ok i was out at glamis last weekend and my bike was running fine i went for a ride stoped for a break and bike would not start again put in a new spark plug checked wire connections and now there is no spark.
My brother has the same bike so we changed out some parts the cdi box the ignition coil and it still has weak or no spark. what could be wrong.

its a 1986 250r

make sure your on off switch is ok, i know it sounds stupid, but i had a 88 trx kick my ass, and that what it ended up being

CHECK ALL YOUR CONNECTION FROM THE CDI BOX TO YOUR MAIN HARNESS
THEY SOMETIMES LOOK THEY ARE CONNECTED BUT ACTUALLY AREN'T. THAT HAPPENED TO MINE ONCE AND THAT WAS MY PROBLEM

Is the cap staying in place? My friend has always had aa problem with his 86 R cuz the spark plug cap keeps coming loose no matter what he does. The only thing htat has worked is to keep it taped down there

stator, samething happened to two buddies of mine.

I'll second Mackmans advise..... Check the connections very carefully
Then check the output of the stater with a voltimeter.

With old bikes like the 250R's the wiring connections get corroded for being in the elements. Pull all your contact points and clean them good then use WD-40 or a electrical grease when reconnecting. Pull your switch assembly off the handle bars and take it apart and clean that up too. Make sure you get to all the contact points.

If that don't work the it's likely your stator is starting to go south on you...

Agree with the conections check them and all the ground wires also. Had a problem with both before on a old trike.

I have had this type of problems before with older atc's. What I had was water in the side covers.just a little puddle on the bottom, but, when the bike got hot, I am assuming it turned to vapor, which was then activating the rust on the stator, which was then, grounding out the electrical/weak spark. I pulled the cover, sprayed the insides with wd-40, and was back in action. The only way to find your issues is with a voltmeter, otherwise, your just blindly turning wrenches, hoping to "accidently" come across the problem...