Vacuum gauge reading fluctuating...

[sasilverbullet]

70 460 Lincoln engine:
At idle a vacuum gauge connected directly to manifold vacuum is jumping between 15-17.

A website I read said that's probably worn valve guides, you guys concur with that?

 

[Eddie]

YES but, you may want to check for vacume leaks. Sometimes a small slit will open & close during pressure pulses.

 

[Cause4Alarm]

Weak valve springs will cause it, but the one thing I would check after after making sue there's no vacume leaks is the timing chain

 

[punisher]

Couple of things on those. Verify timing is to spec at idle.

When you look at a vac gauge you can usually tell by the needle movement whether you have a problem on one cylinder, or multiple cylinders. One cylinder causing needle movement= problem.

Some of those 460s had cams that were really retarded due to emissions concerns (on a 1970 model, maybe, maybe not). If the timing chain/gears have been replaced, the cam might be retarded as the replacement gears were often set up that way (my memory could be off on that). Same goes for a sloppy timing chain. Retarded cams cause fluctuations in the vac readings.

 

[Radman]

Originally Posted By: punisher
Couple of things on those. Verify timing is to spec at idle.

When you look at a vac gauge you can usually tell by the needle movement whether you have a problem on one cylinder, or multiple cylinders. One cylinder causing needle movement= problem.

Some of those 460s had cams that were really retarded due to emissions concerns (on a 1970 model, maybe, maybe not). If the timing chain/gears have been replaced, the cam might be retarded as the replacement gears were often set up that way (my memory could be off on that). Same goes for a sloppy timing chain. Retarded cams cause fluctuations in the vac readings.


Punisher is spot on!! I would be leaning towards the sloppy timing chain as I have seen this discrepancy a few times.

 

[sasilverbullet]

The timing chain/sprockets are fairly new. Timing is set spot-on.

 

[lumpy_grits]

IF your are carbed..
Make sure BOTH of your idle mix. screws are set the same.
FWIW: a 2 point swing is not a big deal.
Respectfully,
LG

 

[sasilverbullet]

Unfortunately, yes, I'm "carbed"

I have had an issue in the past with the idle mixture screws not changing anything when idling. I took them out and blew compressed air and that seemed to fix it. But I'm still not impressed with how they work on this car. The carb is a recently new Edelbrock 1406...

 

[Rickey]

Originally Posted By: sasilverbullet
70 460 Lincoln engine:
At idle a vacuum gauge connected directly to manifold vacuum is jumping between 15-17.

A website I read said that's probably worn valve guides, you guys concur with that?


More info please:

Does the engine skip or shake at idle?
If so does it smooth out on the road?
Is the vacuum gauge needle actually jumping at a rate consistent with the (mis)firing of a particular cylinder?
If so grounding or removing spark plug wires one at a time may reveal which cylinder has a problem.
IE: Removing or grounding a good cylinder's spark plug wire "should" have more effect than removing or grounding a bad cylinder's spark plug wire.

Is there any unusual valve train noise?

As others have mentioned sticking and/or burned valve(s) can cause this. as can a broken valve spring. I'm sure it is possible but I have not seen a timing gear/chain cause this type problem. Valve guide(s)?....not my first choice.

HMMMMMM '70 model engine......originally speced for leaded fuel only?
Possible exhaust seat erosion due to lack of lead in fuel.
If so that could explain the symptom.

Very few iron head engines of that era had hardened exhaust valve seats.
One possibility is a burned exhaust valve. Quite frequently a burned exhaust valve will create a peculiar "suck back" in the exhaust at idle.
A sticking intake valve or one with a broken spring will frequently cause backfiring through the carburetor on acceleration.

Don't overlook the simple: if not already checked....
Firing order (I made this mistake once resulting in a similar symptom and the engine shook badly).
Fouled plug(s).
Bad plug wire(s), distributor cap, rotor button.

I hope this is helpful.

Rickey.

 

[sasilverbullet]

Rickey,
Thanks for the help, I'll try and answer all your questions inline below:

Does the engine skip or shake at idle? - doesn't seem to be missing or shaking at idle.

Is the vacuum gauge needle actually jumping at a rate consistent with the (mis)firing of a particular cylinder? - runs smooth at idle, runs smooth at cruise. I don't think it's anything with the ignition system, I have a new coil, cap, points, condenser, wires, plugs. Dwell is perfect, timing is spot on.

Is there any unusual valve train noise? - nope

HMMMMMM '70 model engine......originally speced for leaded fuel only? - yes, but the previous owner and I have been adding the lead substitute. So that's back to 1990 or so. From 1970 to 1990, haven't a clue.

A sticking intake valve or one with a broken spring will frequently cause backfiring through the carburetor on acceleration. - no backfiring, it has lots of gitti'up and go.

 

[cchase]

I'm far from an expert on this subject but a 2 inch variation doesn't sound significant enough if the engine is not having any issues, to be something that shows a "problem".

 

[mechtech2]

It also greatly depends on WHERE you connect the gauge.
If it is seeing an isolated runner, it will pulse a lot more.
So check out a few spots.
vacuum gauges are dampened - usually with restrictors in line.
You may need a bit more
That could also be all that is wrong.

 

[Grebbler]

A 1406. I have one on a stock 350. Could it be on the verge of being a bit too small? If you have the idle speed screw raised enough to be off the idle circuit and into the cruise circuit, the mixture screws will have little effect.

FYI: Those carbs also have a soft body and the throttle shaft often ovals out the shaft holes and creates a vacuum leak there. having brass shaft bushings installed is the only reliable fix that I have seen for that. If it's really worn it takes a strong return spring to get the butterflies to seat and idle somewhat properly (shaft leak still in effect) . At this point the butterfly plates are gouging into the bores.

 

[sasilverbullet]

Grebbler - you may have hit the nail on the head. I will try that tonight. I'm pretty sure I have the idle speed a little high. It's only a 4 month old carb, so hopefully nothing is worn yet!

 

[Rickey]

Originally Posted By: mechtech2
It also greatly depends on WHERE you connect the gauge.
If it is seeing an isolated runner, it will pulse a lot more.
So check out a few spots.
vacuum gauges are dampened - usually with restrictors in line.
You may need a bit more
That could also be all that is wrong.


I forgot about individual runners having pulses in vacuum.
Connecting the vacuum gauge to the manifold at the base of the carb should minimize this effect. If the the throttle shaft bores are in trouble; Idle speed/quality would likely be inconsistent from one throttle closing to the next.

If it idles and runs correctly, I wouldn't worry over it.

Good luck sir.

Rickey.

 

[sasilverbullet]

Btw, I hooked up the vacuum gauge directly to the port under the carb and voila, almost a perfectly steady needle!

Thanks for the pointers everyone!