Black smudgy spark plug

 Suzuki Boulevard S40 650 single:

Motor's been running a bit rough so I pull the plug and the contacts
are all over black furry stuff. Hate to think what the upper cyl
looks like. Actually the electrode is shiny porcelain white on one side
and all carboned up on the other. What's that all about. Anyway
I replaced the plug with a hotter one mentioned in the manual.

The gas tank has to come off to access the spark plug. Who's brainy
idea was that I wonder. To get the tank off, you have to lever it
up and sideways and backwards and kind of corkscrew it off. Just
about then a big washer fell out of somewhere. Couldn't see where
it came from so it's still out.

Now I'm gonna fire it up and see if it runs any better.

SQ - '06 S40

 
:Suzuki Boulevard S40 650 single:
:
:Motor's been running a bit rough so I pull the plug and the contacts
:are all over black furry stuff. Hate to think what the upper cyl
:looks like. Actually the electrode is shiny porcelain white on one side
:and all carboned up on the other. What's that all about. Anyway
:I replaced the plug with a hotter one mentioned in the manual.
:
:The gas tank has to come off to access the spark plug. Who's brainy
:idea was that I wonder. To get the tank off, you have to lever it
:up and sideways and backwards and kind of corkscrew it off. Just
:about then a big washer fell out of somewhere. Couldn't see where
:it came from so it's still out.
:
:Now I'm gonna fire it up and see if it runs any better.
:
:SQ - '06 S40

Four possibilities:

1. Your bike is running too rich.
2. You air filter may be clogged.
3. You have a problem causing oil to get into the cylinders.
4. The spark plug was old or improperly gapped.

There are other things, but one of those probably covers it.

Is that a carb or fuel injected bike?
If carb, it may be jetted improperly.
If EFI, someone may have modified the exhaust or air box, screwing
up the mixture or you may have an oxygen sensor that isn't
functioning properly.

Do a plug chop.
1. Put in a new plug, properly gapped.
2. Ride at 60 mph for about three miles.
3. Pull in the clutch and hit the kill switch and pull over.
4. If the plug is brown, it's OK. If black, it's too rich. If white,
it's too lean (main jet if it's a carb model).

Do the same thing at 25 mph to test the pilot jet (assuming carb).
___
Cogito Ergo Spam - I think therefore I ham

 

Normally that's a sign of a leaky valve guide...

Rich is sooty black all over, and old spark
plugs don't have pristine porcelain sides to them.
__
Steve
.

 
:Normally that's a sign of a leaky valve guide...
:
:Rich is sooty black all over, and old spark
:plugs don't have pristine porcelain sides to them.

Yup. That used to be quite common but not so much any more. A recent
Suzuki with a leaky valve guide is rare. I'd want to run a new plug
to see if I could duplicate the problem before believing it.
___
Cogito Ergo Spam - I think therefore I ham

 

Nowadays brown deposits on the spark plug mean you're adding an octane
booster to the gas, or you're using a little oil between changes.

But, back in the 1970's, when leaded gasoline was still available, we
did look for light tan lead deposits on the porcelin after running
hard.

However, this is the 21st century, and we're running unleaded gasoline
now.

The correctly-jetted/mapped engine with the right heat range plugs
will deposit a narrow ring of black soot deep inside the spark plug
shell, down where the porcelin meets the steel.

The knowledgeable tuner has a magnifying flashlight for looking down
in there.

If the ring of soot extends up near the tip of the spark plug, the
engine is running too rich.