Yes, I know it is out of production.

But I got a used one, adjusted the valves, and tested it.

Master Airscrew 9x6 Classic, Powermaster YS 20/20, OS "F" glow plug. 85°F OAT.

10,800 RPM peak.

http://media.putfile.com/OS-Engines-FS-20-Four-Stroke

This is a very timely post. I recently obtained a used OS FS-20 and plan to run it for the 1st time next week. Will be using home brew fuel - 10% nitro, 20% oil (castor/synthetic) and a standard MA 9 x 6 prop.

Mine has the incredibly tiny but cute muffler. Will post RPM results when complete [8D]

I don't know what muffler you are referring too. But I would guess it is actually a way to provide a pressure nipple for the fuel tank? I am curious to know if you get more rpm by having fuel tank pressure. Keep us posted!

Correction on the prop. I thought the MA standard black prop was the "Classic" and I have found out it is not. The Classic is a wide blade prop. I was running the normal, narrow blade MA prop.

Okee dokee - ran the little beastie at the field today. Home brew fuel mixed this morning being 70% methanol, 10% nitro, 15% red Klotz & 5% castor oil (Castrol M). Brand new OS "F" plug, air temp approx 75F & dry.

The RPM Results:

Zingali 9 x 6 = 10,300 (NB: not leaned out as engine was VERY UNHAPPY with this fibreglass (?) prop, made dreadful mechanical noises - perhaps resonance thru the prop [:'(])
MA 9 x 5 = 11,400
MA 10 x 4 = 11,100
Bolly 10.5 x 5 = 9,000
MA 10 x 6 = 8,400
APC 10 x 6 = 8,500 (NB: used the APC prop simply to annoy FBD )
MA 11 x 6 = 7,400

Will post a pic of the tiny muffler later [8D]

LOL!!

Your engine sounds like a sweetheart! [sm=thumbup.gif]

Here is the pic of the OS 20FS muffler that came with my engine......as you can see, it's very tiny - 30 mm long & only 13 mm at its widest point. Placed alongside a AA battery for size reference

It's neat! I want one!