Lawn mower will not stay revved up while mowing

Ten year old mower. Always started good runs good. Well maintained and stored inside. Fuel stabilizer etc. Yesterday it started this problem of not staying wide open throttle while mowing. You have to keep pushing the throttle open to keep it revved up. Something has changed on the cable or carb or something. But I can’t figure out what it could be. Help? My grass is growing Ineed it repaired