390 Rough Idle
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Senior User
Joined: Nov 2007
Posts: 120
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From: New York City
390 Rough Idle
My truck's been idling rough for quite a while...enough to shake the image in the mirror in idle. When in cruise, its got power and vibration goes away. I thought that maybe it shakes because one of the cylinders is missing at idle, so I decided to check this issue by disconnecting one spark plug wire for each of the cylinders and see if there isn't any vibration increase for the troubled cylinder. This is what I found:
#1 no vibration increase
#5 vibration increase
#4 no vibration increase
#2 vibration increase
#6 no vibration increase
#3 vibration increase
#7 no vibration increase
#8 vibration increase
My current timing is set at 13. Could this be due to the distributor points or the rotor contact? Or does my timing need to be adjusted? Or is this just normal and I should sit back and relax?
#1 no vibration increase
#5 vibration increase
#4 no vibration increase
#2 vibration increase
#6 no vibration increase
#3 vibration increase
#7 no vibration increase
#8 vibration increase
My current timing is set at 13. Could this be due to the distributor points or the rotor contact? Or does my timing need to be adjusted? Or is this just normal and I should sit back and relax?
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
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From: Long Island USA
Notice how those cylinder numbers seem to be all in one "plane" of the intake manifold?
1-4-6-7 are all in one plane, I think.
That means, one of the two primaries in the carb is not idling correctly.
Did you set the idle mix screws at all yet?
1-4-6-7 are all in one plane, I think.
That means, one of the two primaries in the carb is not idling correctly.
Did you set the idle mix screws at all yet?
Tuned
Joined: Feb 2010
Posts: 277
Likes: 21
From: Western Montana
I'm pretty sure the cylinders are laid out in numerical order with the FE blocks...1234 on one head and 5678 on the other. He did lay them out in the firing order though.
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
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From: Long Island USA
I'm talking about the two plane intake manifold. One plane feeds two cylinders on one side, two on the other. Usually the middle two on one side, and the outer two on the other.
Which is 1-4-6-7 and 2-3-5-8
Which is exactly how his tests went. And the reason why ONE idle screw was causing the issue.
Tuned
Joined: Feb 2010
Posts: 277
Likes: 21
From: Western Montana
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FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 423
From: Long Island USA
Not a problem - it's a good way to diagnose these sort of things. Even when pulling the plugs, if one bank is lean/rich, it's obvious.
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Senior User
Joined: May 2010
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From: Florida
I am having a similar issue. Warming the engine runs fine after this it seems to miss at idle and will go away after you pass about 25 miles per hour or get on it. A lean condition will cause this? I contacted Holley today and they told me they have never heard of a carb causing a miss.
Senior User
Joined: Feb 2011
Posts: 243
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From: Spokane, Wa.
First thing that comes to mind is possibly the points if you still have the old points system. Only enough I was having the same kind of issue and checked the points gap. It was narrower than the specs called for, but after slipping the feeler gauges in there to check I haven't had the problem, even though I didn't actually adjust anything.
Senior User
Joined: May 2010
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From: Florida
I actually found a burned plug wire within the boot. The wires are new made by Petronix but i have been having trouble with the ends. A new different brand name is on the way. I have the Petronix set up in my old distributor. I have some play in the distributor shaft also, i am replacing it as well.
Senior User
Joined: Feb 2011
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From: Spokane, Wa.
Ya, a burned wire could cause some problems. Any idea why it burned? Did they screw up when they crimped the connector or was it a matter of not getting seated on the plug right? I only ask cus if its not one of those then I would do some investigating as to the cause so it don't happen again.
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Joined: Aug 2010
Posts: 113
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From: Moreno Valley
My truck's been idling rough for quite a while...enough to shake the image in the mirror in idle. When in cruise, its got power and vibration goes away. I thought that maybe it shakes because one of the cylinders is missing at idle, so I decided to check this issue by disconnecting one spark plug wire for each of the cylinders and see if there isn't any vibration increase for the troubled cylinder. This is what I found:
#1 no vibration increase
#5 vibration increase
#4 no vibration increase
#2 vibration increase
#6 no vibration increase
#3 vibration increase
#7 no vibration increase
#8 vibration increase
My current timing is set at 13. Could this be due to the distributor points or the rotor contact? Or does my timing need to be adjusted? Or is this just normal and I should sit back and relax?
#1 no vibration increase
#5 vibration increase
#4 no vibration increase
#2 vibration increase
#6 no vibration increase
#3 vibration increase
#7 no vibration increase
#8 vibration increase
My current timing is set at 13. Could this be due to the distributor points or the rotor contact? Or does my timing need to be adjusted? Or is this just normal and I should sit back and relax?
Thread Starter
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Senior User
Joined: Nov 2007
Posts: 120
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From: New York City
How would the canister purge cause bad idle. I thought that this can be an issue with newer models.
http://i69.photobucket.com/albums/i7...tivesystem.jpg
http://i69.photobucket.com/albums/i7...tivesystem.jpg