Ignition timing on an 83? Both vac lines?

[DavErb]

I'm trying to reset the timing on my freshly rebuilt 83 SC motor . I plan on going fo 5 degrees BTC at idle. All the references I have mention remove vac line to distributer. I have 2, the normal advance and retard. When I remove the advance line no problem however when I remove and plug the retard line the engine speed jumps up at least 500 rpm.

Should I use the adjusting screw on the throttle body to get the RPM down to 950 area again or just go ahead and set the timing at the higher speed given that it'll return to the lower speed once the retard line is plugged in again?

Thanks for any clarification of this grey area

[psalt]

Hello Dave,

The ignition timing is checked with both lines disconnected and plugged. Yes, you will have to reset the idle speed at 950 rpm with the TB bypass screw. If you set the timing at the elevated idle speed, it will be incorrect. You also want to check the high speed timing, 25 BTDC @ 4000 rpm, you can mark the pulley if you do not have an advance timing light. The high speed timing is more important than the idle setting, as the springs in the distributor wear, the advance comes in sooner and the idle reading can be false. The vacuum retard is an early emission control device to reduce HC at idle. It also has the side effects of poor cold running during warmup and it raising the engine temperature, especially at hot idle in traffic. I recommend leaving it blocked if you car stutters and stumbles during warmup or if your oil temp rises above the middle in traffic. You should also try connectings a dwell meter to the lambda test plug under the left side plastic cover in the engine compartment. It will tell you whether the lambda system is working properly and it allows you to set the mixture for best performance, which I have found to be around 30-40 duty cycle.

[DavErb]

Good useful advice psalt. Thanks

I'll see if I can dig out the old dwell meter. Haven't used or seen it for 20 years but it must still be hiding in the garage somewhere

[brads911sc]

On my 83, my vacuum advance must have been replaced at one point as I only have one outlet. I do have two vacuum hoses... 1 is plugged in and the other is just laying there. Doesnt seem to have any effect.. Anyone think I should hunt down the correct vacuum advance part? Our host says they are NLA.

[psalt]

The original 83 cannister has two outlets, maybe your distributor was changed, check the numbers. The front outlet, pointing away from the distributor is advance, the original hose was orange, and it was connected to ported vacuum (no vacuum at idle). The rear outlet is retard, the original hose was grey braided and connected to manifold vacuum (full vacuum at idle). Check the hoses with a gauge while running and put a hose on the cannister, apply vacuum and watch the timing marks. Having a vacuum line "just laying there" is NG if it is leaking. Vacuum advance is a load based system to increase mpg at part throttle cruise around 5%, and has zero effect on performance at WOT. If you are concerned about mpg, vacuum advance is a good thing.

[brads911sc]

Thanks Paul. That is helpful information.

[Gunter]

Originally Posted by DavErb

I'm trying to reset the timing on my freshly rebuilt 83 SC motor . I plan on going fo 5 degrees BTC at idle. All the references I have mention remove vac line to distributer. I have 2, the normal advance and retard. When I remove the advance line no problem however when I remove and plug the retard line the engine speed jumps up at least 500 rpm.

Should I use the adjusting screw on the throttle body to get the RPM down to 950 area again or just go ahead and set the timing at the higher speed given that it'll return to the lower speed once the retard line is plugged in again?

Thanks for any clarification of this grey area

Is your engine completely stock? Type 930/16?
Stock CIS with all components and lines connected?
If so, I would use both vac lines but also make sure that the distributor works correctly meaning: deliver the right curve.

Along with the rebuild, was the distributor refurbished and the curve checked?
Distributors tend to be neglected, weights gummed up, top portion of the shaft gets dry and the springs get tired.
Too often, people forget to put a few drops of oil under the small felt on the shaft and that may compromise the function of the mechanical advance.
You can clean and oil the distributor yourself:

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

But the curve still needs to be checked.

[DavErb]

Yes Gunter, as far as I know its stock. The overhaul was to renovate a very tired engine with the usual complement of broken studs, blow by etc. The CIS is also stock with all the hoses connected along with complete replacement of the smaller hoses and a very close inspection of the bigger hoses (all of which were good BTW, I don't *think* I have any vacumn leaks.

I went through the entire distributer and it was a really gummed up mess. Cleaned it all up and it seems to be working as it should though I'm not happy with the condition of those tiny bushings the advance springs wrap around and there also seems to be several milimeters of axial play on the main shaft. side to side play seems fairly minimal. Did not get it recurved.

Assuming I got all that taken care of; would you time it with the retard line disconnected but idle adjusted?

[Gunter]

With stock Lambda CIS, including OX sensor, I would follow factory instruction.
I believe vac lines disconnected (Plugged even though Porsche doesn't say plugged)
Correct idle ~900 and 5 deg BTDC.

Some people just unplug the OX sensor and play with timing, idle and mix adjust.

I used a Stroboscope to check the curve as follows:
Between Z1 and 5 deg on the pulley is 5 mm.
With a white paint marker, I marked at 10, 20 and 30 mm from Z1.
Start and set initial timing per factory, then connect both vac lines.
Retard should be noticeable.
Engine running, note marks on pulley with Stroboscope for every 1000 RPM up to 5k.
The vac advance should be immediately apparent when coming off idle.
Then see what you get for 2k, 3k, 4k, 5k.
You'd need someone to keep the RPM's steady while you work with the Stroboscope, dark place is best.
Don't get caught in the fan belt.
Post the curve-numbers.

Of course, this is an approximate "home-made" way but should give some indication of what curve you get from your distributor.