Merc outboard runs fine on rear fuel tank but starves of fuel at idle from front fuel tank

[FONT=&quot]I have a 2000 200 HP merc efi and my boat has 2 internal tanks. I call them the front and rear tanks. I have developed an issue with the front tank. When running on the rear tank all is well. It will continue to run at any RPM for as long as there is gas.[/FONT]
[FONT=&quot]On the front tank it will starve of fuel after 5 minutes at idle but will run fine at 4000 RPM's.[/FONT]
[FONT=&quot]It has once stalled on me at 3200 RPM but rarely. The symptoms are as follows[/FONT]
[FONT=&quot]Running fine(5 minutes)[/FONT]
[FONT=&quot]Starts to slow down(2-3 seconds)[/FONT]
[FONT=&quot]Dies and will not restart until I pump up the fuel bulb until tight.[/FONT]
[FONT=&quot]Then it will happily idle for 5 minutes then die again.[/FONT]
[FONT=&quot]Switching back to the rear tank after pumping up the fuel again I have left it idle for 15 minutes without it starving. Switching to the front tank and it will die again after 5 minutes after I have pumped the fuel bulb tight.[/FONT]
[FONT=&quot]The tanks are run through a 2 tank rotary fuel switch that feels like it is new even though it is[/FONT]
[FONT=&quot]as old as the boat(21 years). It is a wood/epoxy center console that my wife and I built.[/FONT]
[FONT=&quot]I am looking for suggestions as to what may cause it to run on the front tank at high RPMs but starve of fuel at low RPMs.
[/FONT]I have made sure the vent is not clogged.
The fuel is brand new non-ethanol mid grade in the failing tank.
I have tightened all the hose clamps. Two on each connection.
Is it possible the anti siphon valve is stuck at low RPMs and opens at high RPMs?
I have rigged up a way to bypass the 2 tank fuel valve but the weather has been rainy and I have not had a chance to try that.
[FONT=&quot]Thanks[/FONT]

I don't believe you need anti-syphon valves with an outboard so I would start by removing them.

If the issue recurs after that change, I'd be inclined to hook up a vacuum gauge at the fuel pump (with a Tee) and see what readings you get - both tanks, idle, mid, and high RPMs, under load.

If the vacuum is excessive, then you have a restriction (vs a leak)...and its possible the hose on the front tank has an internal separation and is causing the restriction. I've never seen it but believe it is possible to have a restriction that will impede fuel flow at low RPM but gets overcome by the fuel pump at higher RPM...

Well, I bypassed the two tank valve and it idled for 20 minutes. So I think that is the problem. I am going to replace the valve and all my fuel lines just to be safe.
I will report back after all that.

did you change out the suspect valve?

Still working on it. Ya it is taking longer than I wanted. Should be able to test it this week.

ok I tested and all is well after replacing the valve and all my fuel lines it idles on either tank for as long as there is fuel.