OS 4 stroke FS91S comes either pumped or not pumped. What is the advantage/disadvantage of either?
Laurence

Pumped if you have to mount the tank far away from the engine or way below the carb.
I have a non pumped OS .91 and have never had a problem with it, runs like a dream. (I even had the muffler come off in flight and the engine kept running and I landed under power, so most models won't need a pumped engine if a non pumped pulls fuel like it does)

I have a OS 91 FS pumped on a balsa USA ΒΌ cub. The only advantage I have noticed is the extremely low and reliable idle.

pumped systems have a bad reputation for not lasting long.. at least at a couple of my local clubs.. they have 2 yr.+ pumped engines and they do not pump any longer.

I have 1 pumped OS91fs, and 3 non pumped OS91fs other than the 100$ difference the performance doesn't seem to differ.
there all very reliable and smooth engines with plenty of power,I haven't run my pumped 91 long enough to tell you about endurance of the pump,from what I have been told to do is run the engine dry so fuel doesn't clog the pump and they last quite a while with no trouble when they get gummed up they break down,the diaphram gets stuck and you need to have a tech.take it apart and reassemble it properly after its been cleaned or they will not work correctly.