The 791-682039 Fuel Filter and Line, filters the fuel in the tank as it is forced into the carburetor up by means of the primer bulb.

To install.
Remove retainer ring from fuel line.
Insert fuel line thru gas cap opening.
Thread fuel line thru hole in bottom of the fuel tank.
Pull fuel line thru to firmly seat against inside tank wall.
Put retainer ring on fuel line,hold line and press ring firmly against the out side of the fuel tank wall.
Then attach fuel line to correct line on the carburetor.

The filter will at sometime get clogged with dirt and debris that has collected in the fuel tank, and will have to be replaced.

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.

The purpose of the 791-181086 Fuel Return Tube is to return the excess fuel back into the fuel tank, after the primer bulb has been pressed and has primed the carburetor to start.

After time the Fuel Return Line will become brittle and crack and will need to be replaced.

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.