Misfire ?

[n/a]

I have a 1967 250SL with a slight misfire between 1500 & 2000rpm. I am trying to locate acccurate technical information for the following. CO @ Idle, correct ignition timing, resistance of the coil, dwell angle etc. I have checked the CO% and found the vehicle is running @ 9.85% @ idle (Too High or what)You can hold the revs steady @ 1500rpm and if you shut the throttle down the car almost stalls, indicating a weak mixture. I would appreciate anyones input regarding this problem. Tonybeamer (UK)

PS Great site

[Cees Klumper]

All information taken from the technical data manual ("Tabellenbuch"), edition 1969:

breaker point gap:  0.3-0.4 mm
dwell angle: 38 degrees +3, -1 degrees
Bosch distributor part #: IFUR 6 0 231 116 051 (MB part no. 001 158 02 01)
timing: 8 degrees BTDC at idle, and 30 degrees BTDC at 3,000 RPM, both without vacuum advance connected
CO value at idle: 3.5-4.5

Don't know the coil resistance but this may be in the technical articles.
Also check your spark plug wire resistance, end-to-end (should be 1K ohms). These wires are sometimes the culprit when we are confronted with Miss Intermittent. Of course, check the vacuum advance mechanism (springs, binding etc).

Good luck!

[n/a]

Hi Cees,
Thanks for the information, it is nice to speak with you ref the Merc 250 SL.
I have carried out all the test as listed, The distribitor, contacts,Dwell Angle & advance & retad mechanism were fine. I reset the ignition timing to 8% BTDC vacuum disconected as suggested. The car seems to idle better & run smoother now, however the plugleads are suspect. Would you recomend genuine Mercedes parts or can you suggest anothe brand of 1KV lead set. I still have a slight missfire and once the leads are replaced I will concentrate on the mixture.

quote:Originally posted by cees klumper

All information taken from the technical data manual ("Tabellenbuch"), edition 1969:

breaker point gap:  0.3-0.4 mm
dwell angle: 38 degrees +3, -1 degrees
Bosch distributor part #: IFUR 6 0 231 116 051 (MB part no. 001 158 02 01)
timing: 8 degrees BTDC at idle, and 30 degrees BTDC at 3,000 RPM, both without vacuum advance connected
CO value at idle: 3.5-4.5

Don't know the coil resistance but this may be in the technical articles.
Also check your spark plug wire resistance, end-to-end (should be 1K ohms). These wires are sometimes the culprit when we are confronted with Miss Intermittent. Of course, check the vacuum advance mechanism (springs, binding etc).

Good luck!

[Cees Klumper]

Here's an earlier contribution by Joe Alexander on the subject of spark plug wires that is on point: "The original spark plug wires are solid copper and very rarely go bad. However the screw-on spark plug ends have built in resistance and these do go bad. Chances are if you replace the whole set, you will never get another set that looks as good or lasts as long as the originals. I advise that you check each wire and replace the bad ends. Finding original ends may be a challenge, but I am sure some listers have spares. Make sure you still have the original solid copper wire and that it has not been replaced by those troublesome carbon resistance wires. If you do have some damaged plug wires, original solid copper plug wire is still available from some motorcycle and farm tractor sources. I usually check all my plug wires and ends with an ohmeter during a standard tune-up. Mercedes used several versions of wire ends during the 113 years. Same vintage Mercedes sedans used the same in many instances and are good parts donors! The coil wire was also solid copper originally, and had a screw-on resistance end. I am not sure what the spec. for it is but I can dig it up if needed. I have seen a lot of beautiful original wire sets discarded just because of a defective end! I would change only as last resort if your concerned about original appearance."

Armed with this information, your usual parts supplier ought to be able to supply you with what you need.

[ja17]

Hello tony,
The early 113 cars with the bakelite spark plug wire ends were as Cees stated, 1000 ohms resistance. The later metal shielded spark plug wire ends were 5,000 ohms resistance.  A small ohmeter can be bought a Radio Shack for $15.00 and it will work nicely for this test. Check each wire from the distributor to the sparkplug. A bad wire or end will be radically different from the good ones. I saw that original solid copper core wire was available by the meter from Mercedes. Tom Hanson has the information but I believe he stated that the plug wiring was #110-159-18-18 available for $10.50 per meter.  The numbers for the wire ends are #000-159-2185 for the early bakelite (1000 ohm),  and #000-156-3210 for the metal shielded (5000 ohm). Don't forget the ends at the distributor cap also unscrew and have resistence built into them also. I do not have all the specs and numbers on these in front of me at this time. I will try to post the information later.

Joe Alexander
Blacklick, Ohio