizzy cap?

[1BAD68]

I went out there after supper last night and pulled the valve cover and started it up, everything appears fine.
Then I noticed that when the rpm's are above 1100 (when warming up with choke on) #8 is firing, the exhaust tube gets hot really quick but when it goes back to idle under 1100 rpm's  it cools down again. All the other exhaust tubes stay hot except #8 when idling.
I'm starting to blame the dist. cap or coil. Anything else I should be checking?

[firefighter3931]

Have a look at the reluctor and see if any of the pointed stars are more worn down than the others. Check the gap at each point with a brass feeler guage...shoot for .008 gap. Sometimes when the reluctor gap widens the spark will weaken. Run a resistance test on that ignition wire (ohm meter) and look closely at the #8 tower on the cap.

Pull the plug from the head and reattach to the plug wire and try grounding the plug to a manifold/header and have someone fire the engine to check for spark. It should be blueish white. If it's dull orange there is too much resistance somewhere.


Ron

[Steve P.]

Can also try switching a pair of wires. Known good one to #8.

Is this a solid or hydraulic cam??

How much oil pressure at idle vs. 1100 rpm??

[1BAD68]

It doesn't have the reluctor like you described, it has a Pertronix ignition. I ohmed the wires and they were all really close to the same, I ohmed the plugs and got the same result although as they cool the resistance goes up... strange.
I checked the spark like you suggested Ron and its nice and blue/white, so I bought a new dist. cap but it didn't change.
Its a hydraulic cam and I have about 30 psi oil press at idle when fully hot to about 32 at 1100 rpm, btw it idles about 800 rpm.
Now I'm starting to think that the fuel isn't reaching that cylinder when at idle for some reason. It has a Edelbrock Performer intake with a Edelbrock 600 carb.
I drove it around yesterday and felt no loss of power or performance only a stumble when at idle.

[Steve P.]

When you pulled the valve cover, did you try to measure the lift of #8 valves  vs. others??

I am thinking maybe a partial  lifter collapse at low oil pressure.


Carb and intake should not be an issue unless you have a vacuum port in that runner and it is pouring open air to that cylinder.. Don't laugh. I have seen this more than once....  I do not like a runner port being used at all..