350 SLC 1972 troubleshooting

I have check again my ignition timing today.


Try 1:
Set the timing to 34° @ 3000 rpm without vacuum.
@ 1500 rpm the timing is 21° with vacuum (21 ° without vacuum)
@ idle (750 rpm) the timing is 19° with vacuum (8° without vacuum)
@3000 rpm with vacuum is 34°
With this setting the engine is lazy above 5000 rpm and do not get above 6000 rpm


Try 2:
Added some more advance by turning the distributor counter clockwise.
The engine is getting at 6000 rpm with no problem.
The idle is a little bit more rough.

@3000 rpm the timing is 38° with or without vacumm
@idle (750 rpm) the timing is 23° with vacuum (13° without)
@ 1500 rpm the timing is 26° with or without vacuum.


The behavior of my distributor is strange.
May be it is related to the two vacuums pipes of my distributor.
One is not plugged.

Hi,
did you try to remove the cap the inner vacuum pipe is sealed with?

Maybe this makes a difference.

Regards
Norbert

Hi Sylvain,

it looks like nothing is wrong with your distributor. Its rev change is within limits and its vacuum advance also.

BUUUTTT !!!!!!!!!!!!!!!!!!!!!!!!!!

Your throttle flap only has a retard vacuum connection as did all early 350 SL/C . When your distributor was replaced it was correct to only connect one vacuum line on your distributor and seal the other one. However it is now connected to advance input and not to retard input. Change the vacuum line to the other connection and be happy

Then you will have ignition timing of 0° at idle.

Hello Norbert

I haven't try it yet.
But i assume that the two pipe must be connected wich is not my case.

The one above wich is on every distributor is the retard, used mainly for idle.
It brings between 8 an 10 ° retard, as described in the mercedes document below.
It is consitent with my tests above.



for the distributor with two vacuum pipes :
0 231 403 001
0 231 403 005
0 231 403 009

the second vacuum pipe (below, the one sealed on mine) should bring 8-12 degrees advance at 3000 rpm.

I will try to plug the advance vacuum on my distributor. to see if it is consitent with the mercdes document above.

The big question is where should i plug the Advance Vacuum on the engine ?

As i understand the Retard Vacuum is plugged on the throttle body.


There is no other vacuum line on the trhottle body.




I have check the other vacuum lines on the firewall.
One is not used.


Maybe it is the place to plug the Advance vacuum.

Hello Volker,

you mean that the above connection is for advance
and the below connection is the retard ?
And mine was misplugged by a previous owner / mecanics ?



When the two vacuum are plugged there should be two vacuum pipes on the throotle body ?

Hi,

yes that is waht I mean and have also decribed earlier here

And I do not know why you put a Pertronix / Fullmax / Powerspark contact when you anyhow already have a transistor ignition system. Makes no sense. Most probably previous owner installed it due to his mistake on vacuum connection.

Check that he did not also change power resistors and ignition coil. Because that is another fake myth that you can then just remove power resistors and car will run smoother. Great bullshit you should not believe in.

Thank you volker,

you are more than right.

I have found the same document in english.


I will try it right away.

That is the problem with 40 years old car that have been maintained by poor skilled mecanics.
After that it takes you more time to identify and correct the issues created by them.

With the spares that came with the car, i had the original contact and condenser.
I will check the coil and may be go back to the original system.

Regards

Volker,

I just did a quick test with the vacuum lines plugged the right way.

I set the timing to 34° @ 3000 rpm without vacuum

@ 1500 rpm i have 18°
@ 750 rpm, idle, i have +2°

It is much better and consistent with the factory setting.
The engine is still a bit lazy at high rpm.

I tried with a little more advance, around 7° @ idle.
It is working up to 6000 rpm fine.


I will investigate why the factory setting is lazy.

May be it is the timing chain that is lengthen and change the timing settings.
I will open the camshaft covers to check the chain and guides.

May be it is related to the pertronix "upgrade".
I will check the coil and resistor values.

May be it is related to the switchgear. I am not very familiar with it and try to document myself on it's behaviour.

regards

Hello again,

I have been investigating a little deeper my ignition timing issue.
I have also notice that the 1st and 5th cylinders were misfiring.

The spark plugs of cylinders 1 & 5 are wet (from fuel ?).
As you can see in the following picture with cylinder 2 plug on the left and cylinder 1 on the right.


May be the plugs are defective.

