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OS 70 S II Question

Old 10-06-2003, 11:04 PM
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Default OS 70 S II Question

Hi guys,

I am having a problem with my OS 70 S II. The engine has ran consistently well for about 12 months, and was recently installed in a Pilot Seduction 3D plane. It ran very consistently here too, untill it developed a slightly strange 'clunk' as it was turned over. The problem was traced to a dry cam. The cams, crank etc. were carefully checked and no problems could be found. The local service center also reported no problems with the engine internally.

However, we are now experiencing a dead spot in the throttle curve which moves from about half throttle to 3/4 throttle depending on the idle mixture adjustment. We have been unable to adjust it such that the dead spot dissappears, even when running the engine on the bench.

This is becomming quite annoying as you really need a reliable engine for 3D.

Any help would be appreciated.

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PeterNzen
Old 10-07-2003, 10:10 AM
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Default RE: OS 70 S II Question

Dry cams are usually caused by low-oil fuels, or possibly a lean run at one time or another.

With most R/C engines, there is a "flat" spot in the throttle curve. They are not quite linear. If it's become exaggerated recently, you may need to check valve settings at the rocker arms, and also check to see if the engine's performance is starting to go. You may be starting to lose compression, depending upon how much use the engine's had.

Other things to check: valve sealing, valve sticking, any changes to fuels used, bearing smoothness/roughness, and so on.

The engine may need a thorough going-over to see what the difficulty may be.
Old 10-09-2003, 05:34 AM
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Default RE: OS 70 S II Question

Hi,

I've been experimenting with the engine a bit more, and have determined that if the idle mixture is richened, the severity of the dead spot is reduced, but then the transition from idle to full power is very slow. If I lean the idle to improve the transition, the dead spot gets worse.

The engine has only been used in our trainer / utility aircraft and has very seldom been operated at full throttle. (Although this could possibly have masked a slightly lean condition.)

The fuel used has always contained 20% oil, being made up of 10% Cool Power and 10% Castor, with 10% nitro.

The dry cam was discovered after a prop strike on landing stopped the engine, and the 'clunking' noise was noticed. No internal problems could be found by either ourselves or the repair agent.

We have also noticed extra fuel spitting from the carburettor at idle. The engine was tached today and registered 9800 rpm, with a 13 * 7 N APC.

Any further suggestions would be appreciated.

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Peter Nzen
Old 10-09-2003, 04:23 PM
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Default RE: OS 70 S II Question

Peter,

Your fuel is good, and the top RPM is in the normal range for that size propeller.

You might try re-setting the needles on the engine to see if there's an imbalance between the high-speed and low-speed needles. We always start out with both the high-speed and low-speed needles quite rich. We start the engine and open the throttle to full and then lean the high-speed mixture to just rich of peak RPM...the normal flight setting.

We then retard the throttle and work with the idle mixture and RPM. Sometimes, we've found that trying to get a too-low idle RPM causes problems in acceleration, even though you can get a good, low idle. We'll accept a few hundred RPM more for idle if we can get better handling. As long as the model can slow enough to land, the idle RPM is OK.

Generally, we consider an idle of about 2,500 RPM or lower as "good", consistent with good throttle handling.

One other thing to consider is a new glow plug. After a lot of use, they do start to go bad and poorer idle and acceleration are the first signs.

Finally, depending upon how much running the engine's had, the piston/ring/liner fit could be wearing out. If the compression's starting to go, then idle and acceleration are also the first things to be affected.

I think that's the list. With some of the engine's performance not being as good as it once was, something's definitely the cause. It's just finding it that presents the challenge.

Hope this helps.

Old 10-13-2003, 10:32 PM
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Default RE: OS 70 S II Question

Bax,

Thanks for your suggestions, they have certainly helped. We flew the plane over the w/e and the engine was running much better.

However, the acceleration form idle is still not good ( possibly too rich), although leaning the idle mixture produces the dead spot again. We have also lowered the fuel tank to the correct level which seems to have helped.

We have also noticed that the engine runs better with the plane inverted (engine upright) causing a surging of the rpm through slow rolls etc.

We have tried a new plug previously, which produced no noticeable difference to the engines overall handling.

Thanks for your efforts.

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Peter Nzen

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