ford 3000 runs about 4 hrs then shuts down, why?

i have a 1969 ford 3000. it has an up draft carb. that has been refab. the tractor will run about 3 to 4 hrs then it will back fire thur carb shuts down and will not start up agin for another 4 to 6hrs. while in this shut down gas drips from under the throat of the cab. replaced the up lift tube that goes from the dist. to the intake manafold above carb. when the tractor is started runs fine can get up to 2100 rpm smoothly