Hi all,
My distributor vacuum advance unit was broken, diaphragm puncture, so I ordered a new one. Checking it out on my bench I see that the lever movement is only a millimeter or two from zero vacuum to full when testing it with my vacuum pump. I would expect much more movement.
Does anyone know what the movement should be from zero to max vacuum? I can check it wit the old unit when I pull out the distributor but before stalling the car for distributor work I was thinking that someone might have the knowledge about this.
Car, DD6 1990, is running fine despite this problem, so when I get into changing the diaphragm I want to do it as quick as possible. Summer is short in Sweden… And, with no vacuum ignition advance I am surprised that the car runs as well as it does…
Hi,
Apparently the manual states somewhere that the vacuum advance contributes 4 degrees. This is at the distributor. So 8 degrees crank. I’m not sure where it operates the movement, but I guess you could convert 1-2mm into actual degrees of crank rotation if you knew the pick up point.
Apparently the difference will be noticed in significant reduction in fuel economy.
Also, there are reports that some units are meant to leak slightly, but not a whole lot. Whatever that means.
Cheers
Mark
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
3
Apparently the manual states somewhere that the vacuum advance
contributes 4 degrees. This is at the distributor. So 8 degrees crank.
Perhaps for the pre-H.E., which I guess this is. The H.E. develops less
centrifugal advance and more vacuum advance to achieve approximately the
same total advance.
I’m not sure where it operates the movement, but I guess you could
convert 1-2mm into actual degrees of crank rotation if you knew the
pick up point.
Yeah, 1-2mm doesn’t sound like enough. I woulda guessed 5-10mm of
movement. Perhaps others with pre-H.E.'s will chime in.
Apparently the difference will be noticed in significant reduction in
fuel economy.
Yeah, lack of vacuum advance will do your fuel economy a world of no good.
Makes zero difference to performance, though, which means some
pre-OPEC cars didn’t even have a vacuum advance, gas was so cheap
nobody cared.
Also, there are reports that some units are meant to leak slightly,
but not a whole lot. Whatever that means.
That’s only true of the H.E. units, I believe. The “leak” is actually a tiny
orifice drilled through the metal hub in the center of the diaphragm. It’s
necessary for proper operation of the vacuum regulator, which won’t work
right with a “deadheaded” vacuum line. But I’m pretty sure that vacuum
regulator only exists on H.E.'s.