Dears,
I bought a new OS 71 4 stroke to mount it on my Funtana s40, the engine was doing great and the plane was really fast, but suddenly after a couple of weeks (after breaking in) the engine started to lose power and now it barely flies the plane, i tried the high end needle but I couldn't get more power, I'm using cool power 5% fuel, please help me, I don't have enough experience with engines.

What type of compression do you have?

Did you check for an air leak in your fuel line? You should not see bubbles in the fuel line while the engine is running.

You should always check the valve clearances after breaking in a new 4 stroke engine. They CAN go out of spec.

My four stroke manual recommends checking the tappet clearance after something like the first 20 hours of flight.

It may be getting hot

Take the cowl off and see how it runs

use good 15 to 20%,check the valves,make sure prop is correct size.Perhaps change the glow plug.
5% is not enough.get good fuel,yours may have gone bad over the last bit of use.are you using a filter??

Sounds like your valve clearances are off to me, as my son had a similar problem, then he regapped the valves and all is well again. You really should go to 15% nitro too, as nitrowing suggested. That is what my son and I run on all our OS four strokes and it makes them happy. Jon

Thanks guys for all your help, I don't have a filter so this may be the reason, and about the 5% nitro fuel the engine was doing great with it at first before it started to lose power, I think I'll also change the prop. its a 13x8 size may be I need a smaller one.

A 13 X 6 would make the needle valve easier to adjust

Ok. Changing the prop to a 13x6 is a good idea as that is about as big as I can go on my Saito .72 without lugging it down and overheating it. By your own admission you have stated that you don't have enough experience with engines so is it possible to get an experienced modeler to help you?? Also, please heed the opinions of the experienced people on this forum who have suggested to "CHECK THE VALVE CLEARANCES". It's not a difficult thing to do but it is necessary to do at least once after breaking in the engine. I'm sure that is stated in your owners manual. Obviously something has changed for the engine to go from running great to barely running in the same plane. Valve clearances, bad fuel, high end needle valve way out of adjustment or using the wrong plug are the usual reasons.

A 13x8 is for a .91 - 4stroke in general...prop down a bit ,and do all of the above.Hope it works out.Get a filter for sure.Do the valves again,its likely they are out due to the strain of the larger prop.Learning how to set them is important and good knowledge.I used to reset mine often,because I abused the motor somewhat

I went today to one of the most experienced modelers in my area and we changed the prop to 13x6, we checked the valve clearances and timing, we checked the carburator and fuel line, then we tried the plane in the field; it was a little bit better but still not the power as it was before.

He told me that he thinks its not bad, but I'm sure that power has gone much weaker than before.

are you running all synthetic oil. if so throw some castor in the mix and that might help. ringed engines need some castor to begin with

I would suggest you put a filter on. Since you were running it without one I'd check the needle to make sure something didn't get in there.

Had junk get in my needle valve before. Now I run a filter from the jug to the plane and from the tank to the needle valve. Maybe over kill but I don't have that issue anymore.

Is it like sandy desert where you fly or grass?

If the carb is good and clean,runing a tank of new higher % fuel should help