No ported vacuum on Holley carb
David Salerno
used a Holley rebuild kit.) It's a Holley 4160 (750 CFM, vacuum
secondary), List #3310-2, with #2726 stamped below the list number on
the air horn. The problem I'm having is that the vacuum port on the
side of the primary metering block, which is normally supposed to be
"ported" vacuum for the distributor's vacuum advance, is in fact
manifold vacuum. I checked with my vacuum gauge - it reads about 18"
of vacuum at idle and drops when you goose the throttle, which is the
opposite of what you want for the distributor vac. advance. Other than
this, the carb seems fine. Interestingly, this is the ONLY vacuum port
on the entire carb, there are NONE on the throttle plate (which would
be manifold vacuum anyway). I took it apart again and followed the
passage from the port, through the metering block, and it goes right
to a hole in the body that comes out on the bottom of the carb, below
the throttle plates. Do you think the carb has the wrong metering
block, or the wrong gasket, or is this normal for this carb? If this
is the case, I guess I'm screwed for vac. advance. I'd like to be able
to have ported vacuum for my distrubutor (which I realize is a whole
'nother discussion, based on my newsgroup reading!) If it helps, the
numbers on the metering block are: L33102 5271 5275.
Thanks for your help,
Dave
Rex B
Why wouldn't that work? The normal 18 hg of vac holds the advance diaphragm in
the normal position. When you dip into the throttle you get a momentary drop,
which eases the diaphragm and advances the timing.
The main reason for the port as opposed to a tap in the manifold, is that being
close to the throttle plate you get that pronounced dip in vac when the plate
opens. Regular manifold vacuum drops, but not as quickly/sharply, so it's
useless for the purpose.
As for manifold vac, you can either get a spacer plate like the older Fords
used, or tap the manifold itself. I've also seen some thin spacer plates -
3/16" or so - with a vacuum tap.
Good luck
David Salerno
Thanks for the reply, but I'm afraid you have it backwards. The
distributor advances when it receives vacuum, not when there is none.
That's why you have to unhook the hose to set the static timing, in
case there is some residual vacuum.
Dave
Rex B
That may be true for some cars, and it's logical to assume that.
But think about it: that ported vacuum is a transient, momentary thing when you
dip into the throttle. It really doesn't produce a high-volume vacuum pulse
sufficient to move the diaphragm and breaker plate all by itself.
By using constant vacuum to hold the plate in the normal position, you can
introduce a "hiccup" of less vacuum to let the diaphragm relax momentarily and
advance the breaker plate, then return to normal as the vacuum returns.
So look at your direction of rotation, pull in the vacuum diaphragm with your
finger, then look at which way the breaker plate would rotate vs the distributor
rotation when you release it.
It's been a while since I ran through this exact same excercise, but I don't
think I've lost all my marbles since then ;)
Rex B
I misstated this. The "dip" in vacuum retards the timing.
From ClaasicChevyTrucks.com:
"When the engine is lightly loaded, such as at idle and when cruising, manifold
vacuum is high and ignigtion timing can be advanced without causing detonation.
The high manifold vacuum pulls on the diaphragm. In turn, the diaphragm plunger
pulls on the breaker plate, causing it to rotate. Since the ignition trigger
(breaker points, magnetic switch, and so on) is mounted on the breaker plate,
it begins to open earlier, relative to the distributor shaft. And this causes
ignition timing to increase.This increase in ignition timing subsdtantially
improves fuel economy. If the engine is accelarated, manifold vacuum drops. This
allows the vacuum diaphragm, and in turn, the breaker plate, to ease back
toward their neutral states, reducing ignition timing."