I imagine all the fuel is evaporating and/or siphoning back to the tank during the lengthy no-run periods. You probably have to draw fuel all the way from the tank and fill up the carb bowl every few months when you start it, which is quite a bit more than 'several times' with the hand lever.
An electric pump would save you the time of refilling all the lines and bowl by cranking or hand-priming. I would think there'd be a check valve somewhere in the system, but there may not be, so you might consider installing one before the mechanical pump. Either that or go out and start it every few weeks.
In electric fuel pumps, I prefer the Facet cylindrical pumps. That's what the 2.6L 109's use for a boost pump. Lots of aviation and marine applications for these pumps as well. Mine's been going for almost 40 years now.
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).