MOGAS Fuel

Another thing. I really appreciate that Service Ceiling chart, showing at what altitude and temperature the various fuels can vaporize. It's a very informative and helpful chart. However, that's based on ambient pressure at that given altitude and temperature, right? My thought is that, in the fuel system of a low-wing airplane, the fuel on the suction side of the Rotax pumps is being subjected to far lower than ambient pressure, meaning it can vaporize at temperatures far below those in the chart. Am I wrong in that understanding?

You are correct!
The fuel will vaporize at the "Pressure Altitude" it experiences within the fuel line.
The chart is rounded down aggressively to make it very conservative.
A very crude rule of thumb is that every foot of suction head before the fuel pump is equivalent to 1000 ft of altitude.
It is always recommended to install an AUX Electric Fuel pump as low as possible in aircraft with low fuel tanks.
If you can arrange for the fuel to flow DOWN to the Aux Pump,  this can prevent any part of the fuel hoses from being below ambient pressures.

For those that can't find other postings of the Chart, here it is again.

Thanks. I do have an aux boost pump before the Rotax pumps, but I still had that vapor/low pressure issue that one time. I thought about a design where the AUX/boost pump was right at the fuel tank, so it would always be under positive pressure. But, you’d need one on each tank, and a mechanism to switch one on when the corresponding tank is selected and off when it’s deselected. Quickly gets more complicated than it’s worth. Easier to just use AvGas. 

One thing you can do to know the pressure drop at the pump inlet is to fit up a tee in the line right at the pump and attach a fuel pressure/vacuum gauge. You can get one for about $20, and it’s best to get one with a low scale like -5 to +5 because it will be more accurate in the range you are dealing with.  I did this on my aircraft and found a 1/2 HG drop with one pump running and 1 HG drop with both pumps running (duplex pump on 912iS). Since 1 HG = 1000’ of altitude, I guess I should have just used Bill’s rule of thumb!  But, my aircraft is a Bristell, which has the pump in a compartment below the fuselage (level with fuel tanks) so there is virtually no elevation change to creat a suction head.  But even so, there is some suction created, at least with the injected engine pumps which move a lot of fuel.   

It is a very crude rule of thumb.
Even with No elevation change, there will still be resistive losses.
The 912is pumps move 1/2 gallon per minute at all times.
That is about 1.5 ft/sec in a 3/8" fuel line.
It takes some pressure to keep it moving. 
1" hg = ~1/2 psi =  ~900ft at MSL.  Close enough!

It would appear that the Aux pump is not able to deliver the 30 gal/Hr the Injector pumps need.