Only 3 things required for the motor to run:

Air

Fuel

Spark

If you have all three, then the bike should run!

As Dave says, clean the plugs (make sure they are nice and dry), and while they are out, kick the motor over a few times. If the motor's not been started for 18 months, try spraying a little carb cleaner down the carbs, just to help it along a little.

I instantly thought the same as Dave, fouled plugs!

Andy.
NSR-WORLD.COM