Need info on old OS 120 four stroke
10-17-2011, 04:12 PM
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Need info on old OS 120 four stroke
I recently acquired an old OS 120 four stroke engine. It has dual rocker covers, and the crankcase breather nipple is on the rear cover. I'd really love to get some info on this engine. It seems to be weaker than my Saito 120s. Thanks in advance.
10-17-2011, 06:51 PM
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RE: Need info on old OS 120 four stroke
Not a powerhouse, but no 4 strokes in the early 80s were. Dont lug it too much and dont make it scream. Good for sedate airplanes like Cubs or a big Telemaster. Not a 3D or race engine.
10-17-2011, 07:14 PM
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RE: Need info on old OS 120 four stroke
I had the oportunity to operate one about a year ago... I borrowed it from a friend.
I really couldn't make it run right, but it may have been a bad glow plug. The problem I had was that I set the needles and seemed to run right...about 5 seconds on idle it started to load up too much...like if it was very rich... leaning the lsn further caused flame-outs during transition..so in sumary, I couldn't adjust the low needle.
It averaged about 8300 RPM on a graupner 15x8 on both 10% nitro and 5% nitro... I actually switched from 10% to 5% since I heard it knocking often with 10.
Its a very nice engine indeed, so sad that I had to return the engine and couldn't tweak it a bit little bit more.
I really couldn't make it run right, but it may have been a bad glow plug. The problem I had was that I set the needles and seemed to run right...about 5 seconds on idle it started to load up too much...like if it was very rich... leaning the lsn further caused flame-outs during transition..so in sumary, I couldn't adjust the low needle.
It averaged about 8300 RPM on a graupner 15x8 on both 10% nitro and 5% nitro... I actually switched from 10% to 5% since I heard it knocking often with 10.
Its a very nice engine indeed, so sad that I had to return the engine and couldn't tweak it a bit little bit more.
10-18-2011, 02:21 AM
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RE: Need info on old OS 120 four stroke
Thanks guys for all the info. I seem to have the same issue with idling. If I slow it down to an acceptable idle, it loads up and quits after a little while. If I lean the low end any more, it won't transition. Landings should prove interesting with such a high idle speed [X(]. Oh well, it'll give me something to tinker with.
10-18-2011, 02:46 AM
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RE: Need info on old OS 120 four stroke
ORIGINAL: RevGQ
Scott, emailed you regarding manual. Send me you address if attachment does not appear here.
Scott, emailed you regarding manual. Send me you address if attachment does not appear here.
10-18-2011, 03:41 AM
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RE: Need info on old OS 120 four stroke
First thing I would do is change all the o-rings, cheap and easy to do. Just a couple months ago I had an old Enya 4S that had the exact same problem. I was about to give up when I noticed that there might be an air leak between the carb and intake manifold. Replaced the o-ring there and the engine has been running perfect since with about 50 flights on it. Could be an o-ring somewhere else like the needle valve. This year I had to replace the needle valve o-ring on an OS 91 FX and an OAS 91-FS. Both started to act up. They were impossible to set. I was making the high end more and more rich, but they would still lean out and overheat after a few minutes flying. I'm not sure how the old OS carbs are built, but the newer ones have a set of o-rings on the low speed adjustment screw. May be worth changing those while you have it apart.
10-18-2011, 05:29 AM
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RE: Need info on old OS 120 four stroke
Hi!
I had one of those in a 1/4 scale pylon racer (Old Tiger) built from plans in MAN years ago.
Best prop sizes for it is 15x8 or 16x6.
I had one of those in a 1/4 scale pylon racer (Old Tiger) built from plans in MAN years ago.
Best prop sizes for it is 15x8 or 16x6.
10-18-2011, 12:19 PM
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RE: Need info on old OS 120 four stroke
ORIGINAL: smithcreek
First thing I would do is change all the o-rings, cheap and easy to do. Just a couple months ago I had an old Enya 4S that had the exact same problem. I was about to give up when I noticed that there might be an air leak between the carb and intake manifold. Replaced the o-ring there and the engine has been running perfect since with about 50 flights on it. Could be an o-ring somewhere else like the needle valve. This year I had to replace the needle valve o-ring on an OS 91 FX and an OAS 91-FS. Both started to act up. They were impossible to set. I was making the high end more and more rich, but they would still lean out and overheat after a few minutes flying. I'm not sure how the old OS carbs are built, but the newer ones have a set of o-rings on the low speed adjustment screw. May be worth changing those while you have it apart.
