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Old 01-16-2007, 05:07 PM
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Diagnosing week cylinders 350 SBC

I have had a "thing" going on for some time, I have a 1st gen 350 and #2 has caused a misfire at idle since I built the engine 5-6K ago. I have taken compression tests and they all look good, adjusted and readjusted the valves and nothing changes. Sure new plugs, wires and cap as well. The cylinder isn't dead it just doesn't fire well at idle and doesn't contribute its share at higher rpm. The odd thing is the plug gets blacker than the rest and If I put a new one in it runs better not quite the way it should but better for a while. Within 30 or so miles it blackens the plug again and misfires at idle.
The engine is fuel injected, Edelbrock older system based on the LT1. I have moved injectors around and nothing changes. I am running a mild cam with good vacuum at idle above 18". Sure it gets lower when the cylinder starts to misfire but you get the idea.
I am at a point where I may have to look inside the engine. With compression looking good and assuming the intake gaskets are good (remember the plug is black, looks like fuel not oil) I'm thinking the camshaft didn't fair well during brake-in. The engine doesn't ever pop through the intake under load as flat cams will do.
Need some help here, hate to ask

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Old 01-16-2007, 07:06 PM
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SBC Miss

Sounds like you have a real puzzler on your hands! You mentioned having gone over the valves/adjustment a time or two. Did the rockers on that cylinder loosen up? If not, I have my doubts that the cam lobes are wiped. You could check that by dial indicating rocker movement compared to known good cylinders.
If the plug is fouled by fuel, and your compression tests good, kind of points to either ignition/spark problem or excessive fuel problem. Why it is only the one cylinder is awful strange though!
Will do some more "brainstormin"

Chris Laarman
Ottawa Engine and Performance
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Old 01-16-2007, 07:28 PM
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Like cj11l said, check the lift at the rockers, etc.

I've had one engine with 3 bad lobes, 2 intake, 1 exhaust, that did not affect the engine sound or vibration whatsoever. It was smooth and even with 1/8 inch gone.

The only other thing it could be is that the computer is pulse firing that injector too long.

You might be able to crutch it by going 1-2 heat ranges hotter on that plug. Just to see what happens as a test.

You also said the plug is "blacker than the rest"....... fuel injection plugs will burn almost white with unleaded fuel; almost no color at all, especially if there is O2 sensor feedback.
SO....
You might try backing the fuel pressure down 2-3 pounds as a check, then maybe another 3. Watch the other cylinders to see how they are changing. I guess you have a manifold pressue referenced fuel pressure regulator. Be sure and read your fuel pressure at idle and with the hose off for zero vacuum, also when the engine is revving under full throttle load in 2nd gear.

Since it is one plug only that is very wet, I'm guessing a pulse width problem.
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Old 01-16-2007, 07:31 PM
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Quote:
Originally Posted by cj11l
Sounds like you have a real puzzler on your hands! You mentioned having gone over the valves/adjustment a time or two. Did the rockers on that cylinder loosen up? If not, I have my doubts that the cam lobes are wiped. You could check that by dial indicating rocker movement compared to known good cylinders.
If the plug is fouled by fuel, and your compression tests good, kind of points to either ignition/spark problem or excessive fuel problem. Why it is only the one cylinder is awful strange though!
Will do some more "brainstormin"

Chris Laarman
Ottawa Engine and Performance
Thanks Chris & Xntria for your thoughts,
the valves never loosened up I just did a quick re-adjustment, What you said sure makes sense about the cam lobes.
There are some more things to consider, one the engine runs without a thermostat and runs on the cool side along with taking some short drives as well. Two, it seems the #4 cylinder may be a bit week as well. It too gets a little black like the #2 cylinder.
I have this in a Porsche 914, mid mounted and running an external belt driven water pump without a water pump bypass so if you run it up a bit before the thermostat opens the pump impeller bends, ya nuts because I did it last year.
I guess the injector feed could be skewed slightly, its not sequential injection, one bank fires at a time like the LT1. I could just jumper a new wire in there from the computer since #6 & #8 are fine.
I don't know what else to look at, since the plug gets blackish maybe there is a restriction in the intake, who knows a socket or something? I can't Imagen that but I have seen some strange things.

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Old 01-16-2007, 07:35 PM
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Is it possible you have a bad plug wire? I had a plug wire that had arced completely through the silicone jacket inside the boot at the distributor end of the wire. The engine still ran fairly well but kept fouling one plug over time. You may also want to check the terminals inside and outside of the distributor cap.

