Why is #3 cylinder not firing?
 

This is a new build, and #3 cylinder is not firing. I readjusted the lifters on #3 intake and exhaust, I put in a new spark plug on #3, I put a different plug wire on it, I inspected the distributor contacts and all look fine. I removed the plug wire and started it up and re-attached the plug wire while the engine was running and no difference. I put the #3 plug wire close to ground while the engine was running and it's throwing a big ass spark (MSD). All the plugs look good except number 3, I verified the plug is firing but the cylinder is dead. I checked the compression and it's the same as the other cylinders. Could this be due to a bad hydraulic roller lifter?

What head is this?
What is the sparkplug? #?
What is gap?
Try a compression test on that hole?

Oh, yeah, correct firing order for plug wire?

I'd try a smaller gap and see what it does.
Also make sure there is no paint, crud etc in sparkplug hole. (needs ground)


:confused:

What is plug gapped at now?

Not trying to be rude but did you read my entire post, it pretty much answers your questions. The plugs are Autolite 25 in a 6X head but none of that matters since all the other cylinders are firing fine except number 3. Firing order has been checked. All plugs are gapped to .045, as you can see I already did quite a bit of trouble shooting. One thing I did not check is if there was some paint in the spark plug hole but this would be unlikely since the plug has been in and out a few times but it wouldn't hurt to check it.

First you want to do a compression test on #3 to confirm that it IS fit to fire off.

Testing a plug removed from a cylinder and seeing it throw a spark is very very different then throwing a spark in the dense atmosphere of TDC, so you may just have a bad plug.

Also you can kill power to the dizzy and crank the motor for ten seconds and you should see signs of the plug getting wet, no fuel will = no fire.

It would not be the first time that a forgotten shop rag was found in the intake tract of a motor.

Swap THAT plug with another cylinder
 

Like Steve25 says, sometimes the plug just isn't quite up to the turbulence of TDC.

If the dead cylinder moves with the plug, you found the issue.

Not sure where it is written for the plug or plug gap or heads?

Anyway, try a .032" gap. It may also need a hotter plug.

That plug looks really wet with fuel/carbon.

If there is no rag or vacuum leak in that port on the intake, probably need more info. Hard to do this stuff over the internet.

One other thing to try is rotating the distributor one plug post and moving the wires one plug post opposite to it.
(to see if the misfire follows the cap)

:confused:

Crank engine over to watch valve action on that side?

Compression test was done see post number one, number 3 has the same compression as the rest of the cylinders. I already changed the plug see post number one. Plug gap .045 is fine for an MSD my race engine with 12.5:1 uses .045 gap and all the other plugs are firing just fine. Those things have been eliminated and the engine ran fine on the dyno. I have observed the valves open and close that looks fine too. I suspect maybe a vacuum leak there is no rag in the intake this engine has about 10 dyno pulls and about 20 miles on it.

I know you mentioned you changed the plug and the wire, so I assume that was with known good units.

As someone mentioned though, maybe simply swap #3 plug & wire with #5 plug and wire, and see if the problem follows. If it stays, you can rule out electrical. Then you know it's mechanical.

Just more process of elimination as you go. That's the only suggestion I can see at the moment, and what I would try next.

Hope you find the issue easily! :)

Check cap and rotor to see a crack/ carbon tracking? Maybe that cyl has no problem firing until it has compression to fight?

I appreciate everyone's suggestions I will check those things and report back. I did pull the dist. cap already to look at the contacts but I will look at it again. I replaced the number 3 plug wire with a known good wire and replaced the plug with a new plug. I will double check the firing order and try a different distributor cap too. This is a total mystery.

It’s really baffling to me it kind of don’t make sense. If the compression is good what it looks like for some reason the spark plugs not firing. Even though it seems to test that it is firing.

Vacuum port on the runner for #3 cylinder? A big-enough vacuum leak can make the mixture too lean to burn.

OK I just looked at both pictures at first I only looked at the first picture. The second one doesn’t look very good. Just out of curiosity I’d like to see what the other plugs look like. There might be a clue as to what’s going on. Looking at the plug is Definitely high heat range plug I’m not saying that that’s it but I would like to see the other plugs to see what they look like.

curious what a running compression check # is vs normal compression

do you have headers , probably a dumb idea but spraying water with a little water bottle is a way of seeing if it's heating up and actually firing. or temp gun

gosh you wouldn't think any carb circuit or carb issue would cause this.

vacuum gauge at idle doesn't produce any weird erratic needle movement ?

just some ideas but man that is strange.

To double check a compression tester result just kill the power to the Dizzy or coil and then with a charged battery crank the motor and listen for a change in cranking speed, as this would then indicate cylinders with much lower compression then others.

From your trouble shooting efforts there isnt much left thats not covered

I would add that perhaps there is something going on during run that doesnt show up at cranking like a rev limiter gone bad canceling out #3 only

My next step would be to replace or remove the MSD box back track to a different good iggy system

I agree here. If everything checks out good pull the schrader valve and do a dynamic compression check with the engine running. If the compression drops off/ goes to zero, take a good look for any broken or weak inner/outer valve springs for that cylinder.

I wouldn't expect this with a hydraulic roller lifter but several years ago we had a solid flat tappet cam lobe get wiped with a bad lifter. That cylinder stopped firing. Hope this is not your problem but could be checked by rotating the engine and seeing if the rocker arms on #3 are moving as they should.