How to check vacuum advance unit?

[moosthuizen]

Hi,

I have searched a lot of posts, but could not find any explanation on how to check whether the adv/retard unit is working.

I have a '80 911 sc, Euro model no emission or o2 stuff, standard.
I have a single line from the advance unit, which connects behind the airfilter, left (if facing the engine) to the body housing the buterfly.

When I pull this line at the advance unit, at idle, I can barely feel any suction at the unit itself (where the hose connects), or on the pulled line, which is then still attached on the other side.
If I ref the engine there is also not any major difference, though I probably did not ref it past 4000rpm.

How do I check to see if the advance/retard *(is that what it is called) is working?

Marius
'80 911 sc

[john70t]

You hook up a timing light and point it at the marks on the crankshaft.

If you rev up the engine, the mechanical advance should start increasing the advance marks around 2000-2500rpm and stop around 4000rpm (this is average, not specific).

If you disconnect the small vacuum-retard/advance(?) hose from the distributor to the intake manifold and apply vacuum to it (a Mityvac brake bleeder hand-pump unit works well) the timing should change as well. With the engine off, take of the distributor cap and watch the rotor turn.

The advance plates can stick with corrosion/rust and have to be lubed once a year(the felt pad under the rotor). I use motor oil and a few drops of Marvelous mystery oil.
Next option would be removal, thorough cleaning, and possible rebuild.

[Wavey]

You can check it 2 ways, with the engine running or not running. With the engine off, pull the end of the hose at the throttle body, pull the distributor cap, suck on the end of the hose and watch if the advance mechanism moves smoothly. You can do the same thing with the cap back on and the engine running - the idle speed will change if the vacuum advance is working. If you're just sucking air and the mechanism doesn't move, either the vacuum line has a leak in it or the diaphragm is bad. If the mechanism is sticky or jerky, it probably just needs to be cleaned and lubed. You won't feel any suction from the advance unit itself - the vacuum comes from the engine, not the unit. The suction you feel on the end of the hose will still be pretty weak - it's a small line and it doesn't take much vacuum to pull on the diaphragm.

To answer your advance'retard question - if the advance unit has one hose, it's strictly a vacuum advance unit, which is probably what you have. With that system there are springs that pull the advance back when the vacuum lets up. If it has 2 hoses, it's an advance/retard unit, where the advance and retard are both controlled by vacuum (from different sources).

[john70t]

If the vacuum line is coming from the intake manifold(manifold vacuum), it should be high at idle and low at wide open throttle.

If the vacuum line is coming from the side of the throttle body, it should increase progressivly with rpm.

[SpeedracerIndy]

Good timing on this thread. I've been inspecting my vacuum advance too, and have a euro motor. I found that the wrong vacuum line was attached to the advance unit on the distributor, and was causing a very bad stumble when taking off from a stop light. So bad, that I had to rev to ~2500 and slip the clutch to take off from a dead stop. I put the correct vacuum line on the dizzy and it fixed it for the most part. I still get a little hesitation though when the engine is up to operating temp and I really lug it from a start. I am thinking that the advance unit needs to be lubed. When I pull the cap off and turn it by hand it turns freely though. Where is the proper location to put the oil?

[GrantA]

Good thread which leads nicely into my problem (or not). I have a 78 911SC, the motor was rebuilt 20k ago with hi-comp pistons to give 204bhp. The original dissy was replaced but vacuum advance has not connected it as the mechanic says it give 3deg adv at idle and 24deg adv at 4k revs and that is fine. He is a well known and respected independent Porsche specialist here in UK so I am loathed to question his work. Advice I have been given is "if it aint broke don't try and fix it". Question is, is it normal to run these engines without vacuum adv? Will it do any harm long term? Car runs fine, not as quick as my sons 964 Turbo, but what should I expect.
Grateful for any thoughts on subject.

[Wavey]

I'm guessing that he wants the vacuum advance disconnected because it may provide a little too much advance for use with the high compression pistons - leading to detonation / spark knock. I suppose you could try two things - connect the vacuum advance and see if you get spark knock, or bump up the advance a couple of degrees at idle and listen for the same thing. The US spec engines age good for about 28 degrees total advance without any problems, but they have a lower CR. Generally - more advance = better accelleration.

[Grady Clay]

Guys,

Most OLD automotive shops have a “distributor machine”. With it you can measure the actual advance curve and the effect of the vacuum advance/retard. You can make yourself an actual measured curve and compare it to the specs.

Find some friendly guy who will let you bring your distributor in early on a Saturday morning. Bribe him with whatever.

Best.
Grady

[GrantA]

Thanks Guys. Gives me a good reason to play with the dissy (a few degrees each way - vacuum on or off) and see what happens.

Grant