OK...I'm really unable to visualize the complete plumbing and reason for this observed behavior.  Help me out, and don't judge me harshly.

When I prime the engine prior to cold start, and a really cold start might require 12-15 sec of prime, I can observe (if I were outside looking under the nose) fuel exiting the overflow port.  Why?

This engine is the Lancair Fast Built, so I don't feel I understand as thoroughly as other systems.  The primer switch turns on the boost pump and also opens a solenoid that bypasses the mixture control valve, allowing direct access of pressurized fuel to the fuel spider on the engine's top.  Regardless of whether the mixture control in OPEN or CLOSED, the results are the same, and the fuel flow sensor (located after the mixture valve, before the spider) indicates 2.2 to 15 GPH flow into the spider (longer prime, higher fuel flow).

Is there no pressure relief valve on the overflow line?

Is there any way to prime but not have fuel outported to the ground under the starting engine?  It really needs a longer prime in cold weather, so a shorter prime is not the answer.

Maybe I'm just dense on this subject.

Jeff Liegner
LIVP