I have put the cylinder 1 plug to the cylinder 2.
And the cylinder 2 plug to the cylinder 1.
Restart the engine. Let it idle a little bit.
And check the plugs again.
The new cylinder 1 plug (previously cylinder 2) is wet.
The new cylinder 2 plug (previously cylinder 1) is dry.
It is not coming from the spark plug.

May be it was coming from the high tension wires.
I did the same test by changing the cylinder 1 wire to cylinder 2.
And vice versa. The results are the same. cylinder 1 plug is wet, cylinder 2 is dry.
It is not coming from the wires.


I have also test again the ignition trigger wires between the distributor and the ECU connector.
Also tested the injector wiring resistor value between 1.2 and 1.4 ohms on all injectors.
All is fine.

I checked the distributor head.
There was some cracks in it.


There is also some marks, like if the rotor was touching the head.


On the opposite side of the distributor (wich are cylinder 1 and 5) it is like the rotor was touching below the contacts.
There is also some minor scartch marks just below the contact points.



I had also 2 used distributor heads as spare.
I try one on the car and start the engine.
I had the same behaiour with misfire (no fire) on cylinder 1 and 5 (wet plugs).

Today i descided to remove my distributor to check it outside the car.

I have notice some play (and no it is not the centrifugal advance) in the rotor.
Remember my distributor is equipped with a pertronix ignitor instead of the points and condenser.

When i remove the ignitor magnet, and put back the rotor.
Ther is no more play.




With the magnet in position, the rotor is not in the correct position on the distributor shaft.
It is not low enough on the shaft. You can also see some orange bits inside the distributor.


With the ignitor. the rotor is at 53.44mm.



Without it is at 52.19mm.


You can aslo see some scratch inside the rotor.


The play in the rotor may be the source of my ignition timing delay.
Remember i had to add 8° to get a fully reving engine.

With the car spare parts, there was a used points and condenser.
The condenser is the bosch reference 1 237 330 306, and it had the typical mercedes green wire.
So i assume it is the correct parts for my car.
There is no reference on the contacts, only 'VEMO'.

I will try tommorow to install it back on the distributor, set the dwell angle and set the timing to the factory value to see what happens.

If anyone as got some picture of the original contact install within the distributor it might help.

Hi Sylvain,

i will add a photo of a disassembled distributor here. I do not understand why people would install a Petronix on a car with ignition control module. Maybe someone was desparate with the wrong retard connection. However, I recommend to use Bosch rotor and cap and NGK spark plugs and Beru ignition cables. Beru has sometimes caused troubles with rotor and cap. While Bosch is known for trouble with spark plugs from free market.

Hello Volker,

Thank you for the picture.

I have NKG plugs already.

The current rotor (VK283Pip) and cap (VK283Phv) are made by BERU.
I will try to find Bosch parts.

I will check the ignition cables tomorrow.

The ignition cable are from Beru (ZLE 121).

I have put back the contact points and condenser.
I set the points to 0.4 mm and verify the dwell angle when started.




The condenser wire have been modified but it should not be a problem, I hope.

I will use the following diagram to connect the condenser / point wire to the switchgear.

The distributor is rebuilt.
Just need to test it.

The distributor is back on the car,
The dwell set to 34°.

The engine is not working very well.
Poor idle and no reving.

I still have to set the timing.

But i have just checked the spark plug 5 is dry.



Spark plug 1 is still wet. Strange.

I set the timing to 34° @ 3000 rpm.

Same behavior as few days before.
Poor reving.

Cylinder 1 and 5 Are not working.

The cause was not the pertronix.
With the factory points and condenser, the engine is working only on 6 cylinders.
Just as before with the pertronix but as the spark might be more powerful it was less noticeable.


I did an injector swap between cylinder 1 and 2.
Same behavior, cylinder 1 and 5 not working.



Remember few weeks ago I did a spray test with my other ECU and all 8 injectors were working.

This time I did a single spray test on injector 1.
I put some extra fuel line and start the engine with injector 1 out.
No fuel was coming out.




Few days ago I tested the trigger contacts wire to the ecu there were OK.
I also tested the wire to injectors group 1 & 5 it was OK.

I checked the duty cycle of the trigger contact group 1 (injector 1 and 5) with the engine running.
It is around 50%. It is OK

Can it come from the ECU ?

Yes the ECU is guilty.

I put back my old ECU and ... voilà.