You don't say just how much voltage you get while bike is running. Sounds like a battery that's going bad. Just enough to run but not enough to pull higher cranking current. Just to make sure, put a meter on it while bike is running ( about 1500 rpm or higher. ) Should be 13 - 14 volts with normal running loads. If lower, like 12 volts or lower, than I'd check the alternator output or voltage regulator. To check the battery for cranking power, put volt meter on battery and observe while cranking. Should not go much below 10 - 11 volts while cranking starter. Better if holds at 12 volts while cranking. If 10 volts or lower, the CDI probably will not put out, even if you have a good one. Also, it would be a good idea to disconnect the battery terminals and take a resistance reading from the Pos. battery lead ( not terminal ) to the bikes chassis. You should not have a short circuit reading. Make sure all switches are in OFF position. If you do get a low resistance reading, check for stuck light switches, shorts in wiring, bulb sockets shorted, etc. Here's a link to the NX's ignition section in the shop manual. http://powers31.info/1988_NX250_Ign_Sys.pdf May help you in trouble shooting.