Generally a rebuild in the past was for leaking gaskets, and often for the accelerator pump losing it shot ability about 10 yrs max for a daily driver...but with today's gas being highly evaporate, less use of our vehicles, it seems 5 yrs is the max you might want to let a carb go before it needs just a general overhaul and cleansing to freshen gaskets and hardened/worn rubber components along with deposits