You are correct that it is a ported vacuum . I have not tested the
advance
beyond cruise ie 65-70 mph, but up to and including that rpm, (I did
not
compare this using a tach) it made no difference with the vacuum line
connected or not. Same speed, same advance. Maybe my spot is sweet
through
out the range.
How did you test it?
My suggestion for a quick test would be to run the ingine up to an rpm
equal to 65 mph in your GMC with the vacuum advance disconnected &
plugged. (You'll need an adjustable timing light for this.) Read your
timing.
Now do the same with it hooked up. You should see an additional 10-15
degrees of advance. If you don't have an adjustable timing light, you
should be able to see on your tach, or hear an increase in rpm when you
hook up the vacuum line.
If there is no difference, check out your advance can.
HTH,
Steve F.
advance
beyond cruise ie 65-70 mph, but up to and including that rpm, (I did
not
compare this using a tach) it made no difference with the vacuum line
connected or not. Same speed, same advance. Maybe my spot is sweet
through
out the range.
How did you test it?
My suggestion for a quick test would be to run the ingine up to an rpm
equal to 65 mph in your GMC with the vacuum advance disconnected &
plugged. (You'll need an adjustable timing light for this.) Read your
timing.
Now do the same with it hooked up. You should see an additional 10-15
degrees of advance. If you don't have an adjustable timing light, you
should be able to see on your tach, or hear an increase in rpm when you
hook up the vacuum line.
If there is no difference, check out your advance can.
HTH,
Steve F.
