>how can i test the vacuum advance. i dont have a vaccuum pump? they cost=20
>30.00 if i need to buy a pump i can just buy the vacuum advance for the=20
>same price.
>thanx'
>chris 75gb
Here is the procedure that I used to check both the vacuum and centrifugal=
=20
advance in actual operation. In order to check the timing more easily I=20
painted the harmonic balancer black. Then I painted white lines at 8, 18=20
and 27 degrees BTDC. I calculated the positions of these marks by measuring=
=20
the circumference of the harmonic balancer (20.5") and dividing by 360.=20
This gives the length, in inches, of one degree on the surface of the=20
harmonic balancer. The zero degree mark (TDC) is the reference line that is=
=20
machined into the harmonic balancer. I cranked the engine so that the zero=
=20
degree mark is on the drivers side (looking up from under the engine). I=20
then painted the BTDC marks in order toward passenger side as follows:
The 8 degree mark is at .46". This is the timing at idle with the vacuum=20
advance disconnected.
The 18 degree mark is at 1.03". This is the timing with the engine at 2000=
=20
rpm and the vacuum advance disconnected. It is also the timing at idle with=
=20
full vacuum advance.
The 27 degree mark is at 1.54". This is the timing with the engine at 3100=
=20
rpm and the vacuum advance disconnected.
I used white paint to make lines to mark each of these timing positions on=
=20
the harmonic balancer.
I disconnected the vacuum advance from the carburetor. Using a timing=20
light, I set the timing (at idle) to 8 degrees. If the lines on the=20
harmonic balancer are painted correctly the 0 degree mark (the machined=20
notch in the harmonic balancer) will line up with the 8 degree arrow on the=
=20
engine and the 8 degree mark (my first painted line on the harmonic=20
balancer) will line up with the 0 degree arrow on the engine. Note that the=
=20
zero degree arrow is the first point on the drivers side. The other arrows=
=20
on the engine are spaced 4 degrees apart (4, 8 and 12 degrees).
To check the centrifugal advance, I ran the engine up to 2000 rpms and=20
checked the timing. As the speed comes up you should see the timing begin=20
to advance at about 900rpm. It should show about 3 degrees at 1100rpm and=20
at 18 degrees at 2000rpm. If the centrifugal advance is working properly,=20
the 18 degree mark on the damper (the second painted line) will be lined up=
=20
with the zero degree arrow on the engine at 2000rpm. Then I ran the engine=
=20
up to 3100 rpms and checked the timing. If the centrifugal advance is still=
=20
working properly the 27 degree mark on the damper (the third painted line)=
=20
will line up with the zero degree arrow on the engine.
If the curves are off more by more than a couple degrees, especially at the=
=20
lower engine speeds, then its probably worth sending the distributor in to=
=20
be repaired and recurved. A number of people have told me that Bob Lamey=20
(909-982-7747) does a good job rebuilding and re-curving GMC distributors.
Before checking the operation of the vacuum advance be sure that the idle=20
timing is set to 8 degrees when the advance hose is disconnected. Then=20
unhook the hose that goes to the transmission modulator from its port on=20
the intake manifold and connect the vacuum advance hose from the=20
distributor to the transmission port on the intake manifold. This will=20
apply full vacuum to the vacuum advance and should pull in 10 degrees of=20
advance. Use the timing light to verify that the 18 degree mark (second=20
painted line) on the damper lines up with the 0 degree arrow on the engine=
=20
(8 degrees idle plus 10 degrees vacuum advance).
If you see more than 18 degrees then you have the wrong advance and should=
=20
replace it with the correct one (Delco 1973577). If you see less than 18=20
degrees then the advance is defective, the distributor plate is stuck,=20
there is a vacuum leak or there is a blocked hose.
Hope that helps...
Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI/160=B0)
