I have a 92 250X that will start when it is cold but won't start when it gets warmed up or when its hot.
I've adjusted the valves (exhaust valve was too tight), checked compression which was ok, rebuilt the carb and cleaned it throughly. and I've tried different jets. none of this worked.
Could it be something electrical like the CDI box or the coil?
I'm at the end of my rope here please help.

 

Have you checked your spark when the quad is hot?

You can test the coil also when it is hot by reading resistance through the HT lead to the coil ground (8.8k - 14k with plug cap on, 5.8k - 7.2k with cap off [this will test condition of cap]), and primary (0.1 - 0.2 ohms, disconnect blk/y and grn wires from coil, HT wire from spk plug, and read through blk/y to grn wire on coil).

You also have an exciter coil (blk/red wire from "flywheel" cover, disconnect and resistance from exciter coil to ground should be 100-300 ohms), and a pulse generator (blue/y wire from "flywheel" cover, disconnect and resistance from pulse generator to ground should be 290-360 ohms).

It is a good chance it is your CDI causing the problem, but there are no "published" tests for the CDI on the 250X that I am aware of, and from personal experience, I don't trust the test results anyways due to so many false readings that either said the CDI was good or bad and in fact was just the opposite of the test results.