Dancing Vacuum Needle
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Quite true.
It's really amazing how a tiny exhaust leak can affect the way an engine runs, particularly at idle. Just a little spit, that can barely be heard, can cause the engine to flutter like a misfire, and that will show up on a vacuum gauge.
I can't tell you the number of times I have seen guys trying to chase down a phantom misfire, when it turned out to be a very minor, sometimes nearly inaudible, leak at an exhaust manifold or donut gasket.
Kinda funny how the minor loss of exhaust velocity, what little there is at idle, can cause the whole engine to flutter.
My 300 does this, and has for several years. The manifold has a tiny leak at the top/rear of cyl 2 or 3 (I forget). Can only be heard at idle, after the engine has reached full operating temp, and the alt is not having to supply much charge. Any little load on the engine, and it goes away. I never fixed it, as I am concerned that disturbing the bolts/nuts will result in having one (or more) break, or replacing the gasket will just result in the leak moving somewhere else.....
It's really amazing how a tiny exhaust leak can affect the way an engine runs, particularly at idle. Just a little spit, that can barely be heard, can cause the engine to flutter like a misfire, and that will show up on a vacuum gauge.
I can't tell you the number of times I have seen guys trying to chase down a phantom misfire, when it turned out to be a very minor, sometimes nearly inaudible, leak at an exhaust manifold or donut gasket.
Kinda funny how the minor loss of exhaust velocity, what little there is at idle, can cause the whole engine to flutter.
My 300 does this, and has for several years. The manifold has a tiny leak at the top/rear of cyl 2 or 3 (I forget). Can only be heard at idle, after the engine has reached full operating temp, and the alt is not having to supply much charge. Any little load on the engine, and it goes away. I never fixed it, as I am concerned that disturbing the bolts/nuts will result in having one (or more) break, or replacing the gasket will just result in the leak moving somewhere else.....
Elder User
Joined: Oct 2008
Posts: 708
Likes: 1
I wonder if it's possible that one of the two main venturis on the carb is gunked up. I'm sure your carb is oriented right, so one main venturi is basically fueling cylinders 1,2,3 and the other is fueling 4,5,6. If say the main jet or booster passage or something is clogged on the 4,5,6 venturi then cylinders 1,2,3,4 might be fueling off the good venturi and cylinders 5,6 may be running too lean to even be ignitable.
If your problem comes back, try pulling off the air cleaner and checking to see if gas is dripping from the firewall side venturi booster.
Have you taken the carb off and cleaned it since buying the truck?
If your problem comes back, try pulling off the air cleaner and checking to see if gas is dripping from the firewall side venturi booster.
Have you taken the carb off and cleaned it since buying the truck?
Thread Starter
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Moderator
Joined: Dec 2007
Posts: 7,987
Likes: 104
From: Boise, Idaho
I've had two different carbs on it over the last few weeks. The Holley 390 that's been on it for the last two years, and the Holley 600 from my other Bronco. Both have been rebuilt within the last few months, and both run top notch (and pull 20hg) on the other engine. I'll check it for gas dripping though, who knows.
The problem has definitely become more intermittent. And...sometimes it goes away altogether (which is a huge relief, meaning if I can track it down, I'm good to go). When I finally got it to sit at 18hg again, I pulled all 6 spark plugs one by one and all of them responded the same and as they should.
Then, I'd rev it up and when it'd come back down to idle, the vacuum would be at 15 or so, it'd run rough, there'd be a tick from the engine, etc. etc.
Just need to track down what it is, but I'm not sure where to start. New intake/exhaust manifold, fresh (4 months or less) rebuild on the carb, new carb base gaskets, and my only vacuum lines are the ones running to the PCV, booster, and vacuum advance.
I can try tightening the manifold to exhaust pipe bolts a little bit more, but the rest of the exhaust is sound. They're definitely pretty tight though, and the EFI manifolds don't use an exhaust donut.
The problem has definitely become more intermittent. And...sometimes it goes away altogether (which is a huge relief, meaning if I can track it down, I'm good to go). When I finally got it to sit at 18hg again, I pulled all 6 spark plugs one by one and all of them responded the same and as they should.
