Another ignition prob.

[mulrik]

Here's the facts. Car is smoking black on acceleration. Cam, crank, injection and ignition are all in alignment. Compression is fine. When I put my timing light on any of the leads it flashes irregularly, some more serious than others. Checked the timing light on another card and it works fine. Dweel angle is fine too. I am offcourse going to do a full check on the ignition system working my way from the sparks towards the battery. However I pulled the distributor cap off and it was full of carbon tracks (see picture) Could this explain my irregular ignition or would you suspect something else? Also is there a difference between testing spark plugs outside and their function inside the cylinder?

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'67 250 SL Papyrus White 113043-10-000023

[hands_aus]

Hi,
I suggest you replace the distributor cap, they are inexpensive.
If there is too much oil inside your distributor, clean it with a dry cloth.
Add one or 2 drops of engine oil on the felt wick under the rotor.
Similarly apply a couple of drops to the external oil inlet of the distributor.

Also what sort of economy are you getting?
Have you removed the plugs and looked at the colour of the tips? The colour will tell you lots about the air/fuel mixture. The Haynes manual and the BBB have pictures of ideal plug colours.

The general thoughts about plugs and leads is that they should NOT have any resistor in circuit.
The recomended total resistance of the leads and plug tops is approximately 1K ohms.

Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[mulrik]

We tested the ignition system here's the data:

Lead going from coil to distributor: 3000 ohm
Each plug cable: 2000 ohm (1000 from each end and 0 in the wire it self)
Primary coil: 2,5 ohm
Secundary coil: 8000 ohm

BTW we also changed the distributor cap and the rotor.
The condensator was also tested and worked fine.

However, I still get intermittent flashes with my timing light and the engine runs badly with a lot of black smoke.

What are your suggestions. Change the coil? Change the leads? Change my car....?? ANd what readings are you getting??

I'm lost here....

Ulrik


'67 250 SL Papyrus White 113043-10-000023

[graphic66]

I think you should replace the condensor, it is cheap and easy and could cause the problem you describe. How did you test the condensor?

[erickmarciano]

had the same problem I was runing to rich

1971 280sl
1962 VW
1954 Allstate vespa
1958 gs150 vespa
1962 gs160 vespa

[George Davis]

Mulrik,

it almost sounds like your ignition wires are cross-firing, i.e., the current in one wire is inducing current in other wires.  I woulnd't expect this with wires that are used, but one never knows.  A quick check is to run the engine some place where it is very dark, and watch the wires to see if sparks are visible between them.  If you see sparking, probably need new plug wires.

George Davis
'69 280 SL Euro manual

[mulrik]

Here's the latest news (from the BBC...)

Tested each cable with the same spark plug. The flaches from the timing light corresponded with the spark generated by the spark plug when it was tested out of the cylinder on all 6. As soon as I put the spark plugs in the cylinder flashes from the timing light became very irregular most often not flaching the timing light. I pulled each plug and noticed that 4 and 6 were not working at all. When I removed these spark plugs they were black and wet with gasoline I presume whereas the others were dry black. Now I am going to weaken the fuel mixture as I suspect that the presence of too much fuel in the cylinder actually quenches the sparks. Anyone has any thoughts on that. Also I was suggested to by a "Colortune", which is a see-thru spark plug to help you adjust fuel/air mixture.

Here is how you test the condenser:

Testing the condenser. (capacitor) Open the points and set the meter on ohms. The meter should start out at several megohms and drop to 0 when hooked up to the wire and to the case of the condenser, then start rising again. Reverse the leads, and it should do the same again. A steady low reading means a shorted condenser. No reading at all, (OL on a digital) means an open condenser.

'67 250 SL Papyrus White 113043-10-000023

[George Davis]

Spark plugs do go bad occassionally. I'd suggest replacing the plugs before changing the mixture.

George Davis
'69 280 SL Euro manual

[mulrik]

Well thanks. but these are brand new Bosch W7DC.

'67 250 SL Papyrus White 113043-10-000023

[Ben]

Have you checked the engine earth strap ?

I would also recommend running it in a dark environment to check for arking (sp?).

Have you managed to get the car to run long enough that it warms up ?

Reading the plugs is the best thing to do but if the cold start system/warm up device is operating it may give a false i.e rich reading. However when you start from cold you could disconnect the rod at the venturi and open it a touch to give more air and see if things improve as a misfiring car or one with a fluctuating idle will look very irregular through a timing light.

Does the Cold Start Valve leak ?

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[Benz Dr.]

The coil wire is bad or has resistors on each end. You want a wire with no resistance at all. W7DC plugs are fine or engines with perfect compression but are generally too cold for older engines with lower compression and using some oil. Gap the pugs at .035'' regardless of what you may see in some book or what others tell you.

There could be many other reasons why you're having these problems but starting with ignition is the right way to do it.

Daniel G Caron

[mulrik]

The solution. I found out that when I removed the hoods at the end of the spark plug cables and held them loosely over the plug I got regular sparking. I therefore went to extremes and bought a new coil and new cables and a new condenser. That cured the problem!!!!! Turned out the old ignition coil was not as powerfull as the right one from Bosch. All leads have 1000 ohm in resistance and now the car runs fine. Only with a little black smoke over 3500 RPM, but we are looking into that now.

The funny thing about my problem was that the ignition fault did not follow any of the cables or hoods when we switched those from cylinder to cylinder. We believe that power generated by the ignition system was running on its edge causing irregular firing on some of the plugs in an uneven pattern.

'67 250 SL Papyrus White 113043-10-000023