pump failure
Mark,
There are too many fuel starvation incidents. So, wanting reliable fuel delivery and asking questions seems not dumb, but quite smart.
My first inclination would be to do whatever Van said to do. If you don't have a set of early plans, track one down and see what you can find on the topic. While that is what I would do personally, I'd wouldn't pass up the chance to think it through, reverse engineer, and make sure I understand the concern.
So, before deciding we need a second pump, lets back up and ask another question. What is the common failure mode for the pump you do have? Answer- ruptured diaphragm. However, these pumps have two diaphragms. There should be a small drain hose coming down from the pump, and the end should be somewhere that would cause a noticeable drip on the floor or ramp- should there be anything coming out of that hose. The hose evacuates a space between the two diaphrams. If the diaphram next to the fuel fails, then you get fuel out the hose. Diaphragm on the engine side leaks, they you get oil. (There is a restrictive orfice so you don't empty your fuel tanks or pump lots of oil out this hose).
During pre or post flight, these drips should be noticed and the pump then replaced before total failure.
Does having two diaphragms give full redundancy? No not really, but it prevents fuel starvation following a single diaphragm failure.
What about gravity feed? Someone did a real nice thread; a picture dissassembly of the mechanical fuel pump. I've searched the forums, but can't seem to find it. If I recall correctly, there are at least two check valves which may cause difficulty in actually getting fuel to gravity through the pump. The flow may not happen with the small difference in height, there may not be enough head pressure. One could certainly do some experimentation. With the mechanical pump not moving, drain the carb bowl and see if it re-fills from the fuel tank (I don't know you well enough to leave it at that, so please do this safely, get help..... don't get hurt, have a fire....) Honestly, though I don't know what the chances are of a fuel pump not moving when it should. That would be a topic worth researching. Have you discovered the great search tools? There are years of experience, reports of what works and what doesn't.... all available via a little searching.
I've got wing tanks and both mechanical and electric pumps. Design wise, the exact thought process for having the electric is what? Was the intent was to push fuel through a failed mechanical pump, or is the electric there completely for minimizing vapor lock problems and pushing the fuel uphill. I'm leaning more toward the second, but honestly I don't know for sure. Hopefully, someone with a fuselage tank will chime in here on what they have for pump(s). Or, you could do a search for it if you haven't already.
Mark, you'll have to tell us how your getting along with your new to you bird. I saw that you found it last summer.
