Something that's never sat right with me is that the early style tank has no provisions for capturing fuel, like a swirl pot, sump, etc.
I did spend a lot of time researching various options like adding an in-tank pump, etc. but decided to give it a go with the factory setup.
The problem I envisioned was that if the pump starved for fuel, it would be a problem. Carburetors are a bit tolerant of this since there's a small amount of fuel in the float bowl but I couldn't envision how the MFI system would deal with this.
I tilted my (not yet secured) test rig fuel cell so the pump would start to suck air and to my surprise the resulting bubbles went through the pump, to the filter console, and back to the fuel cell. But were NOT delivered to the MFI pump head. Even when startved to the point that the fuel lines were totally frothy, the fuel delivered to the pump head was clear and air free, as seen in the pic below:
So finally a light went off, that the filter console serves the secondary purpose of removing air from the fuel supply! As near as I can tell it works like this:
- Fuel (with air bubbles) is delivered to the filter console and first must go through the filter in a downwards direction.
- Due to the greater cross sectional area of the filter, the velocity going downwards is going to be very low, so much so that the bubbles will float to the top of the housing.
- There is a screen in the filter console that connects the cavities between the filter and the fuel return path - basically the fuel enters the housing, makes a 90 degree turn, then exits through the pressure regulating fitting and back to the tank.
- The differential pressure across that screen is equal to the pressure drop across the MFI fuel pump head, and the hoses and fittings it goes through to get there. This pressure will drive fluid flow through the screen at all times but it's a small amount compared to the total flow.
- One of the banjo fittings (above) has a built-in restrictor, presumably to "boost" the DP/flow through the screen. I always looked at it and wondered why. And indeed wondered if it was installed on the right port of the console.
- Should air become present at the screen, it will pass through much more easily than the fuel. and is swept away and back to the tank.
It's actually a pretty brilliant little bit of engineering! So many times I considered getting rid of it in order to simplify the system, but I now know that would be a bad move.
OK, I have an MFI pump to calibrate!!