Zero Turn Mowers

i am working on an RZ4824 with a B&S endurance engine. recently it began to surge, idling up and down. manipiulating the choke helped get it back to the garage. so i checked fuel flow from the pump, good. then the spark at each plug, good. added some octane boost to the fuel, seemed to help. (though it does get used every week, as in not winterized) the problem is when it has been operating for more than say 20 min it begins to surge again. i loosened the fuel cap, no change. then i blew into the vent hose from the tank to the engine and the idle steadied.

this leads me to believe the fuel pressure regulation is amiss. however i have found on most small engines a surging idle is more common a coil issue.

so is there a most usual suspect on this model?