Spark Plug Coloration Analysis 230SL - Update

[mistertj]

Says RO for sure.

[Benz Dr.]

I the world of spark plugs, I'm going to say that heat range beats out resistor.  "R" is resistor for sure and 7 is too cold for all but the best engines.

[Pawel66]

What should it be then, if not 7? 5? For moderate climate.

[mistertj]

Status update: I re-gapped the points and moved the timing slightly...maybe about 5° less advanced @ 1500rpm. So right not it's at about 16 - 17° BTDC. Ordered the NGK BP5ES plugs and in the meantime put in a new distributor cap and new wires not running them through the wire holder. No joy! Still have slight bucking and exhaust minor popping at low speeds. Did the split linkage test. No rpm change when increasing pump throttle but a slight increase in rpms when I slightly open the air intake butterfly linkage. Anything more than "slight' opening, the engine dies.

[Benz Dr.]

What should it be then, if not 7? 5? For moderate climate.

I use BP5ES in just about everything I work on. If they look too lean I add a bit of fuel at the BC because that's what the engine is calling for. A nice tan brown to slightly white is what you should be looking for.  If you look at the three plugs in this example you will see that number one is clearly dead. The other two appear to be running at close to the right air/fuel ratios but are too cold for that application.

YMMV

[Pawel66]

5 it is then, thank you.

[mbzse]

Those numbers in the spark plug designations can be confusing.
They differ between brands.
Meaning, "hotter" can be higher or lower number in the scale of each plug designation

I use NGK BP6ES or BP7ES myself in my 280 SL

[Benz Dr.]

Those numbers in the spark plug designations can be confusing.
They differ between brands.
Meaning, "hotter" can be higher or lower number in the scale of each plug designation

I use NGK BP6ES or BP7ES myself in my 280 SL

You're right about the number designations. However 7's in Bosch and NGK are close to the same heat range and 5's in NGK are similar to 9's in Bosch. I will only use nickle plated plugs in aluminum heads with a bit of anti seize on the threads. I've found that zinc or black oxide tends to pull threads.

[Pawel66]

I do not want to steal the thread, it is just confusing and thank you for patience and advise to all of you. Parts lists are saying W5 or W6/W7 (US if I read it right) from Bosch, and I am hearing I should use W9, actually, colder ones.

[mistertj]

Got the NGK BP5ES plugs but there is a problem. I ended up with 2140 BP5ES plugs that have a non removable top. Does anyone know the number for the plugs that have a stud top? I looked at the 7832's and those have a screw top but the stud is short.