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.

I showed two plugs, one from number 3 and one other plug. All the other plugs look pretty much like the clean one.

I checked valve operation on number 3 looks normal just like the cylinders

Looks like it's getting fuel. I would try a cylinder leakdown test and a running compression test to verify the exhaust lobe of the cam on that cylinder. I ran into that one time on an SBC....dead hole, but good compression, fuel, spark, no vacuum leak, etc. Turned out to be a flat lobe on the cam, for the exhaust valve on that cylinder.

Mystery Solved
 

Schurkey was spot on in post number 14. I suspected a vacuum leak from the very beginning but for the life of me couldn't figure out where it could be. I even pulled the carb off and made sure the gasket was covering all the right places. I believe I did all the correct trouble shooting steps but just couldn't figure it out until I pulled the intake manifold. When I was elongating the pushrod holes so I could use 1.65 rockers I got kind of carried away and broke through in a couple places. My machinist gave me some two part putty epoxy and he said it would be fine. Maybe I sanded it too thin but obviously it didn't hold. Now I need to fix these holes with something that will stay put.

Yep that picture explains it all. That baby is sucking in some oil ( Big vacuum leak )

If you can, put a sleeve down that hole, only because the Apoxy is a hit or miss. It’ll come out again when you lease expect it

Not sure how I could do that the hole is not round after I was grinding in there. It's a shame I have a lot of money in those heads, porting, valves, springs, milling.

In that case I would grind that all off and get a real good Apox That’s not saying that you didn’t use a good going to begin with. Hey you may have good luck. Hey I go for it

Tim do it right but to do that you need to pull the head(s), that’s the hardest part.

Next drill the hole with a new 1/2”
Drill bit.
Get some 1/2” OD brass tubing from a hobby shop or order it from Grainger or micro mark.

Cut it to length full coat both the hole and tubing with JB weld and install it.
Let it harden for a full day and then cartridge roll sand away what you don’t need.

Sorry for your issue, but atleast you did not harm anything in a new motor.

Here in this photo of a max ported D port 16 head you can see how I did it.

Glad you found the problem. As said, without all the info on your engine, it's hard to determine the cause.
(nothing about head work being done)


:)

Here’s another photo of that same head and on the left port you can just see the tube peaking out and how far back and flat you can get to roll back to.

If you choose to drill & sleeve you will have to drill slowly when you get to the blow out holes because the might really want to walk on you.

Steve
Thanks for the great info on how to fix this. Fortunately, only one head will need to come off the other head is OK. Can this be drilled by hand or will it need to be done in a mill? I have a drill press but getting the head at the proper angle would be almost impossible.

Hand drilling would be tough, have to go slow and easy. But it could be done.

You have a lot invested in your heads.

Pay a machinist to drill the hole and install the brass sleeve, you’ll be glad you did.

Didn't we go through all of this a few months ago? Different guy, same problem?

https://forums.maxperformanceinc.com...d.php?t=866228

This right here. Its cheap in the long run.

A drill press would be the best way to do it, but you would need to cut a lenght of wood to a 14 degree angle to sit the head on.

I have always done it with a big hand held electric drill that I have which has a side bar handle .
Maybe you could brorow one or get you hands on one from a tool rental shop.

Here's another tip for you Tim.

Use a Qtip wetted with some light oil to coat the whole inside of the tube so that any stray epoxy you get inside it will be far easier to sand out it you should need to.

Is the PCV routed to that runner on the intake?

Lay the spark plug on the side of engine or other ground and crank it over, see if it sparks.

The plug could just be bad.

.

He found the issue gang!

Yes!!! A machine shop has better ways and tools to do this job once and correctly!!!

Enlighten us please.


.

Please explain to me why a machine shop is better equipped to do this repair if you would?

More then anything it takes patience during the insertion and epoxy phase of the repair and time cost money when you have someone else do it.

Nevermind, went back and figured it out.


.

Sorry, I did not mean to ingnor your question.

Steve

I believe the valves are 14 degrees from the cylinder head to block mating surface, but it looks like the push rod holes are 90 degrees to the head mating surface. So why would I need a 14 degree shim under the head to drill the push rod holes on my drill press? Is there something I am not understanding.?

Good to hear. My buddy’s shop is finishing up a 426 Chrysler rebuild that started off like this. Big dollar factory Hemi car just purchased at auction. Several months later the owner couldn't figure out why it ran poorly. Long story short the heads had been hogged out and there was a quarter sized hole between two of the intake ports. There were traces of epoxy where someone tried to fix their mistake. Needless to say both heads ended up being junk. Good to hear yours are salvageable.

Can someone describe to me exactly where the vacuum leak was? I'm facing an identical issue on #4 cylinder. 461 stroker, brand new RA III cam and lifters, 1970 HO intake manifold. I'm not clear on whether it was in the intake or head and where.

Thanks in advance.

Tim if your doing it not by drill press means the 1/2” bit will just follow the hole there now and you will be fine & dandy.

Dog mister, Tim had ground thru the push rod buldge in a intake port and the epoxy repair that he did failed on him.