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MERCEDES W107 The R/C107 coupe and roadster from 1971-1989 (280SL, 280SLC, 300SL, 350SL, 350SLC, 380SL, 380SLC, 420SL, 450SL, 450SLC, 450SLC 5.0, 500SL, 500SLC, 560SL)

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Old 29th September 2014, 08:40 AM
sean sherry sean sherry is offline
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Location: sydney
Posts: 1,298
Question More K jetronic

my Hot Start is now sorted.......
The Vacuum Retard on the Distributor is still an issue....
With the Air Con on it is cut out at all times ... but with normal driving it is fully operational with high vacuum especially at idle. And cuts out as the vacuum drops on acceleration.

But is the ignition still being retarded, say at cruise, when the vacuum rises again ???
Is engine load, via vacuum , always determining the advance curve by retarding as we drive ??? And thus effecting fuel consumption.
My understanding is that the Retard function was to improve emissions at the expense of fuel consumption.

I can't find any conformation of this anywhere.

I'll be happy if I can achieve 25MPG instead of 22MPG
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Old 29th September 2014, 01:41 PM
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Default Re: More K jetronic

Firstly: how many ports are on the diaphragm?

I think you may be confusing vac ports.

AUX/AC advance works by cancelling the retard.

Following applies to the idle running:
Normally retard is at max due to its port position (hence the 0* at idle). The "advance side" is open to the air via the yellow valve.
When advance is introduced, it actually opposes the retard and bellows are balanced - retard at idle is cancelled hence the ignition is advanced.

Now, the advance valve is a changeover type so it is either applying the vac right across the ports OR it is shutting the vac supply side closed and opens the other port to the outside air.
Imagine what happens if you swapped the sides?

To test it, all you have to do is run the AC. If rpm's pickup a tiny bit... all is set correctly.

During anything else than the idle run, ignition timing is controlled by the weights inside the distributor. They can stick.

Last edited by Afro107; 29th September 2014 at 01:45 PM.
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Old 29th September 2014, 03:08 PM
sean sherry sean sherry is offline
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Location: sydney
Posts: 1,298
Default Re: More K jetronic

Yes I kinda understand what you are saying.

But not the last bit about the mechanical advance wholly controlling the advance curve.
Surely the vacuum to the advance side of the Distributer comes into play after the throttle is cracked open and is then variable depending on engine load reflected in the available vacuum.

My understanding is that regardless where the connectors are , the Retard vacuum is pure manifold vacuum and is also variable dependant on engine load conditions. My question still is .... is the Retard always effecting the Timing during most driving conditions.
Mostly light throttle cruising on a light load ???????

Thanks for your reply......Sean
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Old 29th September 2014, 09:26 PM
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Default Re: More K jetronic

You're right, retard is at manifold pressure.

There could be some modulation of retard via the vac element, as you said, but I think that is only at small throttle valve opening because of its position relative to the valve plate. This kinda gives a smooth advance to default value (about 15* BTDC) as engine changes from idle to drive rpm's

I wish I can find the training manual I have somewhere in storage...

Here's standard version of K-Jetronic advance/retard diagramme.
Attached Images
File Type: jpg K-Jet standard.jpg (43.9 KB, 3 views)
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Old 30th September 2014, 08:27 AM
sean sherry sean sherry is offline
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Location: sydney
Posts: 1,298
Cool Re: More K jetronic

Yes, looking at the diagram with the take off ports on each side of the butterfly , it suggests that both advance and retard are subject to the same vacuum draw when the throttle is opened.....a balance.
From this, I can't see any modulation of the Retard signal ,
( via the Air Con compressor clutch voltage signal) The vacuum valve is either open or closed.
I'm now concluding that the Retard is there at all times, except under heavier load conditions, when the Timing is then purely mechanical as the advance is also cancelled
My further understanding is that a vacuum advance facility is for fuel economy
only, as on light throttle an engine is programed to run lean and the leaner mixture needs and earlier spark to fully ignite this mixture. ????

Thanks again for your insight

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Old 1st October 2014, 04:18 PM
sean sherry sean sherry is offline
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Location: sydney
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Smile Re: More K jetronic

Hi Afro1o7, just a follow... up retard disconnected.....on my SL 280 107

Achieved 24.57 Mpg mostly Freeway , with some Suburban to get there.
A 10% improvement
Timing at 36 BTDC at max Mechanical 3500rpm.
Vacuum 15 inches at idle in neutral...... static at 14 BTDC. 600 RPM in Drive.

A rule of thumb for timing and performance which works for me

.... advance Ign to obtain max Vacuum at idle... then retard by 2 inches of vacuum. and set .
Obtains the same results as above.
Engine now " feels " cleaner with the retard disconnected .

Thank you for your input

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