ORIGINAL: bigtim
here is a couple of photos of my older pumped OS 120 spII it has a metal pump mounted on the back plate pre surpass III plastic pump version.
if you stick the starter on it for a few revolutions it will self prime pretty quick, OS recomends using a electric starter on there newer 4 strokes so priming isn't really a issue at all.
as for advantages the reason I decided to use it in this particular plane was to allow me to mount the tank on the CofG to balance it, and allow me to move the battery and reciever in the normal tank position behind the firewall.
this engine doesn't change in RPM no matter what angle its in upside down or vertical also the tank position like many have posted is never a issue.
pic #3 shows my vent tube mounted into the cowl flush with the outside, I just place a catch cup under it to pick up the overflow of fuel when filling up a big difference is the need to vent the tank with a pumper for fuel flow,for non pumped engines the fuel system needs to be closed for the muffler pressure to be affective pushing the fuel to the carb.
I have one of those older models too. It runs but not very well. I think someone opened up the pump at some point and it is just off some how. I have been using it as a parts engine for others, just screws and what ever I can use, it's a shame because when they run well they are very good engines. Nice to see one still in use.