I have a 18.5hp twin II briggs & stratton it will run but dies in 20 minutes need help ?

Sounds like you are getting too much back pressure. Change or remove the air box and run it without a filter. Could also be a clog in the exhaust. Too much pressure will keep the flywheel from spinning fast enough to start. Remove spark plug and turn key to blow out built up fuel and pressure. reinstall and try to start again.