77, 2.7 L, CIS- Vaccuum Advance

[aricaandjustin]

The thing about old cars is you never know what was done in the past. I was timing my engine after fixing the CIS injection system...not sure if anyone has taken apart their warm up regulator and adjusted it while monitoring the fuel pressure, from cold to warm up (but that another story)...

I took off the vacuum advance line on the distributor and found full vaccuum at idle. As I revved up the engine, the vaccuum decreased. This really goes against my past experience with any other car...

Does anyone else know if this is correct?

[2.7RACER]

The vacuum on the '77 distributor retards the ignition at idle for smog purposes.
To test this connect a timing light and notice with the vacuum line connected the plugs fire after top dead center (ATDC).
Pull the vacuum line and notice the timing advances at idle.
The California spec is 15 degrees AFTER TDC with the vacuum line connected.
You'll need this vacuum line connected up to pass smog.
This from page 955 of the factory manual.
Since I run carbs, I don't have a vacuum line.
Converted from CIS a couple ears ago. Not a legal smog car in California.
In fact I run my car at 10 degrees before TDC, the centrifugal advance takes it to 30 plus degrees @ 4500/5000 rpm.
Yes its backwards from other cars that use the vacuum to advance the timing for fuel economy.

[wowzer911]

2.7Racer, My (smog exempt) 74 CIS is 5 degrees (ATDC) with vacuum line connected.
I did not realize that the retarding of the timing was emissions related.
Is there a performance gain to be found by advancing the timing, and/or disconnecting the vacuum, while keeping the stock CIS? (Obviously this area is not one of my strong points.)

[2.7RACER]

I haven't tried this on a dyno. Could be a small gain, I just don't know.
I would suspect the best thing to do is to make certain your distributor is working as designed.
That is do you get full centrifugal advance at high rpm's?
These 30 year old distributors get awfully gunked up inside.
Worn bushings make the timing erratic as well.
It shows up with a timing light.
The timing mark on the pulley bounces around 3-5 degrees with worn bushings.
Remember to plug the vacuum line if you choose to run with it off.
It'll need to be reconnected and reset to factory spec's before any smog check.

[2ndTARGA]

Not to add confusion to an already confusing (to me) subject, but when I first tested my '77 vacuum advance I found there was no vacuum getting to the disti.

Upon further research, I was amazed to find a tiny ball bearing shoved into the vacuum hose between the throttle body and the disti.

I thought this was a PO experiment until I read my Factory Workshop Manual which states:

TYPE 911 S, 1977 MODEL

Ignition Timing

USA Version (vacuum hose permanently plugged, vacuum advance inoperable)

California Version (vacuum hose connected)

So there are two things to consider here. First, the factory manual uses the term vacuum advance for what it's worth.

Second, if your car is a 49 state version (as is mine) you should have a plugged line and no vacuum advance or retard whatsoever.

I've never learned why the factory spec is for a plugged line. Why didn't they just plug or cap both ends?

Ron

Oh, as to your question about vacuum decreasing with RPM, I believe that's quite normal for every engine I know of. High vacuum at idle is due to the fact that the throttle plate is closed and the pitsons are trying to suck more air than can get in. As you open the throttle, the vacuum decreases.

[RoninLB]

My calif 77 had the retard in use. It's a bad news smog idea.

I disconnected the hose and set total timing at 35 degrees.

Mine is marked on the pully. Initial should fall around 0-5 btdc.

If CIS and hot desert conditions it may ping so pay attention.