I bet that car is a rush to drive... Ever see this page before? www.v8914.com
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Old 01-16-2007, 10:25 PM
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What sort of distributor? too much play in the top bush will hurt a points or reluctor type electronic. Easy to check, 2 or 3 degrees dwell variation at idle will get worse up high as the shaft climbs up the camgear.
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Old 01-16-2007, 10:29 PM
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So I removed the distributor cap and could see that #2 & #4 electrodes inside the cap had more ozone type corrosion. It wasn't something you would replace a cap for so I carefully glass-beaded the contacts of all 8. I then inspected the injector connector, #2 terminals didn't have the same shine the others had. I couldn't tell you the reason so I removed it and replaced it with a spare. I don't expect this to bear fruit but I will start it in the morning with the #2 & #4 plugs rotated to the other bank.
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Old 01-16-2007, 10:33 PM
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Quote:
Originally Posted by IanRiordan
What sort of distributor? too much play in the top bush will hurt a points or reluctor type electronic. Easy to check, 2 or 3 degrees dwell variation at idle will get worse up high as the shaft climbs up the camgear.
Good point,
the distributor is electronic and was new when the engine was rebuilt. I do see your point. Good, another thing to look at if this doesn't work. I sure wish I had a scope
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Old 01-17-2007, 01:09 AM
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I had a single point delco of my own lose #4 off idle and within a year my brother in law's bosch equiped 302c lose a pot. Both top bushes were egg shaped. The 302 dissy was electronic but as clevelands house their lower bush in the block it lost a pot just the same. Yes the second one was easier to track down. Use an analogue dwell meter - if the needle wavers you've a problem.
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Old 01-17-2007, 12:08 PM
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A comment was made in # 4 that your fuel injection is like an LT1 is a bank-fire fuel injection. On a 94-96 model car?

Putting "LT1 fuel injection" into Google I get....

http://www.fuelinjection.com/portinj.html

All the links say it is SFI.

Does your fuel injection bank-fire, or is it like an LT1 ?

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Old 01-17-2007, 02:04 PM
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Quote:
Originally Posted by xntrik
A comment was made in # 4 that your fuel injection is like an LT1 is a bank-fire fuel injection. On a 94-96 model car?

Putting "LT1 fuel injection" into Google I get....

http://www.fuelinjection.com/portinj.html

All the links say it is SFI.

Does your fuel injection bank-fire, or is it like an LT1 ?

I am talking about cars made in 86 or 87, ones with normal distributors located in the back of the intake. The system has only 2 trigger points/wires connected to the ground side of the injector controlled by the ECM. This is bank to bank firing.
I did replace the injector connector but the engine still has the same problem. After driving 20 miles the #2 plug is again black and not firing well.
OK, now the Distributor is coming out...............
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Old 01-17-2007, 02:15 PM
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Have you measured the plug wires? Even MSDīs can fail
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Old 01-17-2007, 02:24 PM
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Quote:
Originally Posted by Pianoman
Have you measured the plug wires? Even MSDīs can fail
He said he has new wires and they are doing the same thing.
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Old 01-17-2007, 02:39 PM
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Not clear if this is a new concern or has been happening since the engine was installed / built. If the valid distributor potentials listed above don't help, I'd lean either 1) in combustion chamber issues (head flash, piston top issues) or the intake runners or gasket having issues. Was this port matched?

Are you indexing the plugs? Does #2 screw in as far as the rest & are you orienting the spark toward the combustion chamber?

Just thinking out loud...figuratively...
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Old 01-17-2007, 04:06 PM
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Quote:
Originally Posted by derm
Not clear if this is a new concern or has been happening since the engine was installed / built. If the valid distributor potentials listed above don't help, I'd lean either 1) in combustion chamber issues (head flash, piston top issues) or the intake runners or gasket having issues. Was this port matched?

Are you indexing the plugs? Does #2 screw in as far as the rest & are you orienting the spark toward the combustion chamber?

Just thinking out loud...figuratively...
I took a look at the distributor, disassembled the unit and theres no problem there.
Leaning out the cylinder may be hard to do even for testing purposes.
I can however lean out the hole system at idle and see if #2 & #4 fire stronger.
I see a black plug which looks to me to be rich, now it is the first 2 cylinders to get fuel, can't see how that could make much difference.
Too much fuel or not enough air ?? Looking into the intake may be my next move. I don't see port matching being a factor, we are talking about idle cfm. I do wonder about the intake gasket? Could it be blocking off the air flow, again not likely to live there so long or to effect the idle given so little cfm needed, WELL maybe? The system will expect X amount of air at idle so maybe it could be a gasket issue or something else in there. I have to look at that next...
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