Motorcycle Repair: honda sl90, vacuum type, piston rings

Question
i have a honda sl90,the only problem of the engine is that the spark plug turn into black & dry in color.the piston,piston rings,valve seal are new.the mechanic adjustd the carb but its the same the spark plug turns into black

Answer
Hello Pablo,

Make sure your air filter is not restricting air.
It may need a new filter.

Check that the cutaway part on the carb slide
bottom is facing the air filter.

Use the correct heat range spark plug, not too cold.

Check the compression and check ignition timing.
The engine may be running too cold.

Check exhaust pipe for obstructions. The burnt fuel has
to be able to get out.

Check fuel petcock if vacuum type. (two hoses)
Some can draw fuel from the tank through the fuel
valve.

The dry black spark plug means one of two
things most likely:

too much fuel or too little air intake.

You may have to use a smaller main jet
in the carb to get the plug color brown.
That is the center jet.

Always do a fast run and shut the engine
off, check the plug color.
If it is black try a smaller main jet
until it reads brown.
Don't go too small or the engine will overheat.

Always check the air filter, timing ect
before changing any carb jets.
Your float level in the carb must not
be too high or the carb may flood the engine
with gas. It should just keep the
carb bowl full, no more.

Make sure the choke is off, lever down.

Good luck!