I've never seen something like this !!!.
There is compression only when you turn the prop in a reverse way, otherwise no compression at all. This is an OS-91 FX that has been running excellent for about 100 flights till now.
I´ve dismounted the engine completely and everything looks fine. Any ideas ?

When you turn the engine over, side-loads are placed on the piston and the side of the piston that is loaded up depends on whch way you turn it.

It's possible that you've got a worn piston or a broken ring that isn't sealing properly when the other side of the piston is loaded (as in when it's turned the right way) but seals okay when it is loaded (ie: turning it backwards).

I had this with a TT91FS once -- but it had *zero* compression when turned clockwise and plenty when turned counterclockwise.

Hey, my 91 FX is like that too. I`ve been meaning to ask the very same question. It runs good by the way (12800 rpm on APC 13-6 with some modding)

Most of the time this is only a function of where the ring gap is.

Just had the same problem on a ringed MDS 148.

Piston ring stuck in the groove. Propably because I use 2 % castor and I had not run it for a while. GUM!

Dismantled the engine cleaned the groove and normal service was resumed. First or second flick starts.

The late Duke Fox (Fox Mfg. Co.) wrote an article about this. He said that when you turn it backwards against compression, you're putting the kind of loads on the piston that are applied during the power stroke. If it runs OK, as with another poster's engine, I wouldn't worry about it. It ought to start if you flip it backwards against compression.

Hi!
Are you sure you have the throttle fully open when you turn the engine over.
No air in... no compression!

On two strokes it usually makes little difference whether the throttle is open or not?

By that you mean clockwise is the correct direction? Some will refer to the direction as they face the engine. You flip the prop counterclockwise as you face the engine, but this actually rotates clockwise. This effect is not common with four stokes, but common with desaxe (spelling?) or off center bored two strokes.

I think a little bit of air might manage to leak in through the exhaust port .

It's quite common for a well run in ringed engine to have better compression when turned backwards but I'd say there's something wrong if it has no compression when turned the normal way.

The usual explanation for the difference is much like Charley said. The ring(s) wear in to give a seal from the side thrust loads caused by combustion pressures. Turning the engine backwards simulates the same direction of loading (but now only with compression pressure) so the rings are sealing the best they can.

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I used to get faked out by engines doing this sort of thing. I'd tear them apart and find nothing wrong. Put them back together and sometimes the same thing would happen, but sometimes it would vanish.

One of the other things that used to make me anxious was when an engine suddenly had a clunky sounding connecting rod. I later discovered that many engines will do this when they lose the oil between the wrist pin and the connecting rod big end. Whew!

Since I'm always riding Hobby People's donkey about charging too much, I thought I would mention that I just ordered a Magnum .91 two-stroke from them. Gotta let people know that I do like the folks at Hobby People and particularly their Magnum engines from time to time.

Ed Cregger