excessive lean condition?
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Senior User
Joined: Jun 2005
Posts: 321
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From: Atlanta
excessive lean condition?
Looking for advice. 77 300 I-6 with Carter YFA.
The engine idles rough. There is a persistent miss. When I pull a plug it is very very light brown, leaning towards grey. Spark plug is telling me that things are on the lean side.
I am assuming that the reason for the rough idle / missing is due to a lean condition (I'm told that what it usually is). Most common reason for a lean condition is a vacuum leak, right? I can't find it.
20" of vacuum on the vacuum gauge, pretty stable.
Plugs gapped right, distributor new and clean on the inside. Coil is less than a year old. Spark box is Motorcraft, less than a year old.
Set the timing using a vacuum gauge because I think the harmonic balancer has slipped some. If I set it by the grid on the balancer the spark is WAY too early according to the vacuum gauge.
Rebuilt the carb about 6 months ago. It should be pretty clean.
Sprayed brake fluid around and didn't notice an RPM change, so I didn't find an obvious leak.
I don't want to spend $300 on a new harmonic balancer only to find out that isn't the problem.
If I had to guess where there might be a leak, I would guess it is on the intake manifold, if anyplace.
What is the most logical way to troubleshoot this problem?
The engine idles rough. There is a persistent miss. When I pull a plug it is very very light brown, leaning towards grey. Spark plug is telling me that things are on the lean side.
I am assuming that the reason for the rough idle / missing is due to a lean condition (I'm told that what it usually is). Most common reason for a lean condition is a vacuum leak, right? I can't find it.
20" of vacuum on the vacuum gauge, pretty stable.
Plugs gapped right, distributor new and clean on the inside. Coil is less than a year old. Spark box is Motorcraft, less than a year old.
Set the timing using a vacuum gauge because I think the harmonic balancer has slipped some. If I set it by the grid on the balancer the spark is WAY too early according to the vacuum gauge.
Rebuilt the carb about 6 months ago. It should be pretty clean.
Sprayed brake fluid around and didn't notice an RPM change, so I didn't find an obvious leak.
I don't want to spend $300 on a new harmonic balancer only to find out that isn't the problem.
If I had to guess where there might be a leak, I would guess it is on the intake manifold, if anyplace.
What is the most logical way to troubleshoot this problem?
Thread Starter
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Senior User
Joined: Jun 2005
Posts: 321
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From: Atlanta
Yes, it has an EGR valve. I forgot to mention that I pulled the valve off and plugged off the intake hole so that it didn't suck air. I still had the persistent miss.
I'm thinking I've got something stuck in the carb that is leaning things out.
My problem is that I think I've tried everything at least twice and gotten no improvement. I'm waiting for one of you smart guys on the site to tell me what I've forgotten (or did wrong).
I'm thinking I've got something stuck in the carb that is leaning things out.
My problem is that I think I've tried everything at least twice and gotten no improvement. I'm waiting for one of you smart guys on the site to tell me what I've forgotten (or did wrong).
Senior User
Joined: Feb 2013
Posts: 266
Likes: 0
From: Idaho
Looking for advice. 77 300 I-6 with Carter YFA.
The engine idles rough. There is a persistent miss. When I pull a plug it is very very light brown, leaning towards grey. Spark plug is telling me that things are on the lean side.
I am assuming that the reason for the rough idle / missing is due to a lean condition (I'm told that what it usually is). Most common reason for a lean condition is a vacuum leak, right? I can't find it.
20" of vacuum on the vacuum gauge, pretty stable.
Plugs gapped right, distributor new and clean on the inside. Coil is less than a year old. Spark box is Motorcraft, less than a year old.
Set the timing using a vacuum gauge because I think the harmonic balancer has slipped some. If I set it by the grid on the balancer the spark is WAY too early according to the vacuum gauge.
Rebuilt the carb about 6 months ago. It should be pretty clean.
Sprayed brake fluid around and didn't notice an RPM change, so I didn't find an obvious leak.
I don't want to spend $300 on a new harmonic balancer only to find out that isn't the problem.
If I had to guess where there might be a leak, I would guess it is on the intake manifold, if anyplace.
What is the most logical way to troubleshoot this problem?
The engine idles rough. There is a persistent miss. When I pull a plug it is very very light brown, leaning towards grey. Spark plug is telling me that things are on the lean side.
I am assuming that the reason for the rough idle / missing is due to a lean condition (I'm told that what it usually is). Most common reason for a lean condition is a vacuum leak, right? I can't find it.
20" of vacuum on the vacuum gauge, pretty stable.
Plugs gapped right, distributor new and clean on the inside. Coil is less than a year old. Spark box is Motorcraft, less than a year old.
Set the timing using a vacuum gauge because I think the harmonic balancer has slipped some. If I set it by the grid on the balancer the spark is WAY too early according to the vacuum gauge.
Rebuilt the carb about 6 months ago. It should be pretty clean.
Sprayed brake fluid around and didn't notice an RPM change, so I didn't find an obvious leak.
I don't want to spend $300 on a new harmonic balancer only to find out that isn't the problem.
If I had to guess where there might be a leak, I would guess it is on the intake manifold, if anyplace.
