RE: OS 70 Surpass Ultimate
Amir, I will assume for the purposes of this post that you are referring to the OS .70 FS70 Ultimate (not Surpass) engine with the blue rocker cover and the pump.
I have worked on just one example. I started this engine for its first run for one of the guys at the field. For its first 15 minutes of runtime, this engine was almost impossible to dial in. It would run OK when started then when you tried to dial in the main needle for a rich full throttle run, it would approach peak then without any warning would drown itself with such a rich mixture, it would stop dead, completely flooded.
After restarting, I tried to redial the engine on several occasions and it was all over the place. Eventually, the owner was keen to get her in the air and I set a very rich mixture that seemed to be sustainable over time (in other words, it didn't drown itself out of the blue as it did in the earlier runs). Amazingly, once in the air, it leaned out some, and ran really well. Two flights later it went in hard (dumb thumbs - no fault of the engine) and snapped the carb mount tang from the backplate.
The owner ordered a complete backplate/pump assy and fitted it. A flying buddy and I ran the engine in a test stand with its new pump and guess what - it ran as sweet as any four-stroke I've ever seen. On 15% nitro, it pulled 10,200 rpm with an APC 12x8 (comparison: 9,900 for a Surpass .70 on 15% nitro and 10,600 for a YS .63S on 20% nitro) and idled at a rock solid 1700-1800 rpm with perfect transition. This engine is about to maiden in an Acro Wot so stay tuned for the report.
So where am I going with this?
Well, the OS .70 Ultimate seems to be a bit of a dice throw - it could run like a dog or it could run absolutely perfectly. Unless you are a real "engine guy" who is not scared of persevering with an engine that is not running right and who takes pride in fronting up to the field with an engine that's a little different, I would not recommend buying one based on my experience.