Can the secondary air inlet of the intake manifold not be opened?

You can't. Secondary air intake is to use the vacuum pressure difference of the intake manifold to connect an air intake device with adjustable air intake from the PCV valve (crankcase forced ventilation) pipeline of the engine, and introduce a proper amount of fresh air to achieve the purpose of improving volumetric efficiency. For carburetor or throttle body gasoline injection engine, intake manifold refers to the intake pipe from the back of carburetor or throttle body to the front of cylinder head intake. Its function is to distribute the mixture from carburetor or throttle body to the air intake of each cylinder.