my 1980 KZ250D is running rich (the new spark plug is very black after a week) how do I adjust the carb?

Question

it was running rich before the change, and it is the right plug.

Answer 1

Was it running rich before the plug change? Is it the right plug? Is it gaped correctly?

Is your choke working correctly?
Is your air filter clean?
The only ( adjustment ) you can do to the carb. is the fuel/air ratio screw. You can try turning it out 1/4 turn ,this will lean it a bit.
Be aware though this screw only affects the idle and very low speed ( Like up to 1/8 or 1/4 throttle).
If your bike ,for no apparent reason , began to run rich, you should be looking for the cause rather than trying to adjust it away.

Answer 2

The only reasons the fuel/air mixture would change, if no performance work was performed on the engine would be:
An air leak - allowing more air, causing it to run lean.
Choke stuck open - allowing more fuel, causing it to run rich.
Dirty air cleaner - restricting air, causing it to run rich.
Fouling (black) spark plugs are caused by low compression, due to worn piston & rings, allowing oil from the crankcase to enter the combustion chamber.
Check the cylinder compression.
To low and the bike needs top end repair.
Worn out valve guide seals.
If the compression is within specs, this would be the next likely problem.

Answer 3

You can change the main jet to a smaller one or make sure you are running the right heat range of plus. You can go to a hotter plug.