Need some help with os 4 stroke motors - RC Groups
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Sep 25, 2012, 11:13 AM
Registered User

Need some help with os 4 stroke motors

I just bought an estate collection of 15 planes, most have OS 70 or OS 90's on them, talking with the guy I bought them from, they have been sitting for about 7 years. I took one and soaked it with fuel for a couple of hours and was able to get it to turn over along with the carb. My question is on the 4 stroke, I have only owed 2 stroke and have no experience with them, the one I messed with seems like there is hardly any compression when turning over, if if you move slow enough you can hear the valves. Is this normal? If i already screwed this one up is there a better way to bring back to life older motors that have been sitting for awhile with out having to rebuild them, some of them look brand new. Any input is appriciated and thanks in advance. Ah sorry I am new here, I flew a bunch in the late 80's and early 90's then joined the Navy, Just retired so I am getting back into the planes. I live in San Antonio now.
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Sep 25, 2012, 12:18 PM
Registered User
Well an engine laid up for 7 years may have a stuck ring. If so, no compression. But I would open the carb up and try it. Sometimes if the carb is closed the engine does not draw air fuel mixture into the cylinder and thus shows little or no compression.

Hearing the valves is not enough, you need to take the valve cover off, and see if there is a stuck valve. You are hearing the push rods and the rocker arms moving but you have see if the valves are moving. And while you are in there with the cover off, dribble some oil down the push rods and get it all over the rocker arms.

If the valves are indeed moving you might take the glow plug out and dribble a little oil down into the cylinder and turn it over a three or four times to move it around, re install the plug and open the carb and see what happens.

Generally all the four strokes are ringed, yes there are some that are not like the OS 70FL. But for the most part they are ringed engines.

The fuel is about the same for a four stroke. I use a blend with some Castor oil in it. Anything with 10% Nitro will work well. Higher Nitro works better but costs a lot more and really is not needed.

Sounds like you got a nice deal.
Sep 25, 2012, 06:56 PM
jeffie8696's Avatar
I would give it a good soaking in fuel then put it on the test stand with a little heat from the heat gun and try to start it if it turns over smoothly.
Sep 25, 2012, 11:41 PM
Registered User

4 strokes

Hi Mike Welcome back!I know the feeling I am slowly working back into the hobby myself.Been awhile for me, but always loved the 4strokes.If it were me, I would start with just one of them, if it was put up with some afterrun oil, you maybe in good shape, if not there is a good chance you might need to replace the bearings in them,a small investment for a engine that will last a lifetime,the bearings can pit,rust,etc. then if they are ran the rust turning loose slowly will act like a lapping compound and wear everything out in a few runs.Like was said before may also need the rings replaced, but should not be to expensive to replace.but sounds like you got a deal! Mark
Sep 26, 2012, 03:49 AM
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kerwin50's Avatar
OS leak alot at the valve seats. I like to lap my valves in with a little valve lapping compound, a drill and some fuel tubing and a piece ofbrass tubing from a fuel tank.
I put the fuel tubing over the valve stem and the other end of the fuel tubing mI put the brass tube in. Chuck it up in a drill and lap the valve in.
I've gotten alot of compression and a much better running engine this way. I've done this on several os's
Sep 26, 2012, 01:44 PM
Registered User
I own dozens of OS 4 cycle engines and none have any problems with poor valve seats. Lapping compound is way too coarse for model engines. If they do need lapping toothpaste with some cleanser is more than enough.

Here's the latest OS 4 Cycle I've purchased.
Sep 27, 2012, 01:14 AM
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Reginald's Avatar
Originally Posted by datsunguy
I own dozens of OS 4 cycle engines and none have any problems with poor valve seats. Lapping compound is way too coarse for model engines. If they do need lapping toothpaste with some cleanser is more than enough.

Here's the latest OS 4 Cycle I've purchased.
Congratulations on your buy. I do have two of these and they are magnificent.
OS engines are my favourite too and I have never encountered problems with valve seats. If one must re-seat do use very fine diamont lapping compound and finish with toothpaste. It's a delicate thing for the un-experienced to do.
Talking bad valve seats : the side valve Vega engines !
Sep 27, 2012, 02:21 AM
Registered User
Thanks for your input guys! As of now I have 5 running, just needed a little lubrication and heat. I have a FS 90 that has stripped bolt on the valve cover, not going to try and run it until i can peal around. I have a FS 91 that needs to be rebuilt, bearings, crank are totally rusted, I started to stripe the engine down but got stuck with the cam, I was able to remove the 2 bolts on the side to remove the access cover, all i can get it to do is spin, it wont come off! What is the trick to removing the piston and cam? I also just pulled a YS 120 from 300 extra, are YS's a good motor? Again thanks for your input.
Sep 29, 2012, 12:57 AM
jeffie8696's Avatar
new YS = magnificent , used worn out YS= PITA.
As far as the FS 90, you may be getting hung up on the lifters, need to remove the pushrods to get at them.

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