Vacuum Retard!

FMAN · Post by **FMAN** » Sun Nov 19, 2006 6:20 pm

Hi,

I have been looking at my distributor which has had the vacuum advance disconnected. If I suck on the vacuum pipe it retards the ignition! So I though I should ask here for a sanity check.
If you look from above, the rotor arm rotates in a clockwise direction. The mechanical weights pull the shaft in a further clockwise direction hence giving more advance.
The Vacuum unit pulls the base plate in a clockwise direction hence retarding the ignition!
This is all confirmed with a timing light, assuming the engine rotates in the clockwise direction when looked at from the rear.
Am I missing something obvious here or is this some kind of Porsche foible?
The distributor looks like the Bosch one in the Haynes manual and it is on a ’72 2.4T.
Should the vacuum unit pull or push with vacuum? I am wondering if the wrong part has been fitted.
Thanks for the help

Neil

butzsi · Post by **butzsi** » Sun Nov 19, 2006 6:36 pm

Hi Neil, it is operating correctly. To set the timing you must make sure that there is a constant vacuum (not fluctuating) supplied to the distributor hose and that the engine is at OPERATING temperature, not just run for 10 mins or so - if the temperature is too low, you will set timing to what appears correct but it will be too retarded when the engine is at normal temperature because the vacuum applied will be greater.

Chris

FMAN · Post by **FMAN** » Sun Nov 19, 2006 7:08 pm

Chris,

Does the vacuum supply to the distributor follow the manifold vacuum?

If so, am I correct in understanding that on light throttle openings the timing is retarded relative to large throttle openings? Do you know why it works this way as it is different to every other car I have worked on!

Thanks

Neil

veryporky · Post by **veryporky** » Sun Nov 19, 2006 8:21 pm

I think it goes like this. With the engine at low rpm the vacuum unit causes a retarded condition. When you give it a large amount of right foot the vacuum collapses in an instant and you get 5 degrees of advance instantly which gives immediate throttle response.......

butzsi · Post by **butzsi** » Sun Nov 19, 2006 8:35 pm

Neil, the timing is only retarded on a closed throttle (idle) once the butterfly has opened up a couple of degrees the vacuum collapses because the vacuum port is now uncovered. It is a strange system but was the way they tried to keep the emissions down, presumably for the US market.........

Chris

FMAN · Post by **FMAN** » Sun Nov 19, 2006 8:42 pm

Hi,

Thanks for the responses.

I am getting spiting back through the carbs at low throttle openings, could this be causes by this not been connected?

Neil

butzsi · Post by **butzsi** » Sun Nov 19, 2006 8:50 pm

Have you got Zenith 40 TIN carbs oe Webers?

FMAN · Post by **FMAN** » Sun Nov 19, 2006 9:34 pm

Zenith 40 TIN carbs

butzsi · Post by **butzsi** » Mon Nov 20, 2006 6:26 am

On the side of each carb there is a vacuum operated unit that wprks in conjunction with the speed sensing relay and the microswitch to control emissions on over-run. These vacuum units contain a diaphragm that very
often has become brittle and has punctured giving rise to the spitting that you mention - the units can be taken off easily for inspection. 901.100.946.00 is the part number of the diaphragm if required.

Another cause of spitting can be high resistance plug caps/ poor insulation on plug leads.

Chris

Gary71 · Post by **Gary71** » Mon Nov 20, 2006 7:47 am

Morning,

Spitting back is normally a lean condition, the split diaphragm will cause this.

I (amongst others!) have stripped all this overrun enrichment circuit off as it does nothing for normal driving. Full details on my site (click the link in my signature

)