OS FS-120 Surpass III Pump issues

My newish FS-120 surpass III with pump is developing about 8500 rpm with 16x6 apc 15% wildcat and omega.

Somehow this does not seem right, check the tappets etc and nothing seems wrong, engine idles and transitions fine.

What else could be wrong ? It was purchased second hand.


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Checked some of my old records on an FS-120 Surpass II, and it appears that 8500 rpm on a 16x6 apc is about correct. That's about what I got on a 15x8 apc, and with a 14x8 MAS it got between 9500 and 10,000.

Olin McDaniel

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Olin K. McDaniel

I have an FS-1200SIII purchased from new about a year ago. When I got around to running it up for the scale biplane that I had been building I stuck a problem:-

It will start OK and I can tune up on the main needle and mixture control valve and it will run for 10-15 mins across the full range like a dream. But on the second tank it then floods and you have to reduce the needle to about a 1/4 turn before it stalls in a cloud of unburnt fuel. The excellent OS service organisation (I am not joking) have replaced the pump and carb without success and we are just about to go onto the third set.

The problem seems to come on with use and only shows after a lengthy period of running. What is more, it goes away if you don't use the engine for a week or so, but then comes back to bite later on.

Anyone had this problem? If so what was the cause/fix.


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What fuel are you running? excess castor will clog system and free up once pump etc is hot causing symptoms as you describe.. check sub sections "Carburettors, Tuning & Performance Testing, Glow Plugs." "Fuel facts & FAQ + Fuel Tank installations" at Alan's Hobby Model & RC FAQ Web Links

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Regards Alan T.

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Thanks, I have tried 5%/18% (nitro/castor), 10%/18% and briefly

20%/unknown. The engine is spec'ed for 10%/18%. I am using standard OS F plugs (several, including two previously unused).

I have had the engine to bits and there was quite a lot of nice clean oil in the case, enough to pour out after removing the back plate. The crank-case rebreather nipples are all as quoted in the parts list and clear and no problems with valves, compression etc.

I wondered if excess oil in the case could cause the pump to play up?


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I have the same engine and i CAN ONLY GET 8100 WITH A 15 X 8

It is very annoying as i need just a bit more pull for the model it is in.

It should go to about 9500 I think.

I am only on 10% but going to try 20% soon but it should not be like that any way.

I think I must have a blockage some where, It is only a just run in engine so I doubt its that. I think there is some thing a miss in the regulator.

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