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OS81 FSa Pumped Main needle sensitivity

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Old 05-13-2011, 05:14 AM
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highside
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Default OS81 FSa Pumped Main needle sensitivity

Hi Folks,

Just a quick "is there anything obvious to check" question.

Engine is a brand new OS81 FSa (pumped), now with 8x12minute runs on it. First three runs were on a bench following recommended running in process. Fuel is 16% synthetic/2% castor & 5% nitro. I'm an experienced user with many four strokes (OS, Saito, YS).

From the start, the engine has shown extreme main needle sensitivity. Leaned out somewhat and four-stroking nicely it produces good power (13x7APC), but literally one or two clicks rich from here and it goes so rich as to be unusable. During running in this problem was so pronounced that the glow igniter had to be left connected.

In the air it runs OK leaned out but is unreliable, and if allowed to go marginally rich in the lowrange or midrange it can be impossible to clear.

No fuel leaks anywhere on the motor, and behaviour is the same on the bench as in the airframe (Rapture 50).

My only other pumped OS is a 120 and that is as good as gold.

Any thoughts appreciated,

HS.
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Old 05-13-2011, 05:55 AM
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Default RE: OS81 FSa Pumped Main needle sensitivity

The FSa-81should not be sensitive on the needle.  LIkely your fuel pressure is low.  You may need to find a way to increase muffler pressure to the fuel tank.  Possibly you need to use larger fuel line.  Check the distance of the fuel pickup from the carburetor.  If it's a long way, that may be a contributor to the difficulty.  Just some things to check.


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Old 05-18-2011, 02:31 AM
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Default RE: OS81 FSa Pumped Main needle sensitivity

Hi Bax,

Thanks for the reply.

I think you are missing the point that this is a PUMPED FSa-81. So fuel pressure should not be an issue (also, on the test bench and in the airframe in question the tank is well positioned even if the engine was not pumped).

We have done some more testing yesterday. We checked the pump by removing the outlet and spinning with a starter, it pumps just fine. Then we created a bypass for the regulator by putting a T-piece at the output of the pump, taking one pipe to the regulator and one pipe back to the tank so that the pump is effectively circulating fuel around. This improved things markedly, so I suspect we have a faulty regulator. This would also explain the highly sensitive main needle.

A
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Old 05-18-2011, 05:46 AM
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Default RE: OS81 FSa Pumped Main needle sensitivity

Missed the "pumped" part.  Must have been one of those days.  Yes, a bad regulator can cause the needle to be overly sensitive.


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Old 05-18-2011, 06:07 AM
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Default RE: OS81 FSa Pumped Main needle sensitivity

Hey Bax,

No problems, I have "one of those days" most days [&o]

Seeing as the regulator is not supposed to be user serviceable, back to the importer it goes I guess.

Thanks for your help, always useful to have a knowledgeable sounding board.

HS
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