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?   If system is functioning right it would seem to me to indicate over priming.  I don't use the "primer" switch.  I turn on pump high boost and watch the fuel rate.  When it stabilizes, I run the pump two more seconds.  This puts fuel in the intake ports and produces excellent cold starts. Over priming is serious and if extreme could cause a hydraulic lock in the engine by putting to much fuel into a cylinder with an open intake valve. This can cause serious engine damage.  I have seen a bent connecting rod as a result.

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.  (no it by-passes the spider or main injectors and injects fuel at the "start injector" or "priming nozzle" only)

 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).  This should not be so.  If the mixture is at idle cut off, fuel should not go to the priming nozzle or the spider. 

Is there no pressure relief valve on the overflow line?  sniffle valves. From Skyranch web site: "Sniffle valve is a fuel purge valve that is used in the induction system and is only open when the engine is shut down. With engine shut down, valve should be open to allow fuel to drain overboard. With engine operating, valve should suck closed. If the valve fails to close, mixture problems will occur, especially in the lower rpm ranges. Excessive lean mixtures at low engine speed will result in rough and erratic idle performance. You can check the valve by placing your finger over the drain tube while the engine is idling; the valve is bad if you feel suction on your finger or the engine performance improves."

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.  Cold starting a TSIO550 should be pretty easy.  I suspect something is amiss in the fuel system. Once the engine will run at a stable idle, from full rich when the mixture is slowly pulled out, the engine RPM should slowly increase 50-100 RPM.  Then as you continue to pull the mixture knob the RPM should start decreasing.  You are going from rich of best idle to lean of best idle.  I idle the engine 50 or so RPM lean of best idle.

Maybe I'm just dense on this subject.

Jeff Liegner
LIVP