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OS 91 SII (non-pumped) Idle Mixture Adjustment

Old 08-26-2015, 08:15 PM
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dasintex
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Default OS 91 SII (non-pumped) Idle Mixture Adjustment

This Engine, I suspect is an older Non-Pumped OS 91 4 Stroke Engine; I bought it used with very little use other than a break in; I have the manual and have read it over and over and need some help with low end or Idle Mixture; it seems that my engine runs great on top end, but what I presume(and could be wrong) loads up on low end and quits if it is left to idle for any length of time IE: Taxying out to the runway or on approach for a landing; from my experience with other engines especially 2 stroke engines; high end is adjusted with the Needle Valve that extends out the side of the Carb and low end Idle Mixture is adjusted with the small adjustment screw inside the throttle arm. It seems that this engine does not have the adjustment screw inside the throttle arm; there is a screw beside the base of the Needle Valve Holder Assembly would this be the Low end Idle Mixture Adjustment screw?

Right now, that screw is turned in completely seated or closed; I had assumed it was some sort of Throttle stop or something; would this actually be the Idle Mixture Screw and I need to turn it out or open it up?

Thanks, appreciate your help

Last edited by dasintex; 08-27-2015 at 03:59 AM.
Old 08-27-2015, 05:51 AM
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We've attached the manual for the original O.S. FS-91 Surpass engine. It shows how to adjust the carburetor.

[ATTACH]2116885[/IMG]
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File Type: pdf
Old 08-27-2015, 06:54 AM
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dasintex
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Bill;

Appreciate the reply, I will have to look at my OS again, I had thought that mine did not have the Idle Mixture Screw inside the Throttle arm; if it does than I am familiar with adjusting it; however, I'm thinking it does not, but had a screw beside the base of the Needle Valve Holder Assembly; I will check when I get home today. The manual I have is different than the one you posted.

http://manuals.hobbico.com/osm/fs-70...i-p-manual.pdf


UPDATE: Checked my Engine, and it does not have the Idle Mixture Screw in the Throttle Arm Hub

Last edited by dasintex; 08-27-2015 at 04:46 PM.
Old 08-31-2015, 06:23 AM
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The manual you referred to is for the Series II engine, which you said you do not have.

Does your engine have a number or number w/letters stamped on it?
Old 08-31-2015, 07:50 AM
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All is good, the engine is mounted in a Cowled Plane, the engine is not readily seen; however, I was searching on other OS 91 threads, and someone else made mention that the Screw I have been referring to is the Idle Mixture Screw.

Appreciate the assistance.
Old 09-01-2015, 07:05 AM
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Got a bit turned around. The idle mixture is the small screw that's to the side of the main needle. Page 15 of the manual tells you how to adjust it, and to get it to its proper initial setting. That's just the starting point, and you adjust it from there to suit the engine's needs.

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