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Old 10-01-2011, 01:00 AM   #1
beatified
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Default No low end power and rough idle

Hi, I have a 77 425 with minimal upgrades.

My problem is that my idle is extremely rough and I have no low end power. I have had this car for a long time and when it was running at its best I could peal out in it all day long, and even before any upgrades. It just wont do that at all anymore.

At higher rpm it runs pretty well and has a nasty (good) growl that is most likely due to the Quadra-Jet oversized valves and free flowing exhaust. My secondaries are deffinately opening although it takes a while for the car to warm up to the point that they will open.

Really when it boils down to it power is good at high rpm but honestly I feel it might be missing a little there as well just because there is an issue at low rpm that I am guessing is effecting high rpm as well just not as noticably.

Timeing seems to be on (had it pinging and backed it of to where it just barely disappeared)
Vacumm is low but just barely at about 16 to 18.

Here's what I've done to it.

Fully rebuilt
Oversized Valves
very mildly ported intake
.30 over
upgraded rockers not rollers or ajustable
Cloes Timeing chain and gears set at normal position not advanced or retarded.
Earlier style exhaust manifolds off of a 472 or 500 cause I had a cracked one
2.5 in Y pipe into 3 in high flow cat then into 3 in single in 2.5 dual out muffler
Stock rebuilt Quadra-Jet

I just want that low end power back. Any ideas?
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Old 10-01-2011, 08:38 AM   #2
Nashalac
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Would ck and verify lifter preload and possible vacuum leak
Really sounds like a vac leak
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Old 10-01-2011, 02:23 PM   #3
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What is the cranking compression of all eight cylinders?

A cylinder balance test would also be helpful. If you have a cylinder balance meter, or as part of an automotive oscilloscope, great. Otherwise, put a bit of silicone dielectric grease on the point of a test light, ground the alligator clip, and then slide the point down between the spark plug boot and the plug wire. DON'T PIERCE THE INSULATION, just slide between the boot and wire. When the point makes contact with the metal end of the plug wire, it'll short the spark to ground, killing that one cylinder.

Make note of how much the rpm or the vacuum (or both) drop when each cylinder is shorted in turn.

When you short out a weak cylinder, the vacuum and RPM won't drop much. Good cylinders will drop vacuum and RPM.

If you have a dual-plane intake manifold, and every other cylinder shows "bad", you've likely got a problem with the idle mixture on the carburetor.
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Old 03-04-2012, 09:18 AM   #4
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Quote:
Originally Posted by Nashalac View Post
Would ck and verify lifter preload and possible vacuum leak
Really sounds like a vac leak
I know this is late but finally did a compression test. I have no more difference than 10 psi between cylinders. Most cylinders are 145 or 150 one is 142.5. From my math that's seems to be a difference of at most 7 percent.

I did have a massive intake manifold leak around the EGR ports to the heads. I have taken care of that already though.

Any clues as to where to go next.

My thought is to do a smoke test on the vacuum system.
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Old 03-04-2012, 11:35 AM   #5
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Quote:
Originally Posted by beatified View Post
Any clues as to where to go next.
Quote:
Originally Posted by Schurkey View Post

A cylinder balance test would also be helpful. If you have a cylinder balance meter, or as part of an automotive oscilloscope, great. Otherwise, put a bit of silicone dielectric grease on the point of a test light, ground the alligator clip, and then slide the point down between the spark plug boot and the plug wire. DON'T PIERCE THE INSULATION, just slide between the boot and wire. When the point makes contact with the metal end of the plug wire, it'll short the spark to ground, killing that one cylinder.

Make note of how much the rpm or the vacuum (or both) drop when each cylinder is shorted in turn.

When you short out a weak cylinder, the vacuum and RPM won't drop much. Good cylinders will drop vacuum and RPM.

If you have a dual-plane intake manifold, and every other cylinder shows "bad", you've likely got a problem with the idle mixture on the carburetor.
What are your results?
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Old 03-04-2012, 12:00 PM   #6
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Quote:
Originally Posted by Schurkey View Post
What are your results?
1 = 150
2 = 145
3 = 150
4 = 147.5
5 = 150
6 = 145
7 = 150
8 = 142.5
As far as the cylinder balance test I did a really cheesy version of it and found that all cylinders are getting good spark and it did make a difference in how the engine ran when I pulled the wire from the plug although it was very hard to detect. Is it really needed to go through all the steps you listed in order to do a cylinder balance test, or is this enough? I will do what ever it takes but if Its not needed then probably no point in repeating. Is there a way to detect a change in idle that's more pronounced? I seem to have problems detecting a change when pulling a wire.

Thanks for all your help,
Stuart
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Old 03-04-2012, 01:55 PM   #7
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Quote:
Originally Posted by beatified View Post
As far as the cylinder balance test I did a really cheesy version of it and found that all cylinders are getting good spark and it did make a difference in how the engine ran when I pulled the wire from the plug although it was very hard to detect. Is it really needed to go through all the steps you listed in order to do a cylinder balance test, or is this enough? I will do what ever it takes but if Its not needed then probably no point in repeating. Is there a way to detect a change in idle that's more pronounced? I seem to have problems detecting a change when pulling a wire.
Use a vacuum gauge instead of a tach.

You pull the plug wire off, you risk getting zapped by the spark. You do it with a test light the way I explained, there's no shock.
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Old 03-04-2012, 09:16 PM   #8
Bruce Roe
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I hope your engine has a replacement timing chain.
The OEM can fail at this age, regardless of mileage.

Bruce Roe
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Old 03-05-2012, 01:36 PM   #9
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Quote:
Originally Posted by Bruce Roe View Post
I hope your engine has a replacement timing chain.
The OEM can fail at this age, regardless of mileage.

Bruce Roe

I do indeed have a new timing chain (Cloyes). I already lost the timing chain years ago and replaced it then.
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Old 03-05-2012, 01:47 PM   #10
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I posted a video of my car starting in the morning and an odd thing it does.

It runs horribly for the first few minutes and at too low of an RPM then the idle rises on its own. I dont know what is causeing this but I wonder if this has something to do with my lack of low end power.

This one is really stumping me so if you have any ideas let me know.

http://www.youtube.com/watch?v=FivX6xeque0

I really wonder if my fuel is not atomizing and its entering the cylinders in big drops rather than being well mixed with air.

The video is pretty much useless until about 3:10 then you see the idle rise and then I release the choke just to show that indeed the choke did engage.
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