Hi Chaps

I have an OS.50 2-stroke engine that's about 18 months old. I have looked after the engine really well, but lately it does not perform as it used to! It does not run badly but it does not peak. When I turn the prop over by hand it feels like there is no compression.

What could the problem be? Would it help if I replaced the gasket seal of the engine head? Or perhaps the backplate gasket?

Thanks

More than likely you will need a piston ring and or liner.

I just went through a very similar problem with my OS 50. I found out I had been running too lean and had actually burned a hole in the top of the piston. DUMB!!! Pull the head and take a look. It may look like a small crater or small cracks. Either way you will need a new piston if this is the problem. I replaced my piston/ring and it runs fine now. I also run it richer now.

Did it look something like this?

More in the center and darker. Took forever to start but it did and even flew but weak. Didn't draw fuel very well, had to prime carb manually. Took it off and replaced with a K&B 61 pumper for a while, until parts came in.

I have an OS 46FX on my flying lawnmower and all summer it has been running great. I use an APC 11-5 prop on it. One day it would not peak in RPM. I tried everything; high needle valve, low needle valve, new plug, another new plug, more adjustments. Nothing worked. After a weeks of thinking about it I decided on less prop since the field temperature was about 50 degrees colder than in the summer. Put on a Mac 10-6. The engine peaks again and the mower flies about the same as it always has. The key was not being able to peak it on the ground. The engine was over proped due to the colder heavier air.

I was asked to rebuild a OS 50 SX that was lacking in power.

I found that the piston was dished. the guy had flooded the engine and forced the piston through top dead center. The resulting pressure caved the piston in. That engine ran, idled great, etc but just din't have much power.

He had a second 50SX that developed the no compression syndrome. His rear bearing had lost the bearing retainer cage. It was plastic. But the plastic parts did something to the ring. I replaced the bearings and the ring and she was good as new.

Enjoy,

Jim

Thanks chap! I'll check out the pistom amd the ring/liner!

Let us know what you find, please.

I have just taken the head off of my OS .50SX. The piston seems to be in good condition except for some carbon deposit. However, there was no gasket or seal between the engin block and the head. The bolts of the engine head were also not very tight. This engine has never been opened before. Do they come without a gasket between the head and the block?

Should I put a gasket there? How do I remove the carbon deposit?

I don't know if you still have your manual so here is a copy of it.
http://www.osengines.com/manuals/50s...1fx-manual.pdf
Your engine is on pg 26.

The head gasket is extremely thin and hard to see. The drawing shows a gasket but I can't honestly say that I have ever seen mine. It may be there, or not.

You may need to go ahead and remove the piston to examine the ring. Be sure to carefully mark all parts so they go back correctly. Oiling holes in the connecting rod and such.

They have a gasket. It is .004" thick and is copper.

Enjoy,

Jim

How would a worn out ring look?

I would say to put some fuel in the combustion chamber and turn the engine through compression to see where the oil goes. Will it come out the head gasket area? Glow plug? blow down past the ring?

Enjoy,

Jim

The ring could be broke in half (not likely), deep scratches, siezed to piston with carbon, sleeve/liner could also be scratched. Crack in piston top could be so small that it is hard to see also. If it is out of engine turn the piston upside down and put in a few drops of fuel or alcohol and see if it leaks thru, mine did.

Hi!
I don't think OS use copper gaskets...the only OS gaskets I have seen have been aluminium gaskets.
The gasket in your OS.50 is only 0.1mm or 0.2mm thick...just use a sharp needle and try to bend it loose from the head...it is usually stuck there.

Regards!
Jan K

It's important, of course, when putting the head back on to get it on nice and square. Then gradually tighten the screws in a criss-cross pattern until you reach final torque.

Jaka, You are just speculating.

I have a OS .50SX gasket in front of me and it is copper and .1mm thick.

Enjoy,

Jim

I have placed the head back on the engine block and torqued the bolts. There seems to be a lot of compression now! Like I said before, the bolts were not very tight when I took the head off. Perhaps that was the problem; it was leaking through the gap between the head and the engine block.

Once I've started it again I'll let u know whether the engine performs better than before.

Let's hope that is all it was. Cheap fix.

Jim in all fairness to JAKA He is in SWEEDEN and we are USA Maybe OS has different shims for engines shipped to europe or usa. I don't know.

O.S. engines usually have aluminum cylinder head gaskets that look like an integral part of the cylinder head. Many times, we've had people comment that their engine didn't have a cylinder head gasket, when they actually did. Some though, like the .50 SX, and others, do have a brass cylinder head gasket.

I ran my engine this weekend. It doesn't seem to peak. The compression seems to come and go as the engine is hand cranked.

I opened it up again and inspected the ring. It seems to have a small indentation on one side, so I guess I'll replace it. I'll let you know what happens.