The blue port is the inlet coming from fuel channel connecting the fuel pump to the metering section and will be under fuel pump pressure when the saw is running.
The red port leads to a passage downstream from the venturi that directs the output from the primer into the intake of the engine. There is a check valve (ball and spring) in that port to ensure the output from the primer is discharged and not simply drawn back into the primer when the spring returns the plunger. This is a true primer that puts a charge of raw fuel directly into the intake.
The body of the primer provide enough resistance to prevent the fuel pump from forcing fuel past the check valve into the engine which would upset the mixture, or leaking past the primer when the engine is stopped which would lead to flooding while setting. Parker had this experience during his early modifications which lead to the search for the higher pressure spring for the ball check valve.
I think the issues with the modern bulb type "primers" is they would not have the needed resistance and would allow fuel from the pumping section to be forced directly into the intake, and the amount of fuel that is displaced by the bulb type primers is so great that one actual discharge cycle would badly flood the engine.
This is all speculation on my part...
Mark
The red port leads to a passage downstream from the venturi that directs the output from the primer into the intake of the engine. There is a check valve (ball and spring) in that port to ensure the output from the primer is discharged and not simply drawn back into the primer when the spring returns the plunger. This is a true primer that puts a charge of raw fuel directly into the intake.
The body of the primer provide enough resistance to prevent the fuel pump from forcing fuel past the check valve into the engine which would upset the mixture, or leaking past the primer when the engine is stopped which would lead to flooding while setting. Parker had this experience during his early modifications which lead to the search for the higher pressure spring for the ball check valve.
I think the issues with the modern bulb type "primers" is they would not have the needed resistance and would allow fuel from the pumping section to be forced directly into the intake, and the amount of fuel that is displaced by the bulb type primers is so great that one actual discharge cycle would badly flood the engine.
This is all speculation on my part...
Mark