RE: tuning a OS 46 fx
Familiarize yourself with the high speed needle valve and the low speed needle valve. Your instruction manual should have a diagram of the engine which will show the needle valves.
I'm assuming that you've already done a bit of break-in running on the engine.
Start it up and run it to full throttle. Stand behind the motor- not in front of it. Slowly turn the high speed neelde in a few clicks at a time and you will be able to hear the engine gain RPMs. Continue to turn the needle in and listen to the engine gain RPMs. At a point, it will stop gaining RPMs and after a few more click it will start to slow down again.
What happens is that as you turn the needle in and the engine gets leaner, it will gain RPMs. At one point it will get to maximum RPMs. This is called Max Lean. After it reaches the max RPMs and you continue to turn the needle in ; it will get too lean and start to slow down and loose RPMs and power.
You need to find the max lean and then back the needle out the other way untill the RPMs get back up to max and then slow down a bit as the engine gets rich.
You can use a tachometer to watch the RPMs if you are unsure about what to listen for.
After you have adjusted the high speed needle, you can check to see if its right. Get an assistant to help you and run the engine up to full throttle and wait for it to get to max RPM. Then, point the nose of the plane straight up towards the sky. Your engine should either remain steady at max RPM or it might speed up a tiny bit and remain stable at those RPMs.
If, when pointed straight up, your engine sags or looses RPMs or dies; then you've not got it right yet. It will most likely be that you've got it too lean.
If it's too lean, just shut it off and set it back on the bench. Adjust the high speed needle out about 3 or 4 clicks and restart it again. Run the engine up to full throttle again and then point the nose skyward for a few seconds. It should run for at least 15 or 20 seconds with the nose pointed up.
A different way to test your engine is to get it adjusted to where you think its right and then simply pinch the fuel line going to the carb. Pinch it with your thumb and forefinger. Pinch it for just a second and then let go. When you pinch the line, your engine should speed up quickly. Whats happening is that your cutting off the fuel and causeing the engine to go lean. As it goes lean, it will speed up.
If, when you pinch the fuel line, the motor fails to speed up and dies quickly- then your too lean. Adjust the high speed needle out a few clicks and repeat the test.
You can use the "pinch test" and the "point it skyward" test in combination with each other to assure that you've got it right.
Always adjust the HIGH speed needle FIRST. Get it right before you attempt to adjust the low speed needle.
Okay, time to adjust the low speed needle;
Throttle the engine back to an idle. It should idle around 1900-2200RPM. Maybe a bit more or less, depending on your altitude and weather conditions.
Let the engine idle for about 10 seconds and then run it to full throttle very quickly. It should zip up to full throttle very quickly without dieing or sputtering.
If it dies suddenly when you give it full throttle- then its too lean. Back the low speed needle out about 1/2--3/4 turn and restart it and try again.
You want to establish a good idle and transition to full throttle with no sputtering or burping.
Let it idle for about 10 --15 seconds and then flip it to full throttle quickly. If the engine sputters and coughs as it gains RPM and eventually gets to full throttle- then it's most likely too rich. Shut it off and adjust the low speed needle in about 1/8 or 1/4 turn at a time.
Its all trial and error with this.
You will need to fiddle around with it for a bit untill it gets just right. Your engine should idle well for at least 15-30 seconds without any trouble. If it idles for a few seconds, say 10 seconds and then starts to slowly get rough idle and sputter, you can bet that its probably rich. If it does this- flip it to full throttle and listen for it to cough and burp as it slowly gains RPMs up to full throttle. Thats a sure sign that the low speed is too rich. Adjust it in about 1/8 turn at a time and continue to try it.
If it idles for about 10 seconds, slowly gains just a few RPM, and then dies suddenly, it's too lean. If its idleing and you try to flip if to full throttle when its too lean-- it will die suddenly before gaining any significant RPMs. If this happens, back the low speed out about 1/8--1/4 turn at a time and try again.
Do this next test with the engine off!!
One trick for getting the low speed needle close is to set your throttle stick at idle and adjust your throttle trim to where the barrel of the carb is just barely open. Just where it would be for a nice slow idle. Attatch a piece of scrap fuel tubing to the carb nipple and try to blow through it with your mouth. You should be able to just barely blow a bit of air through the carb . Thats about all the fuel that going in at idle. You should just be able to blow a small amount of air through it. If you can blow enough air through it to blow out a candle- then its gonna be too rich. Adjust the low speed needle in a bit and try again. If you cant blow any air through it, then its probably gonna be too lean. Adjust the low speed needle out a bit.
This should get you started. I'm sure I forgot something. Others will be able to help you with a few more tips.