Then there is something wrong with your aircraft.
When we do C of A air tests it is a requirement to time how long it takes for the engine to stop using 1200 rpm with the fuel selected Off.
From memory if it is more than 2 minutes then it is a reportable defect.
In that case I would say that most (possibly all) carbureted PA28s will fail a C of A air test and be deemed unairworthy.
