When using the single-vacuum distributor, the vacuum hose runs from the vacuum port on the left side of the carburetor to the vacuum canister on the distributor. In that case, the vacuum port on the rear of the carburetor must be plugged.
For most cool season grasses and for folks living in northern climates, the best time to plant grass is in late August, early September. Depending on exactly the climate you have and the type of grass seed you're planting.
The timing of pre-emergent herbicide application for crabgrass is crucial. Crabgrass seed will only germinate after soil temperatures reach 50 to 60 degrees Fahrenheit for several consecutive days. There is some disagreement in the specific temperature and time period depending on the information source used.