Suzuki: 1984 Suzuki 125R quits running after 4 hours of riding, petcock, dirt bike

Question
I have a 1984 Suzuki 125DR, I have been out on it twice and both times it will die while I am riding afer about 4 hours.  If I let it sit for about 10 minutes I can start it again and go for about 10-15 minutes and then it dies again.  It even backfired a couple of times.  I thought I was fouling out the spark plugs but I put in a new plug on the trail and the same thing happens.  Please any advice helps.  Thanks!

Answer
If it backfired, that would be a glaring symptom of being lean.  I suggest you drain the tank, remove the petcock, and check the filter screen at the petcock.  Does it have and inline filter, and when it dies, is there good fuel flow to the carb.

I don't usually answer bike questions, given that my dirt bike is a 69 Ossa trials bike.