Honda XL185S maintenance

Last year I bought a second XL185S bike that was made in 1981.  In the process of fixing it up, I noticed that it cut out occasionally while driving. There didn't see to be any pattern.  Sometimes it would drive for a while, sometimes it would die in a few minutes.  If I waited about 5 minutes it would restart and drive again.  I cleaned the carburetor twice, made sure the gravity feed was working to the carburetor, and spent a lot of time troubleshooting the electrical system for a healthy spark, timing, etc.  Nothing would fix it. 

My friend came over and listened to the problems. Then he said, 'sometimes the tank doesn't breathe right' and checked the hose on the gas can cap. We determined that the vent valve seemed plugged.  I did a test and unhooked the hose from the carburetor and let gas drain out. After a while, it slowed to a trickle, presumably because the tank was going into a vacuum (the carb is gravity fed).  After drilling a hole in it to make sure air could vent into the tank the fuel ran freely until the tank was empty.  

I hooked everything up again and the bike worked great. I haven't had any cut-out issues since.  It's interesting how this would cause intermittent issues with the bike. During dropping temperatures, a vacuum in the tank would happen more quickly. If the tank was full, a vacuum could occur more frequently.  On a hot day, evaporation in the tank might exceed the drain rate of the tank.  Strange problem to work on.