I just got a used OS 70 FL and when the crank is rotated to when the piston comes up and reaches TDC, the rod is in limbo for several degrees either way until it begins to travel back down. The point where the rod has effect on the piston's movement seems to be very pronounced, almost binding a tad, but I see no marks on the piston or cylinder head. Anyone have any problems like this before with this engine or another one? []

I think, but am not sure, mine does that and there just biult that way.

It just doesn't seem quite right. All my other OS and Saito engines are smooth traveling through the cycle, not that "bump" at the top. Any body else with one of these engines have the same effect with theirs?

I checked my Fl 70 and if we are talking about the same thing mine does it to. It seems to run fine.

Almost kinda likes its binding a little up on the very top of the stroke, right?

Ya, i wouldn't quite call it binding though. The prop just dosn't feal like it is turning anything for like 10 degrees and then it starts turning the piston again. I am not sure what that is.

Well, I'll fire it up tommorrow and see what happens. Thanks for replying to the post here, I appreciate it.

boxbeam, your other four strokes are ringed engines. With the ringless FL you are feeling the "pinch" at the top of the stroke which is normal.

Hello; Another explaination could be the big-end con rod clearance, if you are moving through TDC, the piston comes to the top of the bore, the rod clearance is used up, then when you move away from TDC, it is restored. Just a thought.

It's what TimC described. The FL 70 is different from the other 4 strokes in that particular.

Well, I guess that explains it then. Thanks for all your input guys, I thought I may have gotten ahold of a problem engine for awhile. Enjoy!!

I ran this engine today and I don't think it has much time at all on it. There is definetly a tight spot at the top of the stroke, that must be the "pinch" that was spoken about by TimC. It almost sounds like a piece of nailed wood being pulled away with a hammer. I am just going to break the engine in with some time and patience and see what happens to it. This must be a tappered sleeve and piston assembly if it is ringless and a VERY tappered assembly at that!

Boxbeam, I expect you will notice that immediately after the engine has been run and is still hot the "pinch" will be less pronounced, then return as the engine cools. Sounds as though you got one that is properly fitted-enjoy.

jess

Thanks Jess, that is an encouragement!!

boxbeam or Cambo, I'm curious what the manual says about break-in. I suppose it's different than the normal four stroke. I tried to download the manual but my Adobe is acting up.
The manual finally downloaded and I don't know if I agree with the instructions or not. "At full throttle, peak the engine for 5 seconds then richen 1 full turn of the high speed needle......."
This would result in a very rich condition which goes against the accepted method for breaking in an ABN engine. I'd like to hear what other members here think.

Tim, I never got a manual, I got a basket case. The Tower site says to use no less than 18% lubricant whether it is synthectic or castor and synthetic. I am running it sloppy for several tanks and gradually leaning it out until the idle becomes reliable.

boxbeam, I don't know how to cut and paste from a PDF file, but the manual is available at the O.S. web site. Let me see if I can copy the pertinent section.

This is directly from the manuel

1. Use the same fuel and prop as you intend
for flying your model.
2. Open the needle-valve 2.5 turns from the
fully closed position and start the engine.
3. Open the throttle slowly to the mid speed
position, and disconnect the current to the
glowplug.
4. Now open the throttle slowly to the fully
opened position and run the engine for no
more than 5 seconds with the needle-valve
tuned to produced near maximum r.p.m.,
then, immediately, slow the engine down
again by opening the needle-valve
approximately one turn. The rich mixture, so
induced, will cool the engine, at the same
time providing increased lubrication.
5. Repeat this process, alternately running the
engine fast and slow by means of the
needle-valve, while keeping the throttle fully
open, then begin to extend the short
periods of high-speed operation until two
tanks of fuel have been consumed.
WARNING:
When ground running the engine, avoid
dusty or sandy locations. If dust or grit is
drawn into the engine, this can be a
ruinous effect, drastically shortening
engine life ion a matter of minutes.
6. Following the initial running-in session,
check for any looseness in the installation
due to vibration, then allow the engine a
period of moderately rich operation in flight.
7. For the first flight, have the needle-valve set
on the rich side and adjust the throttle trim
on the transmitter so that the engine does
not stop when the throttle is closed to the
idle setting.

BTW boxbeam.

The engine needs to be run what looks to be relativly lean to get running right. I think mine is around 1 turn out. It might just be mine but i have heard they just run that way also. Just for your information

I am also running 30 heli fuel with great results and alot of smoke!

GoodLuck

Cambo, how are you able to copy and paste from a PDF file?
I think I figured it out Cambo, thanks anyway.

Thanks Cambo. Your needle valve setting of 1 turn is seems to be about right. That is where it was when I got the engine, about 1 turn out. I am going to give it some "rich" time beause I don't know the history on the engine. Thanks for posting the break in procedure from the manual, now I know how it should be done. I use the 15% Wildcat, it has always worked well for all my applications. Smoke is sooo nice!!!

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As has been pointed out, this is an ABN engine, which usually doesn't have that much pinch at TDC, such as with an ABC engine. I doubt that doing as they advise would be as damaging to this engine as it would be in a high ratio tapered engine such as a tight ABC engine.

Conversely, while I suspect that their way might break the engine in a little sooner, I can't see where it would hurt to treat the engine as though it were a pure ABC engine and not quench it with lots of fuel by running ultra rich. It "might" just take a little longer to settle in.

It's just my opinion, but I suspect that OS has chosen to fulfill the needs of the instant gratification crowd, while disregarding those that love engines that will last twenty to thirty years without a major rebuild. Giving someone an engine that is quickly ready to hover can be a serious advantage in today's marketplace.

I doubt that the folks at OS do not know what they are doing. Even the latest crowd. <G>