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O.S. 91 Surpass - New - loads up/stops


Old 06-02-2011, 06:36 PM
Ken Cz
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Default O.S. 91 Surpass - New - loads up/stops

Bax - Need your advice on next steps please.

Brand new OS 91 Surpass
14x6 and 15x7 Prop
Powermaster 15% nitro/20% lub.fuel
Break-in per manual
Mounted in GP Venus 2 - about 60 deg off vertical

I have been having an issue with this .91 that I not experienced with the other 2 new 91's I have or the 3 OS 1.20 4 strokes. The all have run great out of the box with zero problems and only HSpeed adjustments needed.

Problem: The engine starts easily but when idling at about 2400-2600 rpm for 15 seconds or so it begin to 'load up' - lose RPM's/stall out unless I kick the throttle up to 3/4 or WOT for a few seconds. Then it is back to idling OK for 15 seconds or so.

I have tried leaning out the LS needle 3 times now and have gone in about 3/4 to 1 full turn now but the needle seems to be getting tight.
After leaning out the LS I have run the engine up to WOT, peaked the HS and then backed off about 200 rpms - each time. Still won't keep running at idle.
Also, the engine responds instantly to full throttle from the idle position - it definitely does not 'bog-down'.

Not sure what to do next.
As usual, you assistance is appreciated.

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Old 06-03-2011, 06:40 AM
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Default RE: O.S. 91 Surpass - New - loads up/stops

"Loading up" is caused by a too-rich idle mixture. You need to work with the idle mixture and idle RPM. Start out with the idle set to "stupid rich", get the engine to full throttle and set the high-speed needle so that the engine is leaned to just rich of peak RPM. Now retard the throttle and lean the idle mixture as needed. When you find a good idle RPM and mixture combination that works, check it with acceleration.

Many times, "loading up" is caused by trying to have an idle RPM that's just too low. Anythng lower than about 2,500 RPM would be considered good.

You may be trying to use too much propeller. We like to run a 12 x 8 or 10, and prop for full-throttle RPM near 10,000 when the engine's leaned to just rich of peak RPM.
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