Jim Wilson

I just got finished with a quest to destroy vacuum leaks in my shark, hooray! Now I have a different problem.

I had just put everything back together, started it up, and it ran great until I opened the throttle slightly. Lucky for me, I noticed that if I just take off the vacuum advance off of the distributor, it runs fine.

So I timed it. 45-50 degrees BTDC Ouch! the PO must have put the distributor in with the wrong tooth. Got everything squared away, right on 31 degrees now, but only works with vac advance off.

Questions.

The vac advance seems to be doing the right thing, I suck on it, and it advances the timing. Why does that throw it off?

How do I get it working again?

Jim Wilson
'79 Euro 5-speed

Jim V

You did not say so I have to ask;

Did you diconnect the vac hose when timing?

Did you time it at 3000RPM?

I keep my '79 at 35 degrees at 300RPM and it runs fine.

Check to be sure the vac line is on the proper vac
source at the TB.

There is a possibility or the engine damper slipping on it's hub which would throw it way off.
I'm not sure how you check for that on these cars

WallyP

To roughly check the timing marks:

Get a two- or three-foot 1/4" wooden dowel, a socket to fit the crank pulley bolt, and a large pull handle.

Remove the #1 spark plug (right front - that's the car's right, passenger side in USA). Insert the dowel until it bottoms on the piston.

While keeping gentle pressure downwards on the dowel, and keeping it from hanging in the plug hole, slowly rotate the engine (ONLY in the correct direction - as if you are tightening the bolt). The dowel will go up or down. When it goes up, turn the crank until the dowel just stops going up - the "0" or TDC mark should be aligned with the pointer.

Not very accurate, but we can't use the "positive stop" method on a 928 engine. If you need to do this more accurately, you can set up a dial indicator, but that is a lot more complicated.

Jim Wilson

Thank you Jim and Wally,

Jim

Yes I disconnected the vac hose while timing.

I am not sure if it was exactly three thousand because I couldn't set the idle that high, and I didn't have anyone to set the rpm from inside the car, but I think I got it close +-300.

I am pretty sure that the hose is going to the right place, I checked it against the manuals. However, I will check it again. It also has a lot of vacuum when I just slightly open the throttle. Would it be worthwhile to check the amount of vacuum?

What is an engine damper?

Wally,

I checked the alignment of the cams recently using the marks on the crank and the notches on the cam pulleys. Is that procedure something extra to check, or is what I did going to tell me the same thing?


Thanks again,

Jim Wilson
'79 Euro 5-speed

Jim V

The damper is the part with the timing marks on it, The hub of it is bolted to the crank and the ring with the marks is mounted to the hub by a rubber insulator; although it's not a really common ocurance the rubber can fail allowing the ring to slip or fall off.

As far as the advance I'm actually not sure if it's an advanced or retarded by vac on the '79s; the later cars use vac from 2 sources maybe Wally
could clear that up?

The difference would be that if it were a retard system there would be full vac. at idle and the timing would advance as vac. was lost.
An advance system would have little or no vac. at
idle and would advance as vac. increased.

Honestly I don't know which way it works on our
cars.

Dennis Wilson

Jim,

On my 78's there are vacuum advance and retard lines going to the distributor. May want to check the back of the advance unit to see if there is a connection for a retard line.

Dennis

Dennis Wilson

Jim,

You really need an external tach to get the RPM's right. Three hundred to 500 rpm's off may have enough effect on the centrifigal advance to adversely change your timing.

Dennis

WallyP

Ignition timing is fairly complex on the earlier models of the 928, as the engineers were attempting to control the emissions while maximizing power, economy and driveability. The only way that they had to do this was to keep adding controls to the distributor.

Final timing is the sum of:
Initial timing - the mechanical starting point determined by initial position of the distributor at zero rpm.
Centrifugal advance - the advance mechanism steeply advances the timing from idle rpm to about 1300 rpm, where the rate of advance slows somewhat but continues to advance up to about 2500 rpm, where the advance ceases.
Vacuum advance - the vacuum advance hose pulls on the vacuum diaphragm, advancing the timing in proportion to the vacuum applied. The advance side is usually larger, so that a given vacuum level has more effect on advance than on retard. The vacuum source for advance is usually a ported vacuum from near the throttle butterfly valve, so that there is little or no vacuum at idle or overrun, moderate vacuum at low power levels, and high vacuum at cruise.
Vacuum retard - the vacuum retard hose pulls on the other side of the vacuum diaphragm, retarding the timing in proportion to the vacuum applied. The retard side is usually smaller, so that a given vacuum level has less effect on retard than on advance. The vacuum source for retard is usually manifold vacuum, so there is moderate vacuum at idle and cruise, low vacuum at full throttle, and high vacuum at overrun.

To adjust the timing, it is critical that you remove both vacuum hoses from the vacuum unit on the distributor!

Hook up the timing light, and check the timing at idle, then watch the timing as you increase the engine speed to 3000 rpm. The timing should advance steadily and smoothly from idle to about 2500 rpm, then hold steady at 31 deg before top dead center above that.

If the timing is not correct, first make certain that both vacuum lines are disconnected. Loosen the distributor clamp just enough to allow you to turn it with some effort.

With the engine at 3000 rpm, turn the distributor so that the timing is set at 31 deg.

Allow the engine to return to idle. Check the vacuum hoses - one should have vacuum. Attach it to the back (inner connection) of the vacuum unit. Attach the hose with little or no vacuum to the outer connection on the vacuum unit. The engine should slow (when the first line is attached - usually little change with the second line), and the timing should now be zero deg +/- 4 deg.

Rev the engine to 5000 rpm - the timing should be between 31 deg and 36 deg.

The retard function is primarily for emissions performance on overrun (coasting), but it needs to be attached for proper balancing of the vacuum to give proper timing at part throttle.