Is this 351m 'missing' or is it something else?
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Is this 351m 'missing' or is it something else?
I changed all my plugs, wires, cap,and rotor this week on my 77 351m. I noticed the RPMs bouncing randomly. I did have a misfire, which prompted me to replace everything, I don't know if the RPMs were doing this before. While it's doing this the engine feels smooth as far as I can tell. I can hear a little flutter... I think. Or maybe it's in my head. If it's there, it's faint.
I posted a video below. Hopefully it shows up.
First I thought it was a misfire so I pulled each wire one at a time to see if anything changes. You can definitely feel the misfire (from the pulled wire) and the RPMs still bounce, but the RPM bounce doesn't coincide with the misfires. Is this a vacuum issue maybe? I don't know what to check. Is it nothing to worry about?
http://vid139.photobucket.com/albums...psd7201e06.mp4
Here is a video with one plug wire pulled:
http://vid139.photobucket.com/albums...psb653682a.mp4
I posted a video below. Hopefully it shows up.
First I thought it was a misfire so I pulled each wire one at a time to see if anything changes. You can definitely feel the misfire (from the pulled wire) and the RPMs still bounce, but the RPM bounce doesn't coincide with the misfires. Is this a vacuum issue maybe? I don't know what to check. Is it nothing to worry about?
http://vid139.photobucket.com/albums...psd7201e06.mp4
Here is a video with one plug wire pulled:
http://vid139.photobucket.com/albums...psb653682a.mp4
Fleet Mechanic
Joined: Jun 2013
Posts: 1,895
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I would suspect a vacuum leak.. some say spray Carb cleaner some will say starter fluid but my preferred way is to just listen very carefully for the hiss.. I have also seen the old duraspark box do some crazy things but not as subtle an issue as this.
Good luck
Good luck
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
I think your tach is bad. It says the engine is idling between 500 RPM (where it would be stumbling) and 1000 RPM (where it would almost be screaming) and you would feel a +/- 500 RPM jump. That's more than a typical engine jumps going out of gear. The idle sounds smooth in your video and you said it idles smooth yourself. That tach looks cheap and old anyway; I'd check with a second one.
That's the only symptom, right? Just suspicious tach behavior? There's no sense in evaluating what the engine does with a plug wire removed. It's supposed to run poorly that way. Am I missing something?
That's the only symptom, right? Just suspicious tach behavior? There's no sense in evaluating what the engine does with a plug wire removed. It's supposed to run poorly that way. Am I missing something?
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Bad tach. That might be. Never occurred to me. Good call.
By removing one wire at a time I was hoping to find the cylinder that was missing sporadically [if that's what it was] by making it miss every time.
By removing one wire at a time I was hoping to find the cylinder that was missing sporadically [if that's what it was] by making it miss every time.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Hold the phone; you're saying it is missing intermittently? I thought you had a smooth running engine after a tune-up, yet were still pulling plug wires, hence my confusion. In one clear sentence, can you please state the exact symptoms that are occurring at the present moment?
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1. The symptom is the bouncing tach. Which I thought was a miss but wasn't sure. Which is why I started the thread.
2. My first thought was to find the cylinder that was missing if that's what it was, so I unplugged one wire at a time to try and make the random tach jump into a consistent thing.
2. My first thought was to find the cylinder that was missing if that's what it was, so I unplugged one wire at a time to try and make the random tach jump into a consistent thing.
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I'm not the best at feeling a miss so wasn't sure. The engine isn't as smooth as a babies butt. The tach threw me off 
. Making sense?
. Making sense?
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Thanks, got it now. Like I was saying, that big of a jump on the tach when the engine is otherwise operational points to a bad tach (or wiring to it). That tach also has a pretty coarse scale. A miss that causes a 100-RPM flutter would be hard to pick up. I'd swap that tach.
As far as the imperfect idle, a good way to quantify the behavior is with a vacuum gauge as stated above. A healthy engine idles with the needle stuck between 19 and 20 inches of Hg out of gear. If it doesn't, the behavior of the needle can point you where to look. It's possible for an engine to still have a light miss in the presence of a smooth vacuum reading, but a vacuum gauge will identify about 85% of problems. It's always the first place to start.
As far as the imperfect idle, a good way to quantify the behavior is with a vacuum gauge as stated above. A healthy engine idles with the needle stuck between 19 and 20 inches of Hg out of gear. If it doesn't, the behavior of the needle can point you where to look. It's possible for an engine to still have a light miss in the presence of a smooth vacuum reading, but a vacuum gauge will identify about 85% of problems. It's always the first place to start.
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