Cylinder 8 not firing

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Trevor B

Trevor B

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Location
Novato, CA
1973 Duster
318 stock heads/cam
Performer intake/4 bbl

In preparing to do the next stuff (heads/cam), I decided to do a compression test to make sure the engine seemed healthy enough to warrant dumping more money into it.

It sounded like a cylinder was missing - car has been sitting a while but I start it up a few times a month. I pulled off spark plug wires one at a time, listened for the engine to change, and replaced them. #8 is not firing - no change in sound or vacuum when I pull off the wire.

Vacuum is reading 15-16 inches (needle sort of bounces between 15 and 16).

Compression test results:
dry/wet
1: 155/175_____2: 160/180
3: 155/160_____4: 159/170
5: 160/172_____6: 160/169
7: 160/175_____8: 165/177

I feel like the valves must be sealing to put up those compression numbers.

After pulling the valve cover and rotating the engine, I can see both intake and exhaust rockers going up and down like they should.

Ideas?

EDIT: forgot to mention that I got a strong spark from the wire and sparkplug is firing (outside of the hole)
 
Numbers seem good.

- Change # 8 spark plug and wire; you can swap them into another cylinder to see if the problem follows them
- Actually measure the lifters/rockers for #8 to see if they are moving as much as the others; could be a collapsed lifter or bad lobe
- Run a leak down test (with out expecting much bad)
- Check # 8 exhaust port for a mouse nest (getting desperate now...LOL)
 
Plug, Wire, Vacuum leak on the #8 intake runner, Cam lobe. It's gotta be one of those I'm thinking.
 
Check #8 plug. I had a broken center electrode once and it would fall onto the ground electrode and ground the plug out at low speeds and idle. When the RPM came up, the compression would blow the electrode up into the plug, make contact and start firing.
 
On a side note DO NOT check for cylinder firing by pulling plug wires. Instead, do something akin to........

Pull the dist. boots and loosen the wires. Pull the wires up out of the cap, and SHORT the spark out with a grounded probe.
 
Is it ALWAYS just #8 cylinder? Does the distributor shaft precess like a gyroscope, or does it spin true?

Have you verified that there is continuity between the rotor side of the distributor cap all the way to the the spark plug? It could be the plug, the wire, or even the cap. Keep in mind what Rusty said too... Plugs are cheap.
 
And.........vacuum leak. What does the plug look like?
 
Blow some air through the plug hole and see what flies out the exhaust pipe, or the carb.
 
Put a new plug on the wire and got a spark outside the hole.
Then installed the plug and still no firing of the cylinder.
(apparently I should not do this again!)

I do have a spark.

Guess if I can find a leak down kit, that might help show me if there is a vacuum leak on the intake.
 
No, leakdown puts pressure into the cylinder. Gives you clues as to percentage of leak, whether going past rings or valves.

We had a guy on here some time ago who claimed he ended up with a crack in an intake, similar problem. One or two cylinders running lean or not firing
 
i had the same problem on a 273, the pcv valve fitting was on an intake runner, swapped it on to the carb and it was good after that. i figure the "leak" from the pcv was enough for it to get a limpy idle on 1 cylinder, as it came right when rpm came up
 
don't all the vacuum tee's run off #8 I'll start there and also retorque the intake manifold, I've seen them get loose on the end bolts.
 
Trevor B said:
Put a new plug on the wire and got a spark outside the hole.
Then installed the plug and still no firing of the cylinder.

I do have a spark.
Click to expand...
Not really..... being able to fire a spark plug gap in open air is meaningless. The spark voltage needed to fire a spark in a compressed fuel-air mixture is many times higher than in open air. So, it can fire fine in open air but not at all in actual operation. You have MAYBE proven that the spark plug can fire, but not that the plug wire is OK. So actually swap the plug AND wire with another set from another cylinder. Or, connect the #8 plug wire to a screw driver and lay it 1/4"-to 3/8" from metal and see if it jumps a gap of that size with a nice blue spark.

Your spark may in deed be fine but you just have not proven that yet. Then move on with other other checks.
 
nm9stheham said:
Not really..... being able to fire a spark plug gap in open air is meaningless. The spark voltage needed to fire a spark in a compressed fuel-air mixture is many times higher than in open air. So, it can fire fine in open air but not at all in actual operation. You have MAYBE proven that the spark plug can fire, but not that the plug wire is OK. So actually swap the plug AND wire with another set from another cylinder. Or, connect the #8 plug wire to a screw driver and lay it 1/4"-to 3/8" from metal and see if it jumps a gap of that size with a nice blue spark.

Your spark may in deed be fine but you just have not proven that yet. Then move on with other other checks.
Click to expand...

Exactly.
Believe it or not, cylinder compression makes it harder for the spark to jump the plug gap.
The higher the compression the more it resists, and when it resists the spark can actually choose a different path than going to the plug.

A wire that the core is burning out of will do exactly this, because outside of the cylinder there is no resistance and the spark jumps the gap caused by the core burning away and also jumps the gap of the plug just fine.

When in the cylinder that big gap inside the wire is harder for the spark to cross.

If you look close at the ends of that wire it will be evident that the core material isn't there and making contact to the metal terminal on the end of the wire.
It will probably look black in the center of the wire also, if that's what it is.

Swapping two wires is a pretty easy way to check this, and the new cylinder that wire is on would not run.
 
Trevor B said:
Put a new plug on the wire and got a spark outside the hole.
Then installed the plug and still no firing of the cylinder.
(apparently I should not do this again!)

I do have a spark.

Guess if I can find a leak down kit, that might help show me if there is a vacuum leak on the intake.
Click to expand...

By chance are you using any anti seize or are have the plug hole threads been painted on the head? Plug wire arching on manifold?
 
Daves69 said:
By chance are you using any anti seize or are have the plug hole threads been painted on the head?
Click to expand...

This makes absolutely no difference. The spark will EASILY jump that "barrier"
 
Did anyone mention that there has to be a combustible mixture in the chamber? I mean is/are the valves opening? A compression test, and a cylinder leakage test will only prove that they are closing.
I see in post #1, that Trevor says the rockers are going up and down, but are they going up all the way? and is the piston pulling a charge.
If an intake lobe is going away, or it's pulling air from the valley, well .... You get it.

But also from post #1, since the heads and cam are being replaced, and the compression numbers are pretty good, why are we even ..........................
 
SGBARRACUDA said:
That is some funny shat right there!
Click to expand...

Depends, but yea. :D

I compression tested the motor and all were about the same, so I pulled the valve covers and hit it with a remote starter switch and the #8 exhaust rocker only only moved about 1/8 inch.
That answered that.
 
If cylinder #8 is not firing, you should hire another one and fire that one....
 
If it has excessive crankcase vacuum that is a good indication of the intake sucking from the valley. Pull the breather and put your hand over the opening. If it feels like your vacuum cleaner that is an indicator.... It should have a little PCV suction but not a lot.
 
TrailBeast said:
Depends, but yea. :D

I compression tested the motor and all were about the same, so I pulled the valve covers and hit it with a remote starter switch and the #8 exhaust rocker only only moved about 1/8 inch.
That answered that.
Click to expand...

Well yeah I was referring to your comment. I know you finding a rounded off lobe was NOT some funny SHAT, no sir, not at all.
 
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