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OS 91 FX at about mid throttle

Old 12-30-2014, 01:30 PM
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prodgodq
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Default OS 91 FX at about mid throttle

Hi,

I'm sure this has been covered before, but the forum search engine didn't turn up anything directly. Here's my problem:

I have a used OS 91 FX installed on a Sig Four Star. It's no longer covered by the warranty.

The engine starts fine and idles well. When I advance the throttle to just below half, the engine begins to stutter and run erratically. This lasts until I advance the throttle to three quarters. The stuttering then smooths out and the engine runs smoothly.

When I reduce the throttle to the point where the stuttering starts again, I adjust the needle valve to a more lean mixture, which helps to smooth the engine out. But then when I advance the throttle to full, the engine seems to be running too lean and will stall out.

I have gotten a lot of advice from people in my club, but I thought I would go to the source. What do you think the problem is?

Any help would be greatly appreciated!

Thanks,
Dave
Old 12-31-2014, 08:57 AM
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Bax
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Very likely the idle mixture isn't set correctly, and your idle is set to a too-low RPM number. Increase the idle RPM and then adjust the idle mixture to help your engine's acceleration and midrange. Any idle RPM below about 2,500 is considered "good".
Old 12-31-2014, 09:01 AM
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speedracerntrixie
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What plug is being used? A plug that has too cold of heat range for the particular engine will have the same symptoms.
Old 12-31-2014, 10:09 AM
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prodgodq
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Hi speedracerntrixie,

Standard fox glow plug. Do you use something different?
Old 12-31-2014, 12:10 PM
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Propworn
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Try one thing at a time and test it. If you make to many changes at once then you are never sure what you did that actually fixes the problem.

Start out by setting up your carb. Back out the idle screw a bit. Back out the high speed needle a bit. Now take a 1/16 drill bit or wire and place it in the open carb and close the carb gently until it just holds the drill/wire in place. Put a piece of fuel tubing on the spigot. Slowly close the high speed needle until it stops, do not force. Open the high speed needle 2 to 2 1/2 turns. Now while blowing through the fuel line slowly close the low speed needle until you feel the air stop. Back the low speed out until you can just detect air passing through. This is your primary setting.

Start the engine and slowly open the throttle to full. Adjust high speed until the motor reaches full rpm then back it off slightly until you hear a very minor drop in rpm.

Set the low speed by advancing the throttle slowly at first then faster as you zero in on the correct setting. If the motor outright quits suddenly chances youíre too lean open the low speed no more than 1/8 of a turn at a time. If the engine loads up and quits itís to rich and you will have to turn the low speed in 1/8 of a turn at a time. When you start getting close to a good fast transition reduce your adjustments of the low speed needle to no more than a screwdriver blade thickness.

Some have trouble determining if the engine is lean or rich so you can put the glo driver back on and advance the throttle. If it quits its lean if it goes to full throttle or struggles to clear itself then its rich.

The last fine tuning to do is reset the High Speed by finding top rpm and just richening it until you get that slight drop in rpm then rechecking your low speed adjustment.

For best results use a new glo plug and new fuel eliminate the two items that can give you grief.

The Miracle Plug is an ok plug I prefer the MCoy MC14 for all two strokes.

Last edited by Propworn; 12-31-2014 at 12:12 PM.
Old 12-31-2014, 07:23 PM
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prodgodq
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OK, thanks for the great advice. I'm sort of a newbie when it comes to fine tuning. I've been very fortunate in the past in that the glow engines in my planes have required very little tuning. I'll do some fine tuning and post the results!

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