Question: What can cause a cylinder misfire

[Bolisk]

My 1970 350/350 has a misfire (so I've been told by very experienced corvette shop) and it is most noticeable (engine rattles a bit) between 1,500 and 2,000 RPM.

I'm pretty sure it's on the right (passenger side) bank. Don't know which cylinder(s) yet. I was going to have a compression / leak down test preformed in the near future.

I've already replaced the plugs, cap, and rotor. I've recently changed the points to electronic, and the coil out approximately three years ago.

I do know that the valve guides were nureled (sp?) during the last rebuild approx 10,000 miles ago. (previous owner).

Just wondering what the causes are for misfire?

-Bolisk

 

[Rod75]

I'll just address an ignition misfire. I'm sure others will chime in with mechanical causes.

This is where an ignition oscilloscope would come in handy.
The most likely cause would be failure/fatigue of the secondary (high voltage) ignition circuit. This starts at the ign coil & ends up @ the spark plug including all components in between. Coil, coil wire (HEI spring & carbon tip) rotor, cap, spark plug wires and spark plugs.
A quick check would be to observe your engine running in the dark. You'll be able to see arcing or shorts in your wires this way.
You can check your spark plug wires and most components with an ohm meter. Look for deviations in the wire insulation/boots.
Be sure spark plugs are gapped to spec.

For your primary ign circuit, I wouldn't hesitate to suspect your points conversion unit either.
Just because parts were replaced recently, doesn't mean they're up to snuff.

This is where a good repair manual comes into play, giving you a step-by-step process for diagnosis.

Good Luck -

 

[lars]

These are the items that will cause a single-cylinder misfire:

Bad plug
Fouled plug
Bad plug wire
Bad distributor cap
Burned valve
Broken ring
Damaged piston
Flat cam lobe
Bent pushrod
Broken rocker arm stud
Broken Rocker Arm
Bent valve
Broken valve spring
Rocker arm nut fallen off
Blown head gasket
Cracked head
Cracked cylinder

Lars

 

[rkboz]

use your timing light on each plug wire and you'll see the missfire on the strobe. If not and it's not electrical then you may have a flat cam or some other internal prob.

 

[lars]

He can have an arc grounding right off the plug boots, and the timing light will fire just fine...

 

[Bolisk]

Ok, so everyone just give me some information that connected some dot's to a potential culprit.

My engine started running rough immediately after I replaced my clutch. At the time, I thought is was a balancing issue do to potentially shoddy machine work by the flywheel guy. After a while I forgot about it, and just got used to the car running that way.

A month or two later, I changes the spark plugs. . .to those E3 diamond fire plugs. About three weeks after the change, I was at the Vette shop and the owner told me I had a mis fire, and showed me how to look for it. He looked at my plugs and said they might be too hot or cold for the way the car is set up, and suggested putting new plugs of the same temp back in the car. I did that, and the problem seemed to get better. . .but not completely better. So I just assumed that the problem was taken care of. . .and the remaining ruff running was the result of the unbalanced flywheel.

Fast forward to this weekend (approx 8 months later) and when I was at the shop again, I was told that my car still had a misfire. So I'm back on trail.

After asking you all for opinions, you all mentioned plug wires. . .which is the ONLY thing i have not changed since the clutch job. . .and when I think back. . .I distinctly remember pinching one of the plug wires (on the passenger side, which is the side the shop told me the misfire is on) between the bell housing and the block when I put the bell housing back on the engine. I actually tightened the bell housing fairly tightly before realizing I had the wire in between. I was so tight that I could not pull the wire out no matter how hard I tugged. So I went back under the car and loosened the bolts. . .to get the wire out.

At the time I didn't think anything of it because the rubber outer covering was not damaged.

Could that have damaged the internal components of the wire to the point at which I have a mis fire? If so, can I just get a spare "good" wire and rotate it thought he right side of th engine until i find the solution?

Sorry for the story.

Regards,
Bolisk

 

[Bolisk]

These are the items that will cause a single-cylinder misfire:

Bad plug
Fouled plug
Bad plug wire
Bad distributor cap
Burned valve
Broken ring
Damaged piston
Flat cam lobe
Bent pushrod
Broken rocker arm stud
Broken Rocker Arm
Bent valve
Broken valve spring
Rocker arm nut fallen off
Blown head gasket
Cracked head
Cracked cylinder

Lars

Wouldn't some of these (the ones with "Broken" in them) make all kinds of noise?

 

[gmjunkie]

Wouldn't some of these (the ones with "Broken" in them) make all kinds of noise?

No,Not always!~!!I've seen rocker arms,push rod's,bent and broken and even valve springs broken with not even a tic!
This valve spring was quiet as a Church mouse,I chased around a intermittent miss for a month!~!!

