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OS 55ax mid range tuning

Old 10-15-2010, 02:19 PM
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Default OS 55ax mid range tuning

Hi

I would like your advice regarding the tuning of my OS 55ax engine. The motor is new and has been run in and flown with about 10 tanks of fuel. The motor idles well and runs well at high speed ( still set on the rich side ). However I am having problems with the mid range setting.
I have leaned the low speed needle about a half turn from the supplied setting and the motor transitions OK if I open the throttle from idle after about 5 seconds ( as per the instructions ). However when I taxy out to the take off position and open the throttle to take off the motor lags and does not respond well. I have still managed to take off OK but it has caused a few anxious moments ! In the air the model flies well at full power but the motor sounds very rough at half power and lacks power in the mid range settings. Do you think I should lean off the low speed needle some more or is the setting too lean already ? I have tried different plugs and I am currently running a OS No7 and using an APC 12 x 7 prop.

Thanks
Anthony
Old 10-15-2010, 02:57 PM
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Default RE: OS 55ax mid range tuning

I am no expert, but I think 1/2 turn is to lean, check your fuel...switch to an O.S. type F plug. also check the nipple on your muffler..make sure it is not plugged up, might not be getting enough pressure to push fuel to the carb
Old 10-18-2010, 09:15 AM
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Default RE: OS 55ax mid range tuning

Whatever setting works and keeps the engine from being too lean at top-end is the "correct" setting. "Factory" settings are only very rough starting points.

If the idle is OK but transition is rough after doing a lengthy bit if idle running, then the idle needs to be adjusted. Generally, one problem is that the idle RPM is too low. Increase the idle RPM a bit and then readjust.

With any engine (there are few exceptions), you should set the high-speed needle at full throttle first, and then retard the throttle and adjust the idle mixture as needed.

From your description, you need to lean the idle mixture a small amount. Likely, you'll have to increase the idle RPM a bit to compensate.
Old 10-18-2010, 10:28 PM
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Default RE: OS 55ax mid range tuning

Thanks for your advice Bax. I will try a slightly leaner setting next time I go flying. There seems to be some debate about the best size prop to use on the 55ax.
I am using a 12 x 7 as per the instructions and the motor is doing about 10,500 RPM. I have not leaned it right out yet. Some other flyers claim this is loading the motor down too much and use a 11 x 6 or 11 x 7. The 12 x 7 seems OK to me and the noise levels are quite low. Am I doing the motor any harm running this size prop ?

Regards
Anthony
Old 10-18-2010, 11:27 PM
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Default RE: OS 55ax mid range tuning

If a glow motor coughs and spits during the transition it is too rich. If it simply cuts out during the transition it is too lean.
Old 10-19-2010, 08:45 AM
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Default RE: OS 55ax mid range tuning

We'd consider running the O.S. .55AX at 10,500 RPM as being quite overloaded. We'd run it so that it tachs in the mid-12,000's. Of course, that means running a propeller with smaller diameter or smaller pitch. 3-D type flyers like to use a 12 x 4 propeller or so. For most sport flying, you can use an 11 x 7 or 8 propeller...even an 10 x 8 isn't too small, but don't go much smaller unless you're trying to run a racing-type airplane. Then, something like a 9-1/2 inch propeller with 8 or 9 pitch or so....whatever works, really. If you're racing, you'll want RPM as high as 15,000+, and expect a short engine life.

Of course, any time you go to a higher-RPM combination, you get more noise. If that's not a factor, then do so. The engine handles quite well when propped for the 12's.
Old 10-20-2010, 06:24 AM
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Default RE: OS 55ax mid range tuning

Hi Bax

Thanks once again for your prompt reply. Noise is an issue at our field and I would prefer to use the quietest option possible. If I stick with the 12 x 7 and live with the lower power output do you think it will do the engine any harm ? I am not sure when you say "loaded up" if that means the engine will suffer premature wear as a result of the larger prop load ?

Also a couple of people have suggested trying a OS Type F plug in the 55ax. I thought they were designed for four stroke motors and what would be the advantage over a OS No 8 ?

Thanks
Anthony
Old 10-20-2010, 10:04 AM
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Default RE: OS 55ax mid range tuning

You won't damage the engine. As long as it's not running lean of peak RPM, and the engine runs well, you won't hurt it with a larger prop. You just won't be able to get as much performance out of it as with a smaller propeller that allows higher RPM (and more noise, of course).

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