Cylinder number 3 won't fire on an '88 LO5 350, w/a Edelbrock carb.

I verified spark to that cylinder w/a spark tester connected to the cap of the plug for "ground". This shows that there is spark, even when under compression.

Compression is 125ish, similar to other cylinders in the same motor.

Intake and exhaust rockers are moving (cam's not flat).

Replaced plugs twice w/new ones. When they come out, they look dry. I don't get it.

-Tom

 

How do you know #3 is dead?

 

Because I did a "poor man's" cylinder balance test by back-probing the plug wires at the boots on the distributor w/a test light lubed w/dielectric grease.

Cylinder number 3 was the one and only one that made no audible or visual (on a tach) difference.

 

Anyone?

 

Do you have a dead squirrel stuck in the #3 intake port?

If you got compression and a good spark there's not much left other than a vacuum leak near #3 (leaning it out and causing an idle misfire jsut on that cylinder) or a dead exhaust lobe on that cylinder (which won't affect cranking compression but that cylinder obviously won't fire properly).

BTW- don't just go by your idle drop when disconnecting individual cylinders. On a poorly running engine it can be damned near worthless as a diagnostic test. If you have headers an infrared pyrometer (looks like a gun with a little laser on the front to check temps) is a more accurate diagnostic tool to see which ones got fire and which ones don't. If you know someone with an old SUN scope they are still VERY good diagnostic tools for this sort of stuff, even if they don't know anything about fuel injection and computers.

 

Damon, thanks for writing.

LOL about the dead squirl.

I hear you about all your ideas; the scope (none available to me), the lazer pyro, and the point about a rough running engine. This engine has stock manifolds though, and it idles good...except for the audible miss of the one cylinder.

Before the miss started, it was honestly, the sooooooothest running SBC that I've ever seen, carbed or otherwise. I think you could have balanced a quarter on the air cleaner at idle. So the miss is very obvious, and doing the "poor man's cylinder balance test" is effective, in this case.

I've also thought about a vacuum leak on the #3 runner, but the problem w/that theory is that it misses under load too. You can clearly feel it and hear it as you accelerate up a hill, at lower RPM. Still, I think that is about the only place to look at this point. I've got compression, my rockers rock, and I've got spark. It must be either a plugged runner or a leaking one.