 

[JohnZ]

After asking you all for opinions, you all mentioned plug wires. . .which is the ONLY thing i have not changed since the clutch job. . .and when I think back. . .I distinctly remember pinching one of the plug wires (on the passenger side, which is the side the shop told me the misfire is on) between the bell housing and the block when I put the bell housing back on the engine. I actually tightened the bell housing fairly tightly before realizing I had the wire in between. I was so tight that I could not pull the wire out no matter how hard I tugged. So I went back under the car and loosened the bolts. . .to get the wire out.

At the time I didn't think anything of it because the rubber outer covering was not damaged.

Could that have damaged the internal components of the wire to the point at which I have a mis fire? If so, can I just get a spare "good" wire and rotate it thought he right side of th engine until i find the solution?

Sorry for the story.

Regards,
Bolisk

Plug wires are VERY delicate, especially if they're the OEM type (it's just a piece of string with carbon particles on it); if you pinched it and then "tugged" on it, there's no question that it's damaged. Get another good plug wire and install it temporarily over the top to see if the misfire goes away; if it does, then replace the old one correctly.

 

[Rod75]

Ok, so everyone just give me some information that connected some dot's to a potential culprit.

My engine started running rough immediately after I replaced my clutch. At the time, I thought is was a balancing issue do to potentially shoddy machine work by the flywheel guy. After a while I forgot about it, and just got used to the car running that way.

A month or two later, I changes the spark plugs. . .to those E3 diamond fire plugs. About three weeks after the change, I was at the Vette shop and the owner told me I had a mis fire, and showed me how to look for it. He looked at my plugs and said they might be too hot or cold for the way the car is set up, and suggested putting new plugs of the same temp back in the car. I did that, and the problem seemed to get better. . .but not completely better. So I just assumed that the problem was taken care of. . .and the remaining ruff running was the result of the unbalanced flywheel.

Fast forward to this weekend (approx 8 months later) and when I was at the shop again, I was told that my car still had a misfire. So I'm back on trail.

After asking you all for opinions, you all mentioned plug wires. . .which is the ONLY thing i have not changed since the clutch job. . .and when I think back. . .I distinctly remember pinching one of the plug wires (on the passenger side, which is the side the shop told me the misfire is on) between the bell housing and the block when I put the bell housing back on the engine. I actually tightened the bell housing fairly tightly before realizing I had the wire in between. I was so tight that I could not pull the wire out no matter how hard I tugged. So I went back under the car and loosened the bolts. . .to get the wire out.

At the time I didn't think anything of it because the rubber outer covering was not damaged.

Could that have damaged the internal components of the wire to the point at which I have a mis fire? If so, can I just get a spare "good" wire and rotate it thought he right side of th engine until i find the solution?

Sorry for the story.

Regards,
Bolisk

Sounds like you've about found and solved your problem!

 

[Bolisk]

No dice.

Checked all plug wires and plugs, spark to that point is fine, and the plugs are not fouled. (Ran a known good plug wire as a by pass to all plugs) So not sure what the problem is at this point.

-JR

 

[lars]

Wouldn't some of these (the ones with "Broken" in them) make all kinds of noise?

Nope - these failures can be completely quiet. Sometimes they'll make ticking or popping sounds, but more often you will hear nothing.

 

[Bolisk]

Update:

So I took all the spark plugs out and wedged the carb butterflies wide open, then connected a compression tester to each cylinder one at a time. For each cylinder I turned the engine over with the starter untill the compression gague wold not climb any higher.

ALL but two cylinders had the same compression level 200-210.

Cylinder 3 had a PSI of 160
Cylinder 6 had a PSE of 180

We also hooked up a vacume gague to the intake manifold and the vacume bounced. Hight RPM seemed to make the bouncing better, but it still bounced.

So, current diagnostic would suggest that i'm looking at bad valvues on both heads. Does everyone concur?

Could this be causing the misfire?

 

[lars]

Nope.

160-180 pounds of cranking pressure won't cause a misfire - not a chance. You don't get a misfire until cranking pressure drops down into the 60-90 psi range. Although you have 2 cylinders that appear slightly lower than the others (you're still within 10%), your misfire is not caused by lack of cranking pressure. You might want to do a leakdown test rather than a compression test - compression tests don't tell much.. But your problem lies elsewhere, and it's not with excessively leaky valves.

Lars

 

[Bolisk]

good to know.

What will the leak down test tell me.

bad valves
or rings?

 

[lars]

A leakdown test positively identifies if there is a cylinder leakage issue, regardless of cam duration and other variables that can affect cranking pressure. The leakdown test also quantifies the quality of the compression, and positively identifies where any issues are located (intake, exhaust or rings). This will allow you, with confidence, to eliminate any piston/ring/valve issue as the source of the problem.​