Low RPMs?
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New User
Joined: Oct 2008
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Well... I believe I've figured out what's going on. I highly suspect the valves are floating at around 2500 rpm. I guess I'm a little slow, but it never occurred to me that springs could be that bad. Sorry for the poor description of this problem. I expect you guys would have recognized the problem right away of I had said it sounds like it's about ready to blow up at 2500 rpm. Apparently this engine has a LOT of miles on it. I've got some springs on the shelf I can put in it. Hopefully the rings are still in good shape.
We'll see...
We'll see...
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I have the upper intake off, but I haven't removed the valve covers yet. I'm still cleaning gunk off the engine. I absolutely expect the valves to compress by hand... But I'm definitely going to try that. My Haynes manual says the intake valve be between 74-82 pounds seat pressure. Exhaust is 76-84 pounds. That seems awfully light to me. No wonder they're floating. I've been looking for replacement springs and can't find any that light. 
More Turbo
Joined: Sep 2003
Posts: 630
Likes: 4
From: Northeast
I'm Confused....
My Haynes manual says the intake valve be between 74-82 pounds seat pressure. Exhaust is 76-84 pounds. That seems awfully light to me. No wonder they're floating. I've been looking for replacement springs and can't find any that light.
Did I miss something here?
And if the basis for your conclusion that you're experiencing valve float is merely that the 76-85 pound-spec. is "too light," why aren't there myriad other reports of extremely low RPM-induced valve float in our trucks?
I would be more inclined to suspect premature cam lobe round-off, due to the oil companies' drastic reduction of zinc (is it zinc?) additives in motor oils, recently, to prevent premature cat. converter failure, than I would valve float.
Whatever the cause, your situation is definitely a brain-twister, and I look forward to your solving it and publishing your findings.
Good luck with it.
Postmaster
Joined: Jan 2009
Posts: 4,558
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Ya i think were all confused..Never heard what happend to your timing problems...I dont think anyone thinks its your valve springs ??? But really want to hear how it turns out...& what you found....
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From: ohio
I will seriously doubt that u have bad valve springs but im thinkin u have a rounded lobe on the cam. Hook up a vaccum guage and see what it yields. Should be smooth at idle and drop rapidly with opening of throttle. If its jittery then go tearing into motor. Google vaccum tests on a running engine, it should tell u if u have clogged cats,bad valve springs ect.
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Well, you're right... I'm getting ahead of myself a little. I'll take the valve covers off today and see what I can tell about the springs. I already ran a vacuum test and was getting 17 psi at idle. I checked a couple of cylinders and got 125 psi compression in both. Since all eight plugs look identical, they're light tan, I assumed all the remaining cylinders we ok. I'll leave that as something I may want to complete later. The compression suggests to me the timing chain is ok, but I'll take a better look at it and see if the chain stretch is excessive. I should know more by the end of today.
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The mystery appears to be solved. I pulled a valve cover off and the springs felt fine. so I moved the timing mark to zero and had a friend watch the rotor while I backed up the crank. The timing mark moved 10 to 12 degrees before the rotor started to move. That means the valves are 20 to 24 degrees late. That'll do it! It's actually pretty impressive that I got 125 psi with the valves so late. In any case, I should get noticeable better power and noticeably better fuel economy when I get a new timing chain on it.
Thanks for the input guys. And a tip of the hat to Old93junk. You were right on the mark!
Thanks for the input guys. And a tip of the hat to Old93junk. You were right on the mark!
Post Fiend
Joined: Nov 2008
Posts: 23,849
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From: McKenzie River
The mystery appears to be solved. I pulled a valve cover off and the springs felt fine. so I moved the timing mark to zero and had a friend watch the rotor while I backed up the crank. The timing mark moved 10 to 12 degrees before the rotor started to move. That means the valves are 20 to 24 degrees late. That'll do it! It's actually pretty impressive that I got 125 psi with the valves so late. In any case, I should get noticeable better power and noticeably better fuel economy when I get a new timing chain on it.
Thanks for the input guys. And a tip of the hat to Old93junk. You were right on the mark!
Thanks for the input guys. And a tip of the hat to Old93junk. You were right on the mark!
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jimkenzie
1973 - 1979 F-100 & Larger F-Series Trucks
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Oct 18, 2004 11:32 PM
