Several years ago, I purchased an OS 140 RX with the OS tuned pipe &
header.
Due to other priorities, It took me a while to finish my pattern
airplane (USA-Star) and have it ready for flight.
I performed the break-in with the engine installed in the airlplane
using an APC 16x14 prop, using cool power fuel with 15% nitro and 17%
synthetic oil.
In the first flight the engine quit after 2 minutes.... It seemed to me
normal for a recently bronken-in engine, and after a closer inspection,
I saw that the soft mount was too soft, so I decided to remove it. ...
and mounted the engine directly on the mounting beams.
On the second flight, the engine quit again after two minutes...
Right after landing, I restarted the engine and observed that the RPMs
we unstable - the engine would accelerate for a few seconds and then
slow down a few seconds, and then accelerate again, etc, etc...
I figured maybe the engine was overheating, so I took off the cowling
and started the engine with no cowling.
The RMPs keept oscillating as described and the engine shaked violently
at idle.
I took off the backplate and cylinder head and found everything as good
as new.
I checked the fuel tank, fuel lines, tuned pipe, etc and found
everything in order.
I tried again, this time using cool power 15%nitro, 17% synthetic, and
I added 5% castor oil.
The results were the same....

I have a question on wich I would like your comments: Assuming that the
engine overheated during the break-in and the first flight, what damage
could that have done to engine? As I mentioned the engine looks as good
as new, both on the inside and the outside, has no free play, has
excellent compression, and has no friction. - I am totally disoriented
as to what might be the problem.

It sounds like the engine is not completely broken in yet. I would run it for anonther half hour or so on low nitro fuel then see how it performs.
Tom.

If an engine is overheating it loses power at the top end of the throttle. It normally smells pretty burned when you run it on the ground.

Maybe you need to turn it richer, both the low and the high speed needle.
Wy do you use so much nitro, they don't recommend it in the manual.
Check it on their website, [link=http://www.osengines.com/manuals/140rx-manual.pdf]1.40 RX manual[/link]

That prop seems a bit heavy, I would use something lighter.

Anyway, that's what I think of it but I'm not an expert. Maybe someone else can confirm this or advise something else.


Good luck

It sounds way over propped to me....[X(]

FBD.

Dave:

That's not "Way overpropped," it's inexcusably overpropped. If his engine would pull it his airspeed would be around 130 mph. That's just not right for a pattern plane.

Combining too much prop with a fairly new engine it's a guaranteed overheat condition.

Bill.

Yep....with an APC, and Coolpower fuel to boot ! [X(] I'd offer a
recommendation....but nobodies ever listened to me yet.

That OS 1.40 is an expensive engine, isn't it ?

FBD. []

I calculate a little over 110 RPM. You do know that this engine rarely hits above 9,000 RPM don't you? The prop is right in the middle of the range suggested. I would look at putting in a larger or stiffer fuel pick up line. I have had the same thing happen and the soft flexible line was colapsing, when the engine slowed down the line would open up, causing the engine to surge. It could also be caused by trash in the carb or needle. Or the fuel lines are too small.

Sport_Pilot hit the nail on the head! I'll bet your using the medium sized fuel line when you should be using the large. If this is not the case, then it's trash in the carb.

Please keep us posted on what you find.

Sorry for not replying sooner ... had computer trouble that did not allow me to connect...
Thanks for all the information you gave me [sm=thumbup.gif]- I will try as many of your suggestions as I can over this weekend and let you know the results.

There is still one question in my mind on which I would like some more feedback:[sm=confused.gif]

Assuming the engine did overheat in these first 3 flights, what damage could have been done? (would it affect the metallographic composition of the piston/sleeve/ring, etc..? Should I be able to measure any distortion on the internal parts? Other?) - As I said, the engine lloks as good as new on the inside as well as on the outside....

Rotorhead:

If you still have good compression, and you see no scoring on the cylinder walls, it's almost 100% probability that you've done no damage to the engine.

Bill.

OK, I am back from a long weekend ... the engine is now working fine: lasted a full flight without quitting. (Idle is not yet low and reliable, but acceptable.)

Here´s what was causing the problem: the glow-plug. ... going back to my first flight where it overheated (melted the plug filament), I had replaced the plug by an OS#8...
This weekend I tried with an A5 glow plug, and voilá...
Also, I removed the cowling to get good cooling airflow.

Thanks again for all your help.

Don't count on it! I always take heed to your suggestions. We are lucky to have a great resource here; Bill, you, and Jim. NOW DON"T GET A BIG HEAD!!!