One cylinder misfire and weird power balance results?

OK there's two problems with this 460 smog motor I'm pulling from a Thunderbird for my truck. I bought the car for $800. The transmission seems strong and shifts smoothly and the engine fires right up even when cold. It just shivers when it's warm and you can hear a pretty regular misfire from the exhaust.

Don't ask about the rest of the car, the motor is just about the only thing that isn't scrap.

Note: I've already disconnected and plugged all vacuum lines on the carb and intake manifold, so none of those miles of vacuum lines will effect this test. I've also plugged the AIR tube that goes to the exhaust manifolds and removed the pump and vacuum hoses pertaining to that system. EGR was tried with the vacuum hose installed and then plugged, no difference.

I've installed these new parts that I bought in advance; cap+rotor, plugs, wires, coil, ignition module, AND rebuilt the carb and replaced carb/EGR gaskets when I started.

Problem:

There is a regular misfire on #4 cylinder. I did a power balance test and removing #4's plug wire dropped maybe 5 RPM. It runs now and then but not often enough to contribute. The plug is dry. It actually looks no different than the other plugs in the motor!

WEIRD problem!

When I removed #5 spark plug lead the RPM's increased by 30 RPM, from about 800 RPM idle to about 830!

What could possibly be going on here?

I'm embedding a YouTube clip of the vacuum reading with the engine hot and idling at around 800 RPM. I give a few slow revs to see if it shakes worse and if it settles back to normal.

Do a compression test on that cylinder

^^^^^ Same, and you might want to double check your firing order to make sure you did not get a couple of wires crossed.

That was quick.

All cylinders are from 149 to 151psi throttle wide open. This one is 149.

I've had to check that firing order several times to be sure, even second guessing myself 10 times!

What on earth would cause RPM to rise??

In this video, I start by relieving the pressure in the gauge so I can watch it build. It eventually tops out at about 60-65psi at idle. Notice here how the compression builds, stops for a few turns, then builds a little, stops, builds...

I believe that each time compression is going up, that's when that cylinder would fire. I know that I'm getting something from that cylinder SOMETIMES because it is not a 100% consistent miss until I unplug it.

I'd check the valves and springs on that cylinder. It sounds like a sticking valve or bad spring to me. At cranking speed, a slowly closing valve might have ample time to close, producing a good compression number. At running speed, not so much.

I put my thumb on the rockers as they move and don't notice any slack or difference from the other rockers.

What methods would give me a sure answer on which valve or spring if any aren't keeping up?

Wouldn't a weak spring get worse as I rev? The vacuum seems to smooth out.

Intake manifold leak (spray it), brake booster leak (unplug cap and test), sticky valve (hand turn or bump), crossfire cap/wire (separate /spray maybe lights out).

I'm thinking vac leak too.

OK the bump test for the valve, how does that work? I hope it's just a rusty valve guide as some PB blast has helped me there before. Spring...well that would be TOO easy, but can it be a spring if vacuum steadys out as revs increase?

It's not a hose or vacuum fitting to blame. All of those are plugged with rubber caps at the source.

Would a vacuum leak at the manifold gasket cause low running compression? When I do a running test, #4 will never reach more than 65psi. The others will reach about 90psi with a throttle snap test.

As for crossfire, I found that the way to track this is to put the tachometer induction pickup on each spark plug lead and look for the one that doesn't read the same as the others

OK here's an update on that #4 missing.

When the car is idling I get nothing from that cylinder. Dead as a pulled-plug.

When I rev the car ONLY under load, around 4,000RPM I start to get a buzzy tapping noise from the engine.

Could a weak valve spring cause the cylinder to be dead at idle?

If that sounds unreasonable, is it a lifter?

I pulled the pushrods and they're both straight. The springs are properly seated and if I press down on the stems, the valves pop right back with no sticking.

Have did a compression test buy turning the engine over without firing it up. Sounds like it might be trying to burn a valve. A running compression test everything moving fast and will give a higher reading in most cases

Yes, the compression is 149-151 on all cylinders.

You could swap those springs to another cylinder and see if the problem moves.