Then, I'd rev it up and when it'd come back down to idle, the vacuum would be at 15 or so, it'd run rough, there'd be a tick from the engine, etc. etc.
Just need to track down what it is, but I'm not sure where to start. New intake/exhaust manifold, fresh (4 months or less) rebuild on the carb, new carb base gaskets, and my only vacuum lines are the ones running to the PCV, booster, and vacuum advance.
I can try tightening the manifold to exhaust pipe bolts a little bit more, but the rest of the exhaust is sound. They're definitely pretty tight though, and the EFI manifolds don't use an exhaust donut.
FTE Legend
Joined: May 2008
Posts: 56,542
Likes: 0
From: In my own world
After thinking about this,I have a feeling the valves needed to be lapped again, after the valve spring rotator change. Maybe what's going on, is that they are lapping themselves in, through use. When you made the change, it held the valve a little bit different than when the head was first asembled. Continue to run it for a bit, and I think it will heal itself.
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
After thinking about this,I have a feeling the valves needed to be lapped again, after the valve spring rotator change. Maybe what's going on, is that they are lapping themselves in, through use. When you made the change, it held the valve a little bit different than when the head was first asembled. Continue to run it for a bit, and I think it will heal itself.
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Moderator
Joined: Dec 2007
Posts: 7,987
Likes: 104
From: Boise, Idaho
That's definitely an interesting and plausible idea.
I'm struggling to think of anywhere on the engine that'd have an intake or exhaust leak (it's sealed up pretty tight), and I swapped in an entirely known good ignition (minus the ignition module), and it didn't make any difference.
It also does seem to be getting better every day, so "healing" itself seems to be in the works... If (and when) I pull the head again for a full P&P job, I'll be sure to have a lot of attention done to a valve job and lapping the valves.
I'm struggling to think of anywhere on the engine that'd have an intake or exhaust leak (it's sealed up pretty tight), and I swapped in an entirely known good ignition (minus the ignition module), and it didn't make any difference.
It also does seem to be getting better every day, so "healing" itself seems to be in the works... If (and when) I pull the head again for a full P&P job, I'll be sure to have a lot of attention done to a valve job and lapping the valves.
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Moderator
Joined: Dec 2007
Posts: 7,987
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From: Boise, Idaho
Well, if that's the case, that at least puts my mind at ease. Wish I had lapped the valves, but not a lot I can do about that now short of removing it.
Would that explain odd tickings, sputters (misfires?), and stuff that come and go with the smoothness (and vacuum levels) of the engine at idle?
Would that explain odd tickings, sputters (misfires?), and stuff that come and go with the smoothness (and vacuum levels) of the engine at idle?
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Moderator
Joined: Dec 2007
Posts: 7,987
Likes: 104
From: Boise, Idaho
That definitely does help me relax, especially since it is improving, and there are *moments* when it actually sounds like a healthy engine (ticking goes away, idle smooths out, all other weird behaviors and sounds seem to go away), so I know the potential for a successful build is there. 
Next on the list before I can just sit back and enjoy it, though, is figuring out why it bucks, stumbles, pops, stalls, the second it has any load on it under 1500 RPMs.... Talking flat on its face dead with a nice pop out of the carb.
Not sure why yet. It didn't have the problem when I had the 390 4bbl on there, and the 6004bbl I have on there now accelerated like a dream when I had it on my other 300.
Next on the list before I can just sit back and enjoy it, though, is figuring out why it bucks, stumbles, pops, stalls, the second it has any load on it under 1500 RPMs.... Talking flat on its face dead with a nice pop out of the carb.
Not sure why yet. It didn't have the problem when I had the 390 4bbl on there, and the 6004bbl I have on there now accelerated like a dream when I had it on my other 300.
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Moderator
Joined: Dec 2007
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From: Boise, Idaho
Maybe I'll swap the carbs back around so its driveable again. Besides, my wife's missing her big 600cfm power whenever she steps on the gas. Complains the 390 just "isn't as fun". 
I'll probably run it down to my parents and back and put a good 250 miles of interstate driving on it.
I'll probably run it down to my parents and back and put a good 250 miles of interstate driving on it.