I finally had time to take a look at my distributor this week. Here are the
details as to how I checked it and what I found.
The HEI distributor I used to replace my breaker-point distributor was a
1377095 (Cardone #1893). I believe this is the one used on a 75 Toronado 455.
The 1377095 distributor came with a 27 degree vacuum advance. Thanks to the
GMC list I now know that 10 degrees is the correct number so I replaced the
vacuum advance with the correct one (1973577).
In order to check the timing more easily I painted the harmonic balancer
black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I
calculated the positions of these marks by measuring the circumference of
the harmonic balancer (20.5") and dividing by 360. This gives the length,
in inches, of one degree on the surface of the harmonic balancer. The zero
degree mark (TDC) is the reference line that is notched into the harmonic
balancer. When I cranked the engine so the zero degree mark is on the
drivers side (looking up from under the engine), the BTDC marks are toward
passenger side in order as follows:
The 8 degree mark is at .46". This is the timing at idle with the vacuum
advance disconnected.
The 18 degree mark is at 1.03". This is the timing with the engine at 2000
rpm and the vacuum advance disconnected. It is also the timing at idle with
full vacuum advance.
The 27 degree mark is at 1.54". This is the timing with the engine at 3100
rpm and the vacuum advance disconnected.
I used white paint to make the lines marking each of these timing positions
on the harmonic balancer.
I disconnected the vacuum advance from the carburetor. Using a timing
light, I set the timing (at idle) to 8 degrees. If the lines on the
harmonic balancer are painted correctly the 0 degree mark (the notch in the
harmonic balancer) will line up with the 8 degree arrow on the engine and
the 8 degree mark (the first painted line on the harmonic balancer) will
line up with the 0 degree arrow on the engine. Note that the zero degree
arrow is the first point on the drivers side. The other arrows 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
checked the timing. If the centrifugal advance is working properly the 18
degree mark on the damper (the second painted line) will be lined up with
the zero degree arrow on the engine. Then I ran the engine up to 3100 rpms
and checked the timing. If the centrifugal advance is working properly the
27 degree mark on the damper (the third painted line) will line up with the
zero degree arrow on the engine.
The centrifugal advance on my distributor is pretty close. At 2000 rpms it
was right on at 18 degrees. At 3100 rpms it was at 24 degrees. Thats not
quite as advanced as I have been told it should be (27 degrees at 3100 rpm)
but I rarely run much above 2000 rpms so I am going to live with it for now.
Before checking the operation of the vacuum advance be sure that the idle
timing is set to 8 degrees when the advance hose is disconnected. Then
unhook the hose that goes to the transmission modulator and connect the
vacuum advance hose to the transmission port on the intake manifold. This
will apply full vacuum to the vacuum advance and should pull in an
additional 10 degrees of advance. Use the timing light to verify that the
18 degree mark painted on the damper lines up with the 0 degree arrow on
the engine (8 degrees idle plus 10 degrees vacuum advance).
If you see more than 18 degrees then you have the wrong advance and should
replace it with the correct one - 1973577. If you see less than 18 degrees
then the advance is defective or there is a vacuum leak or blocked hose.
I would like to thank everyone who gave me lots of advice and insight as to
how to check my timing curves. If I had more miles on this distributor I
probably would have just sent it in to Bob Lamey. But, since its reasonably
new I thought I would check it first and maybe try to recurve it myself. As
it turns out the curves on this distributor are pretty close (after I
replaced the vacuum advance). Here is a summary of the curves:
Speed No Vac With Vac
-----------------------
Idle 8 18
2000 18
2500 20
3100 24 34
Given that I didn't have to spend the $ on a new distributor, I am going to
take my savings and burn it on a few gauges from Westach. One of them will
be the 2C5-56 air/fuel ratio gauge. I am curious to see just how well my
Howell TBI system really does control the mixture. I will be sure to post
the results when I get them.
Dave
73 Sequoia
PS
If anyone else does this checkup on their distributor, please post the
results (and what distributor you are using if you know) to the GMC list. I
am very interested in seeing the range of variation amongst distributors. I
am especially interested in seeing what numbers you get if you have the
original (correct) distributor.
details as to how I checked it and what I found.
