I have a 1985 250sx. Just rebuilt the carb and I rode it for about 30 min yesterday and it ran great. Today is another story. Started it up and let it idle for a couple minutes to warm up. Took off out of the garage and it stalled about 20 seconds later. Got it running again and rode about 4 or 5 minutes before it stalled again. Started it and it stalled about 5 seconds later. Started it again and it ran fine for another 5 min or so.

What could be the problem? I tried putting around on it AND riding it hard and I can't pinpoint when it will stall. Seems completely random to me.

 

Could be ignition failure. A spark tester between spark plug and the boot while running will verify spark problem. Check kill switch, wires, coils and stater. If everything checks good replace CDI.

 

Just in case anyone else has this problem, I thought I'd drop in and post what it was.

The vent on the gas cap wasn't working. Simply loosening the cap a little allowed it to vent and it would no longer stall.