I have a 1998 CR 250 Just replaced the cyc head , jug along with piston and reeds. Starts and rund just fine till it warms up and it start to cut in and out and the kill swithch will not work. the funny thing is when the bike is cold the kill swith will work fine. Could I have a problem with the coil?

no compression
has the spark plug come loose,
could be bust piston, rings or valves,
right googled it, its a 2 stroke no valves,
moving on, your have to remove the head and barrell to see whats gone, the piston or the rings. its probably the rings.
its not as hard as it sounds on a 2 stroke so even a novice could do it.

T is usually the mark on the flywheel and there's usually a mark on the cam sprocket and head to line up. you can google up a service manual too most likely.

Just cause the piston looks fine dosent mean the rings are still good did you do a compression test if you don't have one buy one they are great to have with 2 stroke bikes put the cylinder head back on and cover spark plug hole with finger it should first **** a bit then should blow finger off the hole the compression should be greater than 120 psi of a 125 not sure on the 250 rule of thumb if you have less than 120 your piston or rings is pooched..

Is the choke stuck in the choke (on) position?

If the bike has been seized up it's important to check out the piston and cylinder head. It's a good learning experience and not that hard a project. Remove the seat, tank, pipe and maybe the carburetor and you should be able to get at the head bolts. Pull the head and inspect the top of the piston and ring area. Rings usually get stuck to the piston so there is not any compression. Pull the jug and more than likely there will be alloy from the piston transfered to the cylinder heard.
Take it to a bike shop or automotive machine shop that can hone it to first over. Purchase new 1st over-sized piston and matching rings. Re-assembly and replace all gaskets.
Thanks for contacting FixYa,
TF

You need to remove the fuel tank and then the engine. You then remove the top head by removing the four bolts holding the head and cylinder jug on. slide the jug upwards off of the piston. To remove the piston you will need to separate your case

yes you hit the nail right on the head there,, its the tem sensor playing you up, its telling the car's computer its warm when its still cold and it cuts back the fule like pushing in the choke,,, this causes the engine to miss fire and lack power till its warm change the sensor

overheating try thermostat

Compression is too low!

Replace the piston rings.

Also check continuity from engine to frame till high-tension coil...
Check if breaker points aren't dirty ( clean out with fine sandpaper )
Check breakerpoint settings

When you rebuilt it, I would assume that you replaced the piston rings. Were they installed correctly? Assuming they were, were the valves cleaned and the valve seals replaced? You obviously are getting oil into the cylinder(s) somehow, and these are usually the main culprits. Unless the head gasket has developed a leak.