Vacuum guage 1 to 1/2" flutter. Ignition?

[JNErotten]

With my vacuum guage on manifold vacuum, I get a slight but steady half to one inch flutter between 18 and 19. I can't seem to be able to smooth it out. It does this on my '73 318 Dart too. Any ideas on how to fix this?

[johnwaynejeep]

If it's a slow wander then your carb may be out of adjustment. If it's a quick instantanious tick then you may wanna check your timing. Your valves may be closing too soon or opening too soon and cause the vacuum to suddenly rise or drop. I wish I could help more but this is all I know.

[JNErotten]

It's a very steady rapid vibration. Like, it will look perfect and steady after a few beers 'cause it vibrates so fast. Once the beer goggles are gone, reality sets in and it's still vibrating. I'll go hit the timing and pour a cap of oil down the carb and see if that makes a difference.

[johnwaynejeep]

If it's not one of those things causing the ticking in the vacuum then I have no clue.

[PhilSine]

Under what conditions does the needle flutter? At idle? Drive with the brake on? Slow acceleration? Is it a constant flutter or a periodic drop every 2-3 seconds or so? 1/2 inch to an inch doesn't help much, what is the low to high range in in/hg that the needle fluctuates between?

[RichCal]

Your intake manifold is one big vacuum chamber (negative air pressure) due to large pistons sucking on a relatively small opening. Each time a piston is done sucking in air, the intake valve closes, sealing off that cylinder from your intake manifold.
Some scenarios which can cause vacuum gage flutter:
1) If an intake valve is leaking, then when that cylinder fires, a small relatively short blast of POSITIVE pressure goes back into the intake manifold..and all the way back up into your vacuum gage for an instant. If you have one leaking valve, the flutter should be fairly steady. If you have multiple leaking valves, you'll see a more erratic flutter.
When accelerating from a stop, the flutter will be greatly increased until reaching a steady speed. Thread a vacuum hose through your firewall so you can see the gage while test driving to see this one.

2) If a cylinder is not firing, then for an instant in time when that cylinder is NOT firing, it is NOT helping to rotate the crank which would cause some other piston to pull in its air at a stronger rate, and for an instant in time the vacuum decreases by a fraction, and is reflected accordingly in your vacuum gage. However, you would also hear and feel a "skip" anyway, and the engine will have an obvious shaking, especially at idle.

3) If compression is low in one cylinder, it will not have as much power as the others and #2 above will apply, but not as much.

[JNErotten]

I used vacuum from the base plate of my carb and it's smooth, I use a port off my manifold then it flutters between 18 1/2 and 19. Maybe the line is to big, or the booster diaphram is interfering somehow?

[FrankenJeep]

<blockquote>quote:</font><hr>Originally posted by JNErotten:
Maybe the line is to big, or the booster diaphram is interfering somehow?<hr></blockquote>

I don't think the problem is in the gauge or vac line. Hate to admit that I own a Chilton's ;) , but this is what they say:

<blockquote>quote:</font><hr>
The following describes the actions of the vacuum gauge:

Needle Fluctuated From 15-20 Hg At Idle - Sticking Valve or Ignition Miss

Needle Vibrates Excessively At Idle, But Steadies As Engine Speed Increases - Worn Valve Guides

Needle Fluctuates At All Engine Speeds - Weak valve springs, worn valve stem guides, or leaking head gasket (vibrating excessively at all speeds

Drifting Needle - Improper Carburetor Adjustment or Minor Intake Leak At Intake Or Manifold
<hr></blockquote>

Hope it isn't a head gasket or valve guide seals ... what a pain :(

Frank

[JNErotten]

Yeah, I've done both before on random cars. If you do the head gasket, might as well do the valve seals too. What would excessively mean to them though? IO still can't help but think it's the ignition.

[Rogue]

probably a broken valvespring, seen it happen just a few times...