ORIGINAL: vertical grimmace
I absolutely loved my Goldberg ARF but dumb thumbed it in. I was planning to put a Saito 125 in it before the Stacking. It had an OS .91 FS.
As for the engine, OS is expesive and not worth it. I favor Saito. Bolt it up run the fule lines and go. My OS .91 with the pump is a Pain in the ***. It is a good thing I did not pay pay for that engine fully as it is really not worth it. With the Saito engines they are simple, not a lot of plumbing.
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Not much plumbing with the OS FS-91 either, if you don't buy the one with the pump.
Lots of folks don't read the instructions that come with the pumped engines. They are necessary reading.
Saito isn't one whit better than OS. I own, buy and fly both brands and am not prejudiced one way or the other. Nor is OS one whit better than Saito. Both have advantages and disadvantages. While the Saito engines are generally lighter, I have found the OS four-strokes to be a little less demanding and longer lasting. Remember, though, I am dealing with small samples of each engine, so that may not be the case when comparing a lot of samples of each engine.
Pumped engines, from anyone, have a reputation for being overly complicated and requiring more maintenance over time (usually the pumps give out). Folks should read the instruction manual for pumped engines before plunking down their money. Most manuals are downloadable these days via the internet.
I try to avoid pumped engines most of the time, but sometimes you really need the pump for specific jobs. Buying used pumped engines is something I usually avoid too, although I have made one recent exception (OS FS-120 Surpass II/P). I'm putting the latter engine in my Goldberg Ultimate ARF - if I ever get around to it. <G>
Saito engines are sweet, no doubt, but so are OS, Enya and YS.
Ed Cregger