O.S. 120 FS III with Pump break in. - RC Groups
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Jan 08, 2013, 01:14 PM
iFlier's Avatar

O.S. 120 FS III with Pump break in.


Ran the new engine for the first time not to long ago to start the break in process. You can see it on :

Hawker Typhoon Engine Test (2 min 9 sec)

I only had a chance to run it once before the winter weather set in. It started great but never got above half throttle. It kept running at the same rpm from half throttle to max. Not sure if its too lean, to rich or just normal for a first run?

www.iflier.blogspot.com has more info on the build.
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Jan 09, 2013, 03:13 AM
You made that out of trees?
Boomerang1's Avatar
I've got some news & it's bad I'm affraid.

My 120 did exactly the same thing. After speaking to a well known aussie engine guru he suggested it may have been a partial blockage in the pump or the regulator attached to the carb.

On his advice I put fuel in a squeeze bottle & force flushed out both units. What came out was like a blue jelly like substance.

After this the engine would run much better, at least it would run up to full throttle & run at the expected RPM according to the prop I was running, but it never ran 100% despite endless adjustments.

After contacting the Oz distributor I sent back just the pump & carb. About 10 days later I got back a pump & carb, don't know if it was the original repaired or new units. From this point on it has run great.

Main needle setting is critical, you have to run it VERY lean, close to cutting out. A click or two rich for good luck just doesn't work on this engine. If run a little rich & you fly around some time at half throttle or so when it comes time to go back to full throttle it will get stuck running very rich & not clear itself & run back up to full revs.

Just my experiences, good luck! John.
Jan 16, 2013, 01:26 PM
iFlier's Avatar
Thank you for the advice. I ran it again last weekend and it is suffering the same ailments.

I have adjusted the valve timing as one was a little out. Going to try it without the fuel filter between the tank and the engine to see if it makes a difference. I will try cleaning out the pump and the carb as you suggested also.

In building I tried to keep it clean from dust and debris etc. Maybe something still managed to get in!
Sep 29, 2014, 09:54 AM
Registered User
Old thread, but I had the same issue with my FS 120 Surpass III on breaking it in. The high end needle at ~3 turns out was WAY too rich and I experienced the same thing. You really need to lean it out by a full turn. I thought it was acting too lean by bogging down, but turns out it was so overly rich it just killed it. Once I leaned it a good full turn, it produced up to max RPM.

I also noticed in the manual a mention of pump break in... running the throttle around 75% pinned there for a while before doing full speed runs. Probably to clean out the pump to start.
Sep 29, 2014, 10:26 AM
iFlier's Avatar
Agreed! It turned out that one of the guys at the field simply leaned it out as you did and the rpm came up as you experienced. I have been running it since without any issues.

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