This 21Hp Briggs V Twin tractor will run great for about 20 or 30 minutes then it stops getting gas, I can take the carburator side of the gas line loose from the fuel pump and crank it until gas shoots, replace the line and it will start and run again maybe another 5 minutes then quits again, what is wrong with thing, one repairman said it might be vapor locking but it don't seem to be to warm, got me stumpted. Thanks for any help