<VV> Vacuum Readings

[Gary Mierzwa]

There are two vacuum sources on the Corvair engine: ported vacuum, and 
manifold vacuum.

SHORT VERSION:
The ported vacuum may be identified with a vacuum gauge.  With the engine at 
idle, the ported vacuum source will have zero, or a very low, inches of 
vacuum reading.  As you accelerate the engine, the vacuum reading should 
increase with engine rpm.  A manifold vacuum source will read fairly high, 
17 to 21 inches of vacuum at idle and drops with an increase in speed.  The 
effect for both sources is most noticeable when you snap the throttle open 
quickly.

LONG VERSION:
Ported Vacuum:
The ported vacuum sources are the vacuum fittings that are standing straight 
up and down on the side of the carburetor, nearest the centerline of the 
engine.  The distributor vacuum advance should be hooked up to the ported 
vacuum source from one of the carburetors, and the ported source from the 
other carburetor is usually plugged off.  On some later cars, the ported 
vacuum from both carburetors is connected to a common " T " and then routed 
to the distributor vacuum advance.  The ported vacuum signal, along with the 
mechanical advance built in to the distributor, is used to modify the 
ignition timing, as the engine rpm and load varies.  The ported vacuum 
signal should normally indicate zero in/Hg (inches of mercury column) at 
slow idle rpms.  The vacuum reading should increase towards 10 to 15 in Hg 
as the engine rpm is increased, depending on engine size, condition etc. 
etc. etc...

Manifold vacuum:
There are two manifold vacuum sources: choke pull-off and balance tube.

The easiest ones to get to are the choke pull-off sources.  The choke 
pull-off sources are the fittings that stick out of the carburetors in the 
horizontal direction on the "corners" of the carburetors.  They should be 
connected to the little vacuum diaphragms near the top of the carbs.  The 
choke pull-off diaphragms are used to allow the chokes to be fully closed 
when the engine is cold and not running.  This allows the carburetors to run 
very rich, for easier starting.  When the engine starts up, and begins 
developing manifold vacuum, the choke pull-offs open the choke plates about 
a quarter of an inch, which prevents the engine from flooding and stalling.  
After the engine warms up for a few minutes the thermostats, in the lower 
engine shrouds, open the chokes further as the engine gets up to 
temperature.  The balance tubes are the fittings that are sticking 
horizontally out of the cylinder heads, pointed towards the front of the 
vehicle, just below the carburetors.  The balance tubes connect the intake 
manifolds of both cylinder heads together to keep both halves of the engine 
operating at nearly the same power levels.  This helps maintain a smoother 
idle and less vibration during acceleration.  There are probably 20 other 
great reasons to have a balance tube, but one is enough for me.  (Maybe a 
thread on VV?).

The manifold vacuum signal should normally indicate a steady reading of 17 - 
21 in Hg at idle/fast idle rpms.  The vacuum reading should decrease as  the 
engine rpm is increased, depending on engine size, condition etc. etc.  
etc...   Normal readings will be one in. Hg lower for every 1000 feet of 
altitude above sea level.

Possible Problems:
1. A Little Low, but Steady Reading
	stuck piston rings
	Late ignition or valve timing
	Worn: pistons, rings, cylinders, blow-by ,smoke from oil filler tube
2. Very Low, Steady Reading
	Ignition Timing
	Loose carb mounting
	Cracked/loose balance tube/hoses
	Leaking choke pull-off diaphragms, hoses
	Leaky: Intake manifold, carb gaskets, throttle shafts
3. Needle floats very slowly, changing by 4 or 5 in Hg around normal
	Carburetor running too rich
	Spark plug gap too narrow
4. Needle fluctuates rapidly at idle speed, steadies as rpm is increased
	Worn valve guides
5. Back and fourth movement of needle that increases with engine speed
	weak valve springs
6. Needle drops to 0 with engine at idle,OR as rpm is slowly increased
	Restricted exhaust
7. Regular dropping back of the needle
	Spark plug not firing
	Valve burned or sticking open
8. Irregular dropping back of the needle
	Valves sticking part of the time
9. Race the engine for a moment, then quickly release the throttle.  The 
needle should increase to around 23 - 25 in Hg as the throttle closes, 
indicating good compression.  If the needle does not rise, time for a 
compression test.

Note! engines with high lift cams and more valve overlap could have lower 
and more unsteady manifold vacuum readings.  Engines with AIR emission pumps 
may also have lower readings.


Gary Mierzwa
West Florida Corvair Club
CORSA Chapter 325

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