Why do I have a Dead Cylinder?

[68.Charger]

I am trying to figure out why I have 1 dead cylinder. I have a 70 Road Runner with a stock 440 6 bbl. and #8 cylinder won't fire.

I pulled the XR5 spark plug for #8 and its very black and has build up all over it. The rest of the spark plugs look nice with a clean tan appearance on the porcelain. I did a compression check and its about the same as the other cylinders ~170psi. I pulled the valve cover and bumped the engine over and the valves seem to be moving the same as the other 3 with nothing obvious looking out of whack. With the engine running I can pull the spark plug wire for #8 and the rpm's don't change - but they will drop for the other plugs. When I am removing or inserting the #8 plug wire from the cap, I can see and hear the spark arcing to the plug wire brass end piece. I replaced the plug with a new one and no change. If I pull the plug and ground it with the spark plug wire attached it is arcing across the tip to ground.

So it seems I have compression and spark so why won't it fire? I used a thermal gun to check the exhaust manifold temps from a cold start and sure enough number 8 lags way behind the other 7.

What am I missing?

 

[pnora]

Vacuum leak. Look that runner over close or any ported vacuum items on that runner. Wire or cap would be next.

 

[pnora]

Look at the back side of the intake. I believe there is a port on the number 8 runner.

 

[dadsbee]

Snag a spare plug wire off something else and run it on #8. Then a new cap next...

 

[68.Charger]

Great suggestions.
There is a vacuum port on that intake runner for the air grabber and the power brakes. I will plug that off and see what happens.
I will also try a different plug wire and report back.
Thanks!

 

[1 Wild R/T]

spay carb cleaner around the port. see if it changes anything...

 

[Geoff 2]

I suspect you are getting oil down the intake guide [ #8 is rear cyl, engine slopes ] which is causing carbon build up on the plug, which then fails to fire.

 

[68gtxman]

@68.Charger Another thought: watch the engine running at night with the area lights off. Look for errant arcing around the spark plug, which should be visible with the lights off. You may have a weak spot in the plug wire that is allowing the spark to arc over to the exhaust manifold.

 

[AR67GTX]

Always a slight chance of a bad plug too - cracked insulator or just bad. Seems to happen.

 

[68.Charger]

spay carb cleaner around the port. see if it changes anything...

You nailed it!! I sprayed around each intake port and sure enough when I hit #8 the rpm's picked up! Gawd I love this site!
Thanks to everyone who responded; much appreciated. Now comes the fun part of pulling the intake off and checking to see what needs to be sealed up. It's always something!

 

[1 Wild R/T]

A new bathtub style gasket & coat each side of the ports on that gasket with this....

Amazon product ASIN B007TWXI1K

 

[68.Charger]

Well if anyone is interested, I pulled the intake today and look at what I found on #6 and #8 ports. No wonder it was having issues. I guess the previous owner had a oops during assembly...

 

[Kern Dog]

 

[Don Frelier]

I've never used the paper gaskets shouldn't need them unless you've got some weird head milling job that was done incorectly.

You probably know but that valve cover bolt is loose...

 

[Kern Dog]

I ran paper gaskets on each side of the valley tin when I used thicker head gaskets. I ran .060 and .075 head gaskets in the past.

 

[68.Charger]

I don't plan to put the paper gaskets back in but will use the 3 Bond or Permatex #2 to seal it up the ports and black RTV for the ends. Does that sound correct?

The valve cover bolts were still loose from when I was checking the rocker/valve action.

 

[Don Frelier]

I like to dry fit the valley pan first.
Take a dull chisel and carefully make a sharper crease where the angles hit the rails in the rear and front.
Careful because you can mess it up doing this.
See if the holes line up nicely where the intake bolts go into the head.
Set the intake on and check the bolts again.

 

[RemCharger]

I like to dry fit the valley pan first.
Take a dull chisel and carefully make a sharper crease where the angles hit the rails in the rear and front.
Careful because you can mess it up doing this.
See if the holes line up nicely where the intake bolts go into the head.
Set the intake on and check the bolts again.

Exactly. I like to have everything perfectly clean, dry fit the intake and make sure it fits flat on the heads at all corners. Then semi snug the pan rail ends, do the dull chisel treatment and commence with installation. A bit of thread paste on the bolts and go around the pattern several times as the intake squishes down and finds its home.
I did a thread on my distaste for the paper gaskets, exactly as seen by the OP.

 

[1 Wild R/T]

I used the paper gaskets once.... Back around 1979.. Every printed manual said they had to be used when using an aluminum intake... I had an early A12 Edelbrock manifold & carbs on my Challenger... It ran so bad I was about to swap back to a 4bbl.... Then I pulled the intake & saw the same thing posted above.... Put it back together with a new stamped steel bathtub coated the port areas with some Yamabond left over from having split the cases on a YZ250... Back in the 70's Yamaha was the only place to get three bond in the US.. They sold it as Yamabond... Later Honda added it to their shelf & labeled it Hondabond... The actual maker is Threebond...

 

[Geoff 2]

A backfire might have caused the gasket to shift...