When the engine is idling low the fuel flow used compared to full power is only approx 15%. Therefore most of the fuel that is sent by the pump will be returned to the fuel tank. When the engine is tuned the optimum efficiency can be obtained with the optimum mixture of A/F ratio. When the fuel pump is too large and the regulator is not strong enough to handle the excess fuel that it is receiving it will then not be able to regulate the flo to allow the fuel to return to the tank. This will cause the fuel pressure to be too high and too much fuel to be pumped into the engine which will lead to over enrichment and poor fuel efficiency and a variety of problems will occur.