Random backfire through carb.

[ric812]

I have a 1979 GMC Sierra 15 with a 250 co 6 cyl.with a Rochester varajet ii. I have had this truck for three weeks now and have driven it twice to work which is over and hour away one way with no problem, as well as driving it locally almost daily during this time. This past weekend I drove it to work, an hour away, I ride a bus during the week, without any problems but halfway back it started backfiring through the carburetor so bad it blew off the vacuum lines. I attempted to drive it another 5 miles down the interstate while having to stop every 2 to 3 mies to replace 5he vacuum lines, in between it would run fine then backfire. I have replaced the spark plugs, wires, rotor, distributor cap, as well as completely rebuilding the carb and replacing the floats and making sure they were at the correct height. This is not a constant backfire , it only happens after driving a few minutes then I stop , shut it off , check everything and when it starts back up it runs fine for a few miles, highways speeds or in town after replacing everything.

I have searched the forums and could not really find anything like this. Please help I am completely lost and looking for a direction to go from here. My internet searches have stated to check everything I have replaced. I have not checked the timing yet.

Thank you for any assistance.