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Old 07-17-2019, 10:09 AM
Brian Yoder is offline
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Help!
four stroke .91 engine not idling

Can someone help me on my O.S .91 four stroke engine, the idle slowly backs off till the plane shuts off, I've adjusted the idle screw and I got it so it doesn't sputter in the transition from idle to full speed but that didn't make a difference that the idle still slowly dies away, however at quarter throttle and above it'll hold the same rpm, any ideas on how to fix this?
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Old 07-17-2019, 10:16 AM
Bunky F. Knuckle is offline
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Clean the carb out. Probably a small piece of crud in there somewhere.
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Old 07-17-2019, 01:34 PM
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Also change the glow plug
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Old 07-17-2019, 05:23 PM
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Ok so I cleaned the carb out good didn't make a difference also changed glow plug, still nothing. Should also mention that it is mounted inverted, running a 15x8 prop, and have a smoke system tied into it. Also if I run it full power and point the plane vertical it seems to slow down a little bit.
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Old 07-17-2019, 06:18 PM
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15x8 is a pretty tall and slanted prop for a .91, I use a 14x6 on a .95.. same for a 1.10a and they freaking scream and are easy to run.

You're smoker might bring it down also. Other than all that I might guess its running lean, but a tach would tell the story.
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Old 07-17-2019, 06:36 PM
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Ok so I cleaned the carb out good didn't make a difference also changed glow plug, still nothing. Should also mention that it is mounted inverted, running a 15x8 prop, and have a smoke system tied into it. Also if I run it full power and point the plane vertical it seems to slow down a little bit.
The tach reads around 9300 at full power
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Old 07-17-2019, 06:46 PM
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The tach reads around 9300 at full power
For how long?

Sounds a tad low on the R's, but you are running a bigger prop. Maybe try a smaller prop and richen it up and see how it goes. These days a .91 isn't a monster by any stretch. A 15" prop just sounds like a load. Let's see what anyone else might say.
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Old 07-17-2019, 07:47 PM
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What fuel and nitro content? What glow plug?

A 91 should run 14x6 or 14x8. Dad had a Saito 100 with a perry pump on it, and it twisted a 14x6 with ease.

Stick a 14x6 on there and try it again. The 15" prop might be acting like a flywheel, and lugging it down.

Or, you are trying to idle it too low. I have seen far too many people try to get these engines to idle reliably at too low of an RPM, and the engine slowly stalls, if not caught soon enough to idle up.

Again, we all are shooting from the hip.
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Old 07-17-2019, 08:15 PM
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The tach reads around 9300 at full power
I'm using 15% nitro, have O.S. type F medium four stroke glow plug, I noticed that if I set the idle a tad high it wont back off, so perhaps I was trying to idle too low, I'll try a smaller prop and then perhaps I can idle it lower then as well. Does anyone know what these engines should idle at?
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Old 07-17-2019, 08:37 PM
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With a 14x6 you should idle about 2000 rpm and see 9700 at WOT.

I'd suggest moving down a prop size to at least a 14x8 or 15x6.

OS FS-91 Surpass Second Run (5 min 3 sec)
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Old 07-31-2019, 05:11 PM
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You said you're running it inverted. Check the center line of your fuel tank to the center of the fuel inlet at the carb. They should be close to even with the tank going no more than 3/8" below the carb nipple. If your tank is too high that might be a problem. I'm not positive but I would check.

So your fuel, plug, carb, prop, and tank are all checked and ok. Get the thing running and set the high speed needle running wide open. Now chop the throttle and let it idle. Look at and feel what's coming out the exhaust. Is it spitting raw fuel? Too rich. Make your adjustments slowly, run back up to clean it out, then recheck. Don't go for a super low idle either. If it sounds dry and airy when you throttle up and there is nothing spitting out the exhaust, it's too lean. My guess is your engine is running too rich and that is why the idle goes down. You only move the low speed needle a tiny bit at a time when you make these adjustments. Also, let it idle for a little while to see what it does. Take your time when making low speed adjustments. Lastly, your top end is going to lean out a little when you lean the low end so you may have to open the high speed needle a click or three as you dial it in.

carl
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