In a message dated 9/4/2007 12:51:37 A.M. Eastern Daylight Time,
nick.barb@xxxxxxxxxxxxx writes:
I've
discovered what I thought was a fuel starvation problem may be an
ignition
problem. The vacuum advance on my flathead 6 cylinder
Plymouth
engine is not working. I did a test from a good running car.
I installed a long hose to the good running car and ran it to the
problem car's vacuum advance hose and revved the engine on the good car
and with the distributor cap off the problem car I noticed there was no
movement of the breaker plate. I could also hear a leak from
the
diaphragm of the vacuum advance of the problem car. I plan
to replace
the vacuum advance.
OK, the most telling part of this paragraph is that you heard a vacuum leak
from the car. I would guess that the diaphragm is leaking. (You
could have done all of this by just sticking a much shorter hose in your mouth
and sucking on it!)
For reference purposes, here is how the vacuum advance works on most
carburetors:
Usually, there is a small hole drilled into the throttle body of the
carburetor, just above the upper side of the throttle plate when it is closed,
at idle. This port (hole) leads to the vacuum advance connection on the
carburetor and thence to the distributor. Since this port is above the
throttle plate, it sees no vacuum at idle and you should have a stable condition
at idle. It makes no difference to the engine whether the line to the
distributor is connected or not.
Immediately, as you open the throttle, the port sees both manifold vacuum
and the "Venturi Effect" of air flowing around the throttle plate, and the
vacuum reading spikes, sometimes even going higher than manifold vacuum for an
instant. That brings you to full vacuum advance for part-throttle
operation. At wide-open-throttle, there is no vacuum of any kind, to speak
of, so the centrifugal advance takes over.
As a side note, I do believe that there are some models of cars that do
away with the idle port altogether and just feed manifold vacuum to the
distributor all the time, and, there even are some that use vacuum retard
sometimes, but I have no experience with them.
As far as your original situation, I see no way that the vacuum advance
could effect your idle speed unless it was just kind of flopping around in there
on its' own! To be sure, I'd pull the distributor and check the advance
plate, springs and etc. to be sure that everything is operating as it should
be.
Joe
Savard
Lake Orion, Michigan
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