Spark plug help

[jstuart]

Greetings to all!  

After reviewing past posts I'm confused.  I have a 69 280sl that started missing after leaving the upholstery shop with a new soft top, seat covers w/pads and new carpet.  I bought a set of plugs for the first step, WR7DC, are these the correct ones?  After reading some of the posts I'm not so sure.  The car show's 89K on the odometer...could be 289K for all I know, has the automatic trans and AC.  Tail pipes are showing a lot of soot.

Any recomendations will be appreciated.

Jay

1969 280SL

[Cees Klumper]

The WR7DC plugs will be resistor plugs, and you want the non-resistor kind (W7DC). Here's an old post from Dan Caron (Benz Dr) on the topic: "The Bosch recommended plug for almost every older car is W7DC but I often use the W9DC if the engine runs a bit rich. ... I've been using W9DC for years with no problems. The 7's are the recommended one's to begin with. ... Hotter plugs do in fact run at higher temperatures. The cylinder temperatures are transferred on to the spark plug and depending on how far the porcelain goes down into the plug will give you hotter or colder. Hotter plugs warm up faster and stay that way so that anything that forms on them is burned off. Plug gap is also important. I've seen manuals that say .024 or lower - don't do that. 032 or even a bit more is where I like them. Make sure you have a good working coil and no resistance in the plug wires. Terminal ends at the plugs only should be 1K ohms and no more."

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[jstuart]

Thank you for the clarification Cees.

1969 280SL

[merrill]

Cees,
what is the thread size for the spark plug?

I went to check my plug thread size and the thread size for my compression tester and they both were 9/16 coarse ,  neither would go into the metric check a thread?

Both went into the  9/16 so I guess I can use my compression tester in my sl, just wanted to verify the thread size

Matt
66 230 sl - "white"
78 300 D - Blue
98 C230

[Benz Dr.]

Thread size is 14mm.
 I think it's 1.5 threads per mm.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061