Club 80-90 Forums

Interesting post here on vacuum advance and retard. And Andrew's post here seems to say something similar. The vacuum retard seems to be for emissions, and seems to be designed to retard the timing at idle to improve emissions. The vacuum advance takes over once the throttle opens. So maybe leaving the retard vacuum disconnected, and setting up the timing with only the advance connection attached, will produce the correct curve under throttle, but no emissions-benefiting retard at idle. But since the distributor seems to respond to the difference between the two vacuum signals it's difficult to know whether that difference throughout the rev range can be accounted for by a single timing offset at idle. I suppose it depends on whether the retard vacuum from the plenum is constant across the rev range.

I'd assumed one vac was advance the other retard, they are connected to dirrerent sides of the vac unit.

itchyfeet wrote:I'd assumed one vac was advance the other retard, they are connected to dirrerent sides of the vac unit.

Yes, that's what I understand, and I believe that the lever is actuated by the *difference* between the advance and retard signal - so a differential diaphragm. So when the advance vacuum is negligible at idle the retard signal dominates, but once the throttle is opened the advance signal dominates. But I don't have one - I'm only guessing.

I found this on another post.
If it helps.
The fuel injected DJ has a dual vac dizzy which has, er, dual vac, but that relies on a constant vac signal from the plenum for retard, and a ported vac signal from the throttle body for advance. (Thus, as soon as you open the throttle, the advance vac signal is created and cancels/overcomes the idle retard signal, instantly snapping the timing forward about 12 deg. As you won't have this facility on the carb (no ported signal from throttle, just a single permanent vac), you're better off with a DG dizzy (SVDA)
Written by someone a lot wiser than myself.

Yeah, thanks, that's Andrew's post that I linked to three posts earlier.

What this subject needs is an experiment! Someone with a properly functioning 2.1DJ with a DJ distributor, to measure the timing advance *with vacuum pipe(s) attached*, at a range of engine speeds, say everything from idle to 3000 in steps of 250 rpm. I'll attempt do the same with the DG distributor on my 2.1DJ.

Then we'll know the impact of the vacuum advance/retard issue.

I'l use the idle speed adjustment screw to set the rpm for each increment. Is there a similar adjustment on the injection setups?

CJH wrote:Yeah, thanks, that's Andrew's post that I linked to three posts earlier.

Sorry.
New to the forum thing.
Usually use Fb pages.
Need to pay more attention to previous posts!

CJH wrote:What this subject needs is an experiment! Someone with a properly functioning 2.1DJ with a DJ distributor, to measure the timing advance *with vacuum pipe(s) attached*, at a range of engine speeds, say everything from idle to 3000 in steps of 250 rpm. I'll attempt do the same with the DG distributor on my 2.1DJ.

Then we'll know the impact of the vacuum advance/retard issue.

I'l use the idle speed adjustment screw to set the rpm for each increment. Is there a similar adjustment on the injection setups?

Not sure that will help, you won't open the second 2e3 throttle on your driveway what's happening loaded is what a rolling road is looking at.

some basics first, let me know if I made a mistake

dizzy part numbers
P1080560 by Paul_Barr, on Flickr


you can see vac pipe to advance side of dizzy connects to throttle body
P1060845 by Paul_Barr, on Flickr

P1080540a by Paul_Barr, on Flickr

P1080542b by Paul_Barr, on Flickr

P1080543a by Paul_Barr, on Flickr

P1080544a by Paul_Barr, on Flickr


using a vacuum pump and making on dizzy body ( diameter is 65mm so calculated 0.56mm around circumference is 1 degree)

P1080545a by Paul_Barr, on Flickr


P1080552a by Paul_Barr, on Flickr

centre of hall sensor as a reference

P1080553 by Paul_Barr, on Flickr

Same reference to DG dizzy hall sensor shows rest point is the same

P1080556 by Paul_Barr, on Flickr

This vac unit is leaky so I could not keep the vacuum long enough to take a photo at full advance so it's at rest but shows the vac advance is the same DG and DJ

P1080559a by Paul_Barr, on Flickr

another picture of the DJ on left DG on right

P1080555 by Paul_Barr, on Flickr

itchyfeet wrote:
Not sure that will help, you won't open the second 2e3 throttle on your driveway what's happening loaded is what a rolling road is looking at.

I'm struggling to understand why the load makes a difference. The vacuum comes from the piston being pulled down, so I would have thought the only variable is how fast the engine is turning. What am I missing?

itchyfeet wrote: This vac unit is leaky so I could not keep the vacuum long enough to take a photo at full retard so it's at rest but shows the vac retard is the same DG and DJ

Useful measurements. From memory though, doesn't the rotor arm turn clockwise, so isn't moving the Hall sensor anti-clockwise causing an advance?

Dunno but if the second vac port won't open surly there must be a difference

probably got to do with fluid dynamics, flow and pressure/ vacuum are related, at idle you have much less flow so you may not see the same vacuum

going down a hill throttle closed you have high rpm but high vacuum and no flow

CJH wrote:
Useful measurements. From memory though, doesn't the rotor arm turn clockwise, so isn't moving the Hall sensor anti-clockwise causing an advance?

I don't think so surely it means the vane hits the hall sensor earlier

I misunderstood the terminology
I thought retard was firing earlier but it seems not
will change photos