phminell

Last day I checked and fine tuned my ignition timing because there was some ticking during acceleration.
As expected, there was too much advance (14° at idle w/o vacuum)
I set it up to 10° at idle (without vacuum) which gives me a total advance of 32° (w/o vacuum) at 2500 rpm
The car is now running fine.

The purpose of my post is that I observed a strange behaviour on the centrifugal advance while checking the hig rpm range :

rpms / advance (w/o vacuum)
700 rpm / 10°
2500 rpm / 32°
3000 rpm / 32°
3500 rpm / 26°
4000 rpm / 30°
4500 rpm / 31°

I observed a significant and sudden drop in the centrifugal advance after 3000 rpm (from 32° down to 26°) and then a slow ramp-up again from 3500 rpm to 4500 rpm

Does someone have an idea on where this could come from ??

OMF

I think that your mechanical advance is gummed up or the bushings are worn which are affecting the smooth operation of your advance. Have you disassembled it and checked for worn parts?
Thats where I'd start.....

phminell

This would explain a jump up in advance (let's say for ex from 28° to 32° all a sudden) but not a drop down from 32° back to 28° when increasing the rpms...
I already looked at the ignition mechanism and nothing looks particularly worn out... (it's "only" 15 y old : changed in 1999)

OMF

The bushings usually wear out....were they replaced 15 yrs ago?? I'd look at those.
Is the back down of advance repeatable? If it is something in that system is worn.

phminell

Yes it is repeatable. It does it every time.
I'll have a look this week end

dochorsepower

There are other factors that can participate in distributor position, for example vertical float resulting from worn bushings, remember the distributor has a bevel gear. Also something could be happening with cam location. You don't say how much wear is on your engine.

phminell

I dont believe in the worn parts : I forgot to tell you that previous owner replaced the stock engine with a new ZZ4 in 1999 so it is not that old.

One idea I had yesterday : it is perhaps because I did not plug the vacuum device input when I disconnected the vacuum hose from it.

I don't have time during the week. I'll do the test this week end with the vacuum input plugged.

my 76 ray

You only need to plug the hose that you remove from the vacuum advance, not the advance unit.