Help me better understand SC dizzy advance/retard.

[Gogar]

I decided to check the timing on my SC for fun and I've done it on my 2.4 a bunch, so I'm pretty confident about doing it.

But, if someone could better explain the retard AND advance vac theory to me I'd love it. Some searches gave me some good reading, but no real answer.

On my '72 as I understand it, the Dizzy has a vacuum line that will retard the timing at idle. So with the line disconnected, I set at 5 BTDC (or straight up, I can't remember) and reconnect the line, and get 5 ATDC. Then, at high rpm, the vacuum doesn't apply anymore and the mechanical advance of the dizzy kicks in, and if everything works, hopefully I'll get between 30 and 35 degrees of mechanical advance. Right?

On an SC, the spec is 5 BTDC at idle and 25 BTDC at full (mech) advance (with lines disconnected). The retard line does what I expect when hooked back up.

SO: My question is this:

Does the VACUUM ADVANCE line "further" advance the timing to get it into the 30-35 range? So I set for 25 with no lines connected, and then with the vac advance line connected it will advance further at high rpm?

Or, if I'm not getting the theory, can you tell me more about the "advance' vacuum line?

I guess the only reason I'm asking is if I can set to 30 or 35 to get some more oomph I'd love to.

Thanks sorry for the wordyness.

[rattlsnak]

you are correct on all your theories!

you can also check it with the lines connected to get a good idea of where it is at 5000 RPM with the advance.

you can also remove the cap, hook up a hand held vacuum pump to each port, and watch the plate move when you apply vacuum. You cant measure it this way, but if your not getting full advance, you can see if the plate is 'frozen' in place. (which is very common)

[Gogar]

Thanks.

Well, when I put the retard line back on, it does what it's supposed to, so I assume it's not frozen.

[rattlsnak]

Probably, but sometimes they only move 4-8 degrees or such due to dirt/corrosion, etc. Best way to check, is disconnect lines, rev it up to 4-5000 rpm (only for a quick couple of seconds), see where the advance is, then plug the line is and see how much it advances. It will advance some on its own, due to the centrifugal weights, so this is the best way.

[Gunter]

Service your distributor first the easy way:

Distributor service (Clean and lube) real easy without removing the pinion gear!

Then set your maximum advance:

Maximum advance for SC 3.0 or ss 3.2

Worked real good for me.

[Gogar]

Thanks, Gunter. I am watching your thread too. Good info.

UPDATE:

I messed around again and here's what I've got:

With vacuum lines disconnected and plugged with golf tees, I slow the idle down to about 900 and set to 5 deg. BTDC. Then, when revving to 4K rpm and using my variable timing light, I find that the timing is getting to about 20 BTDC.

Next, I reconnect both vacuum lines and reset idle speed. I find that idle timing has moved to about 5 ATDC (okay, fine, right?)

And now, my 4k RPM timing gets to about 25 deg BTDC.

The SC spec from Haynes is 5 BTDC idle and 25 at 4K rpm, with vacuum disconnected. But I'm only getting to 25 with the lines connected.

My new questions:
1. A-OK?
2. Is my distributor dirty and should I be getting some more mech. advance than I'm getting?
3. Should I just say "screw it" and set to 35 BTDC at 4k RPM and who cares what the idle timing is?

Oh and I'm at 5000 ft elevation, if anybody's into that kind of thing.

Thanks!

[Steve@Rennsport]

Jeremy:

Without a total recurve of the advance mechanism, everything you do will be a compromise of sorts.

My suggestion would be make sure that the advance mechanism is in perfect working order and well lubricated. After that, set the timing to your selected value above 4K and ignore the idle setting.

I do retain vacuum advance on CIS cars and delete on carbureted/MFI ones.

You should be able to get 25-30 degrees total (initial + centrifugal) at 6K.

[Gunter]

Gogar:
Haynes is o.k. but, get the Bentley SC Repair Manual, you'll love it.
Service your distributor first, it's very easy with my instructions.
ATDC is not o.k.
Read what Steve says, then re-read my write-up.
Clean/oil the distributor, disconnect vac lines, ignore the 5 deg BTDC, make a mark on the pulley for 30 deg, rev to at least 4k+, move the distr. to match the 30 mark with the engine case seam, stop engine, tighten distr., reconnect lines.
Timing should be around 8-10 BTDC.
If not disconnect the retard line.
I leave my retard line disconnected and plug the nipple by the TB.
You should get at least 30 deg max advance with an SC distributor provided the weights/springs and vac pod are good.

It takes a bit of playing/adjusting mixture and idle to get the right setting.

Like Steve says: Without a real proper re-curving, it's a compromise but I found that it improved performance without pinging.
BTW: I run 89-91 octane. 89 works fine.

[Gogar]

Why is 5 ATDC at idle not okay? (with line connected)

I know that's the spec for my 73.5 CIS.

How much is the vac retard "supposed" to retard, if I set for 5 BTDC at idle, with line off?

Thanks!

[Gogar]

Huh, huh. I said "retard." Cool.

[Gunter]

I assumed that this was for your '83.
My description, and experience, is for a 3.0 or ss 3.2.
I believe the retard only works on deceleration for the purpose of emission control on the Lambda '80-'83.
Notice that the lines for advance/retard are above and below the throttle plate (Butterfly) respectively.
'78-'79 doesn't have retard.
Does a 73.5 actually have a vac retard?
If it has, I know nothing of the settings for a 73.5 with points.

Have you serviced your distributor?

[Gogar]

Sorry, Gunter. My post IS for my '83, but, I was just giving the only example I know of ATDC. Sorry for the confusion. Yes, a 2.4 CIS has a retard line only, no advance.

The retard line Obviously works at idle, and from what I know it's purpose is soley to retard timing at idle for emissions purposes, independent of the Lambda. But then again, maybe I'm way off.

I guess what I'm hoping for someone to chime in with is:

HOW MUCH should the vacuum lines RETARD or ADVANCE (add or subtract to the total timing) if everything's working correctly?

If mine isn't, then I'd like to get a new diaphragm when I clean my distributor guts.

Thanks!

[Gunter]

73.5 distributor runs cw, SC distr. runs ccw; apples and oranges.
Forget the effect of retard on a Bosch SC distributor '80 to '83 in idle.
There is no retard-effect in idle! Only advance has an effect.
The retard only works on deceleration (when coming off higher revs.) with Lambda '80-'83 US.
Learn about Venturi-effects of vacuum above or below the throttle plate.

I tried it on two different engines with 3 different SC distributors; max advance works.
If it doesn't work in your case, something else is wrong.
Service your SC distributor first, then you have a choice of either setting it the way you want, or you can go by my example of setting to max advance.
Friendly greetings and good luck.

[SP2]

Gunter, like Gogar I am also learning about the SC dist. My 81 SC broke so John Walker loaned me an older SC dist with no retard line so he clamped the line until I can get my dist repaired. I noticed that when the engine is fully warm the idle jumps from 1k to 2k. I asked John about that and since I know so little the answer just went over my head but I thought he said the idle went higher because I no longer had any retard function on the car. I remember he said he could fix this by re-adjusting the mixture, but I declined since I am having my dist repaired and will re-install when ready. Any thoughts?

[Gunter]

You are in good hands with JW.