First thing I would do is change all the o-rings, cheap and easy to do. Just a couple months ago I had an old Enya 4S that had the exact same problem. I was about to give up when I noticed that there might be an air leak between the carb and intake manifold. Replaced the o-ring there and the engine has been running perfect since with about 50 flights on it. Could be an o-ring somewhere else like the needle valve. This year I had to replace the needle valve o-ring on an OS 91 FX and an OAS 91-FS. Both started to act up. They were impossible to set. I was making the high end more and more rich, but they would still lean out and overheat after a few minutes flying. I'm not sure how the old OS carbs are built, but the newer ones have a set of o-rings on the low speed adjustment screw. May be worth changing those while you have it apart.
10-19-2011, 06:31 AM
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RE: Need info on old OS 120 four stroke
Hi!
It should idle fine if you just use an: OS F plug , Use 5-15% nitro, Use a 12-15 oz tank placed according to the tank mounting rule, Preferable use a Uni-flow tank set-up, use a 16x6 prop...and set the low speed needle.
It should idle fine if you just use an: OS F plug , Use 5-15% nitro, Use a 12-15 oz tank placed according to the tank mounting rule, Preferable use a Uni-flow tank set-up, use a 16x6 prop...and set the low speed needle.
10-19-2011, 06:40 AM
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RE: Need info on old OS 120 four stroke
It would idle fine but not transition fine... i had the same issue.... when you tune it for a good idle, wont transition well and detonates when you throttle back.
10-19-2011, 08:39 AM
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RE: Need info on old OS 120 four stroke
check the valve clearance. i have been running two of them for 4 years. when they start acting up, its always been the valve settings have been off.
10-19-2011, 09:18 AM
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RE: Need info on old OS 120 four stroke
ORIGINAL: jorge76471
check the valve clearance. i have been running two of them for 4 years. when they start acting up, its always been the valve settings have been off.
check the valve clearance. i have been running two of them for 4 years. when they start acting up, its always been the valve settings have been off.
10-19-2011, 11:20 AM
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RE: Need info on old OS 120 four stroke
Just got back from running the engine....it runs like a different engine, idles and transitions great. I should have checked the valves to begin with.
10-19-2011, 07:39 PM
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RE: Need info on old OS 120 four stroke
watch the pushrods, they will sometimes lose there temper [harding] on the end of the pushrod and start to wear on the end. the only way to fix that is replace the pushrod. the pushrod are same as newer 1.20 eng's. just use the part# on tower hobby parts serch.
02-11-2012, 11:38 AM
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RE: Need info on old OS 120 four stroke
I have been tinkering off and on with this engine, and still can't get any decent rpm's out of it. With a 15x6 apc propeller, omega 15%, I get 7000rpm, with 10% omega, I get 6500rpm, and with 30%, I get 6800rpm. It will idle down to around 2000rpm with the 15 or 30%, but it doesn't idle well with the 10%. There is a huge amount of "slobber" coming out of the crankcase nipple. I have checked and reset the valves, checked and reset the timing, new glowplug....any ideas, or am I expecting too much from this old warhorse? It idles and transitions great, and runs smoothly throughout the rpm range. I was a little surprised at the drop in rpm's using the 30% fuel. I expected it with the 10%, but not the 30%.
02-11-2012, 02:23 PM
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RE: Need info on old OS 120 four stroke
Hi!
Take it apart and change the bearings!
All old engines that has been laying for some time need bearing replacement!
You should get around 8500-9000rpm with a 16x6 Zinger/Top-Flite using 5% nitro and 20% Castor oil.
Take it apart and change the bearings!
All old engines that has been laying for some time need bearing replacement!
You should get around 8500-9000rpm with a 16x6 Zinger/Top-Flite using 5% nitro and 20% Castor oil.
02-17-2012, 03:24 PM
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RE: Need info on old OS 120 four stroke
I think I may have found the problem. The pushrods are different lengths. The intake rod is 2.777", and the exhaust is 2.827". What length should they be? Could this be my problem?
03-06-2012, 06:29 PM
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RE: Need info on old OS 120 four stroke
i have the twin to your engine and it is a Valve problem try to take off the valve covers and push each valve down and make sure you watch each valve as you push to check and make sure they go all the way up and down and tweek up your fuel settings. my engine has a cylinder head exhaust crack and trying to find a new cylinder head.
Ron G.
Ron G.