Where can I buy the CDI for a Yamaha zeal 250cc

the crankshaft position sensor may be out remove and test
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If you register here you can download the manual...
250cc motorcycle forum

yamaha dealer or ebay, if not there try the yamaha owners clubs

carb needs cleanin or ajust--buy a clymer manual for all procedures

the function of your bikes's battery is signal light's and horn only i guess you already aware of it, other than that your electricity comes from your coil, the coil supply all from CDI to headlight taillight and park light, the CDI supply electricity from Spark, i suggest you check your CDI first and then Primary coil one of them is in bad condition, hope this help

starter brushes could be worn out or a dirty or loose connection!
Starter button may need checking also the solenoid
week compressin or leaking diaphrams in carbs will cause it too
I suggest you charge battery & start at the button & work your way back

The decompression valve has come apart in the engine. Tear it down!