Re:Fuel or ignition?

[jakl]

Hi folks, I'm perplexed.  US '70 280SL.  Two weeks ago car was idling fine.  While driving in a parking lot for 20 minutes at both idle and no faster than 15 mph, car started to run rough.  It felt like dirty fuel or water in gas.  It was missing at idle and on acceleration.  Decided to try Italian tune-up.  Not as noticeable at 65-70 mph, but still present.
     A week later I tried dry gas in the fuel tank.  Drove car 40 miles - no difference, still rough.
     About two months ago I had the exact problem after filling up tank.   After idling 5 minutes, started to run rough.  Added fuel stablizer, drove car - problem alleviated.  I only use Sunoco 94 octane.
        This feels like the same problem, but the remedy is not working.
        The entire ignition system has less than 1500 miles on it.  I'm going to change the fuel filter first.  If this doesn't work I'll regap points.  Any suggestions, similar experiences?
Thanks,
-jakl

[jeffc280sl]

Seems like the experts always start with the spark plugs.  What do they look like?



Jeff C.
1970 280SL 4-speed

[jakl]

Jeff, many thanks.  If not for my stubbornness this problem would have been remedied months ago.  Your simple suggestion proved correct.  The spark plugs had heavy carbon build-up after only 1500 miles.  I cleaned and regapped.  Car runs fine.  Thanks again.
Regards,
-jakl

[Naj ✝︎]

jakl,
This may be a temporary fix!
Why did the plugs carbon up? Plug heat range incorrect or mixture too rich, could be a choked up air filter ?
naj

65 230SL
68 280SL

[mdsalemi]

I had a similar problem and similar solution, just last week.  New plugs did the trick.  On mine, the electrodes looked great, the carbon buildup was around the base.  Italian tuneups do wonders, too!  Let's see whether the NGK BP6ES last longer then the Bosch...

Reminds me of an old repair manual I once had when I spent my time fooling around with the ORIGINAL Mini-Coopers in the 1970's...the book said that "Carburetor" is a French word that translates to "It's the electrical system, stupid!"

Now, nobody take offense!  It's just a funny old book, and how many times have we suspected something else (like the fuel system) and it has been something electrical?  Of course the book was written with "Lucas, Prince of Darkness" in mind, but you get the idea!

Mike Salemi

[jakl]

Naj, I agree it's temporary.  Part of the problem is a fast idle (1200 rpms).  You may remember, this is the car with the incorrect distributor.  I've since purchased a used correct replacement from SL Tech.  I wanted to correct this problem before swapping the distributor.  I'll also try a hotter plug.  I currently use Bosch W7DC - I'll try W9DC.
     Also, is there a way to check the ballast resistors?  They look very old.  There are two, I believe 0.4 and 0.6.  I have a transistorized ignition - '70 280 sl US.
Thanks for your help.
Cheers,
-jakl

[Naj ✝︎]

Hi, jakl,
I go by the assumption that either they work (have continuity) or they don't. Early ones used to be open to the elements at the back and prone to rust but late ones are fully encapsulated so no chance of rust. I have measured the resistance on the 0.9 and 1.8 ohm resistors but without load, the reading bears no resemblance to the quoted rating.
naj

65 230SL
68 280SL