The primer hose is inserted into the fuel tank then connected to the Primer.
By pressing on the primmer a few times this assembly fills the carburetor with fuel.
When the carburetor has enough fuel in it, the excess fuel is forced back into the the tank through the fuel return line.

After time the primer hose will become brittle and crack and have to be replaced.
The primer will also lose its elasticity, and may also crack and not be able to suck up the fuel into the carburetor.
And will have to be replaced.

This is an OEM replacement part,it is sold individually.
The Bumper Head Knob is the part of the trimmer that when struck on the ground, advances the trimmer line in the spool.

To remove the Bumper Head Knob, thread it off the spindle of the gear box assembly.

This is a genuine OEM replacement part, it is sold individually.
Mulching blades have a more curved style surface and frequently include extra cutting surfaces along the blade edges.
This blade is designed for all the models listed below.
If used correctly with the mulching plug that is inserted into the mower, the clippings will stay under the deck longer and get re-cut.

A carburetor basically consists of an open pipe through which the air passes into the inlet manifold of the engine.
The pipe is in the form of a Venturi: it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part.
Below the Venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air.
This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints.

Fuel is introduced into the air stream through small holes at the narrowest part of the Venturi and at other places where pressure will be lowered when not running on full throttle.

This is a genuine OEM replacement part. it is sold individually.
The Bump Head Knob Assembly is the part of the trimmer that you bump on the ground to advance the cutting line from the spool.
Because of the abuse this part gets, it is advisable to order an extra one so you have on on hand.