The HEI distributor I used to replace my breaker-point distributor was a
1377095 (Cardone #1893). I believe this is the one used on a 75 Toronado 455.
The 1377095 distributor came with a 27 degree vacuum advance. Thanks to the
GMC list I now know that 10 degrees is the correct number so I replaced the
vacuum advance with the correct one (1973577).
In order to check the timing more easily I painted the harmonic balancer
black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I
calculated the positions of these marks by measuring the circumference of
the harmonic balancer (20.5") and dividing by 360. This gives the length,
in inches, of one degree on the surface of the harmonic balancer. The zero
degree mark (TDC) is the reference line that is notched into the harmonic
balancer. When I cranked the engine so the zero degree mark is on the
drivers side (looking up from under the engine), the BTDC marks are toward
passenger side in order as follows:
The 8 degree mark is at .46". This is the timing at idle with the vacuum
advance disconnected.
The 18 degree mark is at 1.03". This is the timing with the engine at 2000
rpm and the vacuum advance disconnected. It is also the timing at idle with
full vacuum advance.
The 27 degree mark is at 1.54". This is the timing with the engine at 3100
rpm and the vacuum advance disconnected.
I used white paint to make the lines marking each of these timing positions
on the harmonic balancer.
I disconnected the vacuum advance from the carburetor. Using a timing
light, I set the timing (at idle) to 8 degrees. If the lines on the
harmonic balancer are painted correctly the 0 degree mark (the notch in the
harmonic balancer) will line up with the 8 degree arrow on the engine and
the 8 degree mark (the first painted line on the harmonic balancer) will
line up with the 0 degree arrow on the engine. Note that the zero degree
arrow is the first point on the drivers side. The other arrows 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
checked the timing. If the centrifugal advance is working properly the 18
degree mark on the damper (the second painted line) will be lined up with
the zero degree arrow on the engine. Then I ran the engine up to 3100 rpms
and checked the timing. If the centrifugal advance is working properly the
27 degree mark on the damper (the third painted line) will line up with the
zero degree arrow on the engine.
The centrifugal advance on my distributor is pretty close. At 2000 rpms it
was right on at 18 degrees. At 3100 rpms it was at 24 degrees. Thats not
quite as advanced as I have been told it should be (27 degrees at 3100 rpm)
but I rarely run much above 2000 rpms so I am going to live with it for now.
Before checking the operation of the vacuum advance be sure that the idle
timing is set to 8 degrees when the advance hose is disconnected. Then
unhook the hose that goes to the transmission modulator and connect the
vacuum advance hose to the transmission port on the intake manifold. This
will apply full vacuum to the vacuum advance and should pull in an
additional 10 degrees of advance. Use the timing light to verify that the
18 degree mark painted on the damper lines up with the 0 degree arrow on
the engine (8 degrees idle plus 10 degrees vacuum advance).
If you see more than 18 degrees then you have the wrong advance and should
replace it with the correct one - 1973577. If you see less than 18 degrees
then the advance is defective or there is a vacuum leak or blocked hose.
I would like to thank everyone who gave me lots of advice and insight as to
how to check my timing curves. If I had more miles on this distributor I
probably would have just sent it in to Bob Lamey. But, since its reasonably
new I thought I would check it first and maybe try to recurve it myself. As
it turns out the curves on this distributor are pretty close (after I
replaced the vacuum advance). Here is a summary of the curves:
Speed No Vac With Vac
-----------------------
Idle 8 18
2000 18
2500 20
3100 24 34
Given that I didn't have to spend the $ on a new distributor, I am going to
take my savings and burn it on a few gauges from Westach. One of them will
be the 2C5-56 air/fuel ratio gauge. I am curious to see just how well my
Howell TBI system really does control the mixture. I will be sure to post
the results when I get them.
Dave
73 Sequoia
PS
If anyone else does this checkup on their distributor, please post the
results (and what distributor you are using if you know) to the GMC list. I
am very interested in seeing the range of variation amongst distributors. I
am especially interested in seeing what numbers you get if you have the
original (correct) distributor.
