Boem Boem.. wrong ignition timing?

[jan lauwers]

Hello Friends,
 
I’d like to ask for some assistance on the following problem please: (280SL automatic, from Jan1969, US spec).

A few seconds after releasing the throttle pedal, I hear some explosion-like noises. It sounds like some form of after ‘combustion’ in the exhaust system.

Here is what I have tried so far:

1/Valve clearance adjusted.

2/Static ignition set at 8° before TDC of cylinder # 1. I did this with the help of an experienced friend. We marked the 8° BTDC mark white on the single timing scale and turned the crankshaft until the mark was exactly on the index which sits above the scale. We then connected a test lamp between the coil 1 of the low tension circuit and mass. Then we carefully twisted the distributor clockwise only just until the moment when the test lamp went out. (Contact breaker points closed).
I admit that I am confsued about what the setting should be. On page 93 of the Haynes manual I see a static setting of 8° and of 10 ° BTDC. Which one is right for me? I dont see the difference. My distributor wears this label: IFUR 6, 0 231 116 051.
I have looked on this forum for the correct settings but there is so much info that I dont know what is right for me…. And my skills are very basic!

On top of this boem boem problem, I now think that I have also lost some power on the engine since we have played with the distributor.

Any views on what is causing this sound please? …What is it and how can I resolve this?

Could it be a too rich mixture?

Thanks a lot guys!

Jan

[mdsalemi]

Jan,

I suspect your fuel injection pump isn't correct. You may be too lean?
The backfiring is unburned fuel (caused by the lean condition) getting into the hot exhaust system and then exploding.

It sounds like you need to go through the FI pump adjustments and get the settings correct. If your skills are basic, my suggestion is to find someone with skills on this pump to set it up for you.
The bad news is that finding someone local in Antwerp might be challenging; however nearby in The Netherlands there are a few places that can help. The good news is that once set up properly--with ignition, too--that it will last a very long time.

[jan lauwers]

Michael,

Thanks for your reaction. Interested why you say too lean... Unburned fuel makes me think that the mixture is too rich!?..What is your logic for too lean?
Also, I noticed that the spark plugs are rather black even though they ar fairly new.

Thanks!

Jan

[mdsalemi]

There are backfire conditions that can be caused by too lean and too rich. In the too lean condition the mixture is off enough such that the mixture doesn't ignite with the spark, but does when exhausted. That's the backfire. In the too rich condition the mixture doesn't burn completely, and finishes in the exhaust system. If you think about that of course timing is impotant too...

Regardless, the spark plugs are a sure fire way of telling you that you have the too rich condition. That and the backfiring means the mix is probably off. Theres external adjustment for idle mix; (~900 RPM) and internal for low range (~1500 RPM and above) and high range (above 2500 RPM)

You can try the techniques listed in the tech manual as a start. The Blacklick CO test is one; linkages are another. Or you could take it to Van Dijk in the Netherlands...

[ja17]

Idle and static timing is not that critical. It is a good place to start. Setting the timing at 3,000 rpms will yield best results.  30 degrees at 3,000 is a good place to begin. Some like to go with more advance . Always use premium fuel. Also check to make sure that the vacuum cell on the distributor is functional. Next check to see that the breaker plate in the distributor moves when the vacuum is present. Some USA cars had an emission feature. A switch activated a solenoid on the injection pump which moves the injection pump rack to full lean, when the accelerator pedal is at idle. If your engine is equipped with this feature, you will have two electrical solenoids on the back of your injection pump.

[jan lauwers]

Thank you Joe

[jan lauwers]

Just for info guys, we adjusted ignition timing with a strobo and I have the power back. There is still a little bit of boem boem, but it is so much more of a civilized sound, so I am a happy man.

Thanks for your actions!

Jan

[Shvegel]

Try advancing it a hair more. In my experience the timing spec for the later cars is pretty conservative.  I assume it was to reduce NOX on the US spec cars.

[Benz Dr.]

Try advancing it a hair more. In my experience the timing spec for the later cars is pretty conservative.  I assume it was to reduce NOX on the US spec cars.

It is.  Backfire under throttle is too lean, back fire while coasting is too rich. I set everything to 38 degrees BTDC; anything less won't give you full performance.