I've recently took a 1985 Big Red 250cc and got it going after being idle for 6 years. Believe it or not I just changed the oil and spark plug, repaired the rusty gas tank, stuck in a new battery and it started right up.
It will run great for about 10 - 15 minutes and then it just shuts off and will not start again untill I remove the fuel cap, and it starts up again no problem and runs very well for another 10 minutes or so and then it repeats.
Could this be problem be the tank is getting air locked or something?
Any ideas would be appreciated.

 

Replace the gas cap. There is a vent on the cap and it is probably clogged leave the cap off and see how long it will run.

 

Agreed, pick up a new cap at Honda One, they're not expensive and are usually in stock. If you join newfoundlandmotorsports.com and contact terry through the message forum and you'll get a 15% discount too.

 

Before buying a gas cap, make sure you dont have rust in the carb filter, rust will sit on the botom of the filter screen, after runing for a few minutes the rust bounces around the filter, clogging the gas flow. Also besure your carb is not slimed from old gas just sitting around.