Help diagnosing an engine miss.

[studiog]

Posted this in the C3 forum but since it is relative to all small blocks I thought I'd post it here too. The C1 - C2 forum is where I live and I know there are allot of knowledgeable people here also.

I understand if it gets moved.


Working on my brother in laws' 71. Recent rebuild. Motor has a miss. I have isolated it to the #3 cylinder but can't figure out what is causing it. I pulled one wire at a time from the distributor as it was running and #3 made no difference in motor rpm.
Here is what I've checked so far:
1.) replaced spark plug
2.) replaced plug wire
3.) Compression 145-155 in all 8 cylinders
4.) Cam lift on #3 intake & exhaust is normal


Any comments welcome.
Thanks all.

 

[IH2LOSE]

Was the plug wet your removed on number 3.

Do you have an inductive timing lite to see if the plug is actually firing from the distributer

Have you inspected the cap just to see if that brass post is there and useable

 

[tigernut]

I recently had a pretty bad high-rpm misfire.

Replacing the distributor cap fixed the problem.

Doug

 

[studiog]

Larry:
I do have an inductive timing light and I have checked that the #3 plug wire is firing.

Tigernut: Seems like the dist. cap should be the next move here even though it seems to be in good shape, I guess you never know.

 

[65TripleBlack]

Working on my brother in laws' 71. Recent rebuild. Motor has a miss. I have isolated it to the #3 cylinder but can't figure out what is causing it. I pulled one wire at a time from the distributor as it was running and #3 made no difference in motor rpm.
Here is what I've checked so far:
1.) replaced spark plug
2.) replaced plug wire
3.) Compression 145-155 in all 8 cylinders
4.) Cam lift on #3 intake & exhaust is normal


Any comments welcome.
Thanks all.

What was the condition of the plug? It sounds like you have a wire crossed. You might also have a miss on #4 or #6.

Joe

 

[studiog]

What was the condition of the plug? It sounds like you have a wire crossed. You might also have a miss on #4 or #6.

Joe

The plug in #3 looks pristine and all the other plugs look a little sooty. After testing again it seems that #5 & #8 are also a bit weak. Am I looking at an intake leak?

 

[PTighe]

Working on my brother in laws' 71. Recent rebuild. Motor has a miss. I have isolated it to the #3 cylinder but can't figure out what is causing it.

I'm not sure exactly what terms you mean by cam lift. If either the intake or exhaust rocker arm is too tight and not properly adjusted, this will cause a miss in the engine. Go back and get the #3 valves closed and readjust the rocker arm nut.

Don't ask me how I know this. PT

 

[studiog]

I'm not sure exactly what terms you mean by cam lift. If either the intake or exhaust rocker arm is too tight and not properly adjusted, this will cause a miss in the engine. Go back and get the #3 valves closed and readjust the rocker arm nut.

Don't ask me how I know this. PT

What I mean by cam lift is just that the rocker action seems normal and the cam is not worn "flat".
Checking the rocker arm nuts cannot hurt, and I won't ask how you know

PS: If the rocker arms were too tight would that necessarily keep the valves from closing, and would it show up on my compression test?

 

[65TripleBlack]

The plug in #3 looks pristine and all the other plugs look a little sooty. After testing again it seems that #5 & #8 are also a bit weak. Am I looking at an intake leak?

Is this an LT1 engine? If it still has the original solid lifter cam, then the inlet valve lash should be .030" and exhaust should be .024".
If some of the plugs are black, and some are normal tan/white, then you have a carburetor balance problem, or intake manifold leak.
LT1 engine should register approx. 11 in-hg vacuum @ 800 RPM. If plugs 1-7-4-6 are fouled and the others are not (or vice versa) then you've got a gross imbalance in the carburetor.

Joe

 

[Tom Bryant]

PS: If the rocker arms were too tight would that necessarily keep the valves from closing, and would it show up on my compression test?

Yes and yes. Since you have already stated that your compression is even in all cylinders you can eliminate too tight of a rocker arm. A valve being held open would result in zero compression in that cylunder.

Tom

 

[studiog]

Is this an LT1 engine? If it still has the original solid lifter cam, then the inlet valve lash should be .030" and exhaust should be .024".
If some of the plugs are black, and some are normal tan/white, then you have a carburetor balance problem, or intake manifold leak.
LT1 engine should register approx. 11 in-hg vacuum @ 800 RPM. If plugs 1-7-4-6 are fouled and the others are not (or vice versa) then you've got a gross imbalance in the carburetor.

Joe

This car has the base engine and hydraulic lifters. I am leaning toward a manifold leak.
Thanks for the help.

 

[studiog]

Yes and yes. Since you have already stated that your compression is even in all cylinders you can eliminate too tight of a rocker arm. A valve being held open would result in zero compression in that cylunder.

Tom

Thanks Tom