What is the most logical way to troubleshoot this problem?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
If you have a solid 20 inches on the vacuum gauge, then there is no vacuum leak. I find it really hard to believe that an engine that misses and idles rough has a perfect vacuum gauge reading; usually the vacuum gauge is the tell-all. Is there any chance there is more to the story?
I don't recommend setting the timing with a vacuum gauge; it can come out over-advanced which could explain your lean plugs. I know you don't want to replace the harmonic balancer, but a timing light is the tool for setting base timing. It's also hard to solve problems in the midst of other problems you don't intend on solving.
I don't recommend setting the timing with a vacuum gauge; it can come out over-advanced which could explain your lean plugs. I know you don't want to replace the harmonic balancer, but a timing light is the tool for setting base timing. It's also hard to solve problems in the midst of other problems you don't intend on solving.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
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More Turbo
Joined: Jan 2008
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From: in the middle of nowhere
You could use a piston stop and check to see if your balancer is off. Heck just rescribe it and time it with a dial back .
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Thread Starter
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Senior User
Joined: Jun 2005
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From: Atlanta
If you have a solid 20 inches on the vacuum gauge, then there is no vacuum leak. I find it really hard to believe that an engine that misses and idles rough has a perfect vacuum gauge reading; usually the vacuum gauge is the tell-all. Is there any chance there is more to the story?
....
....
Nothing in life is ever "perfect". Gauge goes to about 19-20 inches and doesn't oscillate. What I mean is, I've seen the web pictures of how to diagnose what you see on a vacuum gauge (e.g. bad valve) and it isn't doing any of those things.It does drop when the engine misses. You hear the engine bump and the pressure will drop a couple of inches when the engine bumps. Then it pops right back up where it was.
Here is my plan for the weekend. 1) use a stick to find TDC on #1. As suggested, scribe the balancer at that point. 2) get the timing light out and see if I can correctly set timing with it. If I can and it doesn't improve things, 3) time to clean, clean, clean and rebuild the carburetor.
Good plan or bad plan?
Cargo Master
Joined: May 2009
Posts: 2,587
Likes: 47
From: Eagle River, AK
Nothing in life is ever "perfect". Gauge goes to about 19-20 inches and doesn't oscillate. What I mean is, I've seen the web pictures of how to diagnose what you see on a vacuum gauge (e.g. bad valve) and it isn't doing any of those things.It does drop when the engine misses. You hear the engine bump and the pressure will drop a couple of inches when the engine bumps. Then it pops right back up where it was.
Here is my plan for the weekend. 1) use a stick to find TDC on #1. As suggested, scribe the balancer at that point. 2) get the timing light out and see if I can correctly set timing with it. If I can and it doesn't improve things, 3) time to clean, clean, clean and rebuild the carburetor.
Good plan or bad plan?
More Turbo
Joined: Jan 2008
Posts: 628
Likes: 3
From: in the middle of nowhere
You would get a more accurate reading with a piston stop instead of a dowel. There should be numerous videos on youtube about degree'n camshafts. Just use the same principle on the crank using the timing pointer instead of the cam.
Thread Starter
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Senior User
Joined: Jun 2005
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From: Atlanta
looked it up on youtube. never seen that before. Learn something new all the time around here. Thanks.
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Senior User
Joined: Jun 2005
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From: Atlanta
OK, when I ask for advice, I usually listen. Ordered a piston stop and timing tape from Summit yesterday. Should be here today. I'll set the timing the right way this weekend and start from there.
PS: After I do, if I see the harmonic balancer really slip, I'll swap it out.
PS: After I do, if I see the harmonic balancer really slip, I'll swap it out.
Cargo Master
Joined: May 2009
Posts: 2,587
Likes: 47
From: Eagle River, AK
OK, when I ask for advice, I usually listen. Ordered a piston stop and timing tape from Summit yesterday. Should be here today. I'll set the timing the right way this weekend and start from there.
PS: After I do, if I see the harmonic balancer really slip, I'll swap it out.
PS: After I do, if I see the harmonic balancer really slip, I'll swap it out.
Then all you have to do is figure out the compression vs. exhaust stroke and you're home free. Since my valve covers are on, I will use the thumb over the spark plug hole method to find the compression stroke.
Tuned
Joined: Feb 2013
Posts: 258
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From: Peterstown
I had a 1989 Dodge with a 318 in it one time that did the exact same thing that you are describing. And I, like you, did everything I could possibly think of, but nothing corrected it. I checked the EGR. The plunger was operating correctly, and it was getting vacuum, at the correct time. Finally, I just decided that I was going to replace the EGR anyway and hope that it would not be a waste of forty bucks. When I got it off, I inspected it visually, and guess what I found. The piston inside of it (you can see it through one of the exhaust holes on the bottom) was completely gone. I guess it must have burnt up or rusted away. Either way, it was gone and I had exhaust gases going through my carb at all times. Anyway, I replaced the EGR with a new one, and problem solved. It ran like a brand new one. So, before you do anything else expensive, check that EGR. Worst case scenario (if it isn't bad) is, you'll have to spend 2 bucks for a new EGR gasket.