83Farmboy wrote:OK here's an update on that #4 missing.

When the car is idling I get nothing from that cylinder. Dead as a pulled-plug.

When I rev the car ONLY under load, around 4,000RPM I start to get a buzzy tapping noise from the engine.

Could a weak valve spring cause the cylinder to be dead at idle?

If that sounds unreasonable, is it a lifter?

I pulled the pushrods and they're both straight. The springs are properly seated and if I press down on the stems, the valves pop right back with no sticking.

How are you pushing the valves open....and how much pressure is required?

I'm holding the rounded end of a ball peen hammer against the stem and smacking it with another hammer. I've seen sticky valves and usually with that test if they're reeeeeally sticky you will feel the stem clack against the ball peen, in the same way too much lash causes a gap to be created before the rocker smacks back down onto it.

Looking at the gauge, could it really be a burnt valve?

460bronco wrote:You could swap those springs to another cylinder and see if the problem moves.

I would swap the spark plug and wire to another cylinder, and see if it does anything. I know you said they were all new but, one could still be bad.

Yep I did that. Have a few new ones, both plugs and wires, hanging around. I've swapped multiple times JUST to be super sure.

I find it interesting that it gained rpm when you killed the #5 cylinder, since #5 and #4 are side by side in the firing order (and the distributor cap). I know you said the cap was new but I'd take a really close look inside that cap, particularly near 4 and 5. Look for ANY flaw, scratch, nick, dirt, carbon track, etc. Swapping wires, plugs, etc., you eliminated another possibility, that of a crossfire between two bad, cracked, or CHEAP brand plug wires. Also note if the cap has brass terminals, as those are best. The cheap crap doesn't have brass.

Btw, one way to look for external crossfires is to look around under the hood while the engine is running in TOTAL darkness.

When all this fails, I'd start looking for a vacuum leak. Incidentally, does this thing have power brakes?

I'm aiming at the valves, springs, lifter etc now.

Boosted brakes, yessir. Doesn't make a difference in this case since I plugged everything at the source, zero vacuum hoses here. Even the advance port on the carb is blocked.

I think I may have figured why I was getting higher RPM from #5 unpugged. Since that spark has to go SOMEWHERE I think it was jumping to the #1 plug in the cap, which is where my tachometer induction coil is. I moved the tach to #2 and repeated the test and it dropped RPM like normal

Hey, say what you will, at least I DID figure it out...eventually

Not knocking you at all. Sometimes these things will have you talking to yourself. It does seem like a valvetrain problem, but I figured you eliminated that possibility. Press on... and update.

Update. Replaced the exhaust spring on that cylinder and no difference.

Here's something interesting. When it's cold, if I unplug #4 it makes no difference, so it's obviously not running. However, it runs much smoother than when it is hot. When it's cold, only a trained eye would see the slight jitter in the engine. Once hot, you can hear the idle drop every time #4's turn comes and if you put your hand on the car you will feel the irregular nudge of a dead miss.

My theory is that a cold engine makes much less efficient power strokes. 1/8 of nothing is nothing, so a miss isn't too bad in the whole equation. As things warm up and become normal, 1/8 of all of that power is a good chunk and becomes apparent.

So what problem would:

Read high equal relative compression on a static compression test
Cause a completely dead cylinder
Darkens plugs
Has perfect spark/dwell
Ticking vacuum signal

But:
No popping, coughing, or surging idle
No tapping noises at or above idle with no load
Isn't a burnt valve
Isn't a weak exhaust valve spring
Isn't a sticking exhaust valve

Lean misfire at idle. Maybe the idle circuit on the secondary drivers side of the carb. is running lean. I would expect it to affect two cylinders though. As mentioned earlier an intake manifold vacuum leak could act the same.

Does this thing have a working choke?

How do you mean? The choke snaps shut when cold and opens as it warms up to a near vertical position. Are you suggesting closing the door to make it rich?

Can you check the exhaust temperature at the exhaust manifold, on that cylinder compared to the others with an infared thermometer and see how much heat it is putting out? I don't know exactly what is going on but, I doubt that cylinder is completely dead.