This is a genuine Ruixing replacement part, it is sold individually.
By pressing the bulb, it will purge the carburetor of air.
Fuel will be forced up from the fuel tank through the fuel line then into the carburetor.
Then the excess fuel will be forced back into the fuel tank through the fuel return line.

At some time the primer will lose its elasticity or crack and need to be replaced.

We have an article and video on How to Replace a Primer Bulb, in our Repair Center. That can be found on the front page of our site.

The Ryobi 310917001 Spool W/Line is the part of the trimmer that rotates at a high rate of speed and releases the line that is used for trimming.
The Spool is inserted into the spool housing and is held in place by the 522994001 spool cap.

This is a genuine OEM replacement part, it is sold individually.
A spark plug is used to deliver electric current from an ignition system to an internal combustion engine.

Over time spark plugs will become dirty or fouled.
It is a good idea to replace the spark plug once this happens.
You will need to gap your spark plug according to the owners manual of the tool you have.

This is a genuine Ryobi Replacement part and is sold individually.
Switches are commonly replaced parts in power tools. This is because switches use mechanical internals, causing them to degrade and wear out over time.
A switch handles the on/off and variable speed functions.
They also have a role in electric breaking; they can momentarily reverse electrical polarity.

Some common signs of a broken or failing switch are:

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 Ryobi replacement part, it is sold individually.
Its purpose it to hold the spool with line into the string head assembly.

To remove the Spool Cap press on the two tabs that lock it into the string head.