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tuning a OS 46 fx

Old 09-22-2003, 12:07 AM
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SBR_RV
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Default tuning a OS 46 fx

i am sort of new to this hobby and i am still not quite sure how to tune my motor.
what do i listen for whenrunning it up
Old 09-22-2003, 12:57 AM
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Rcpilot
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Default 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.
Old 07-17-2017, 07:52 PM
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Default Nice

Very comprehensive explanation on a tough subject to explain. I've been playing with glow and gas for 30 years and that's about as good as it gets. So newbies this should really help.
Old 07-18-2017, 02:50 AM
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Reviving a 14yr old thread are we? I think they call that 'necrobumping'. :O
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Old 07-24-2017, 04:54 AM
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Originally Posted by Mbyford View Post
Very comprehensive explanation on a tough subject to explain. I've been playing with glow and gas for 30 years and that's about as good as it gets. So newbies this should really help.
I agree. Should make that a sticky at the top of the forum.
Old 07-28-2017, 08:41 AM
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blw
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I'm surprised that advice stood all this time without someone pointing out that it is not necessary to do all that on a 2 stroke. Hmmmm, I was here in 2002 along with W8YE and Hobbsy. Oh well.

If you are taking a plane and trying to tune the 2 stroke heres how it needs to perform: it needs to rev as high as possible as smoothly as possible, and it needs to idle smoothly and transition to full throttle without a hiccup or as little hesitation as possible from the idle position. Some will set where they won't hesitate at all. Those are the best, of course.

So, to get all of the above in the most amounts the engine can give, based up manufacturing tolerances, design, materials, operator skill, etc run the engine wide open and set the high speed needle as high (lean) as possible. Then, back off about 400 RPM and leave the high speed needle alone. Don't tweak it and listen, etc. Maybe even fly the engine around for 5 minutes and get the engine up to operating temps, parts all hot and mated, etc. Make sure the hs needle is peaked and backed off 400 rpm. Now, let it idle about 25-30 seconds. This slows the engine down and allows the crankcase bottom design to do it's thing. You're letting the engine crankcase fill with excess fuel as it idles and this is a very important step. Now, push the throttle wide open and listen to the engine. You're looking for the engine to reach WOT as smoothly as possible. If it dies, the low speed is probable set too rich because the case if filling with fuel during the long idle. If it wheezes and acts like it's going to die them it's too lean. 90% of the time you need to keep leaning out the ls until you get the best transition to wot possible. Sometimes, engines like the Evolution 2 strokes need a hotter plug like an A3. But, that's how you do it. When you get your 2 stroke ls tuned you can probably leave it alone for years depending on type of fuel changes, etc. The engine will should become a very reliable engine that you can trust every time.......unless you have problems elsewhere.
Old 01-30-2020, 12:32 PM
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Originally Posted by blw View Post
I'm surprised that advice stood all this time without someone pointing out that it is not necessary to do all that on a 2 stroke. Hmmmm, I was here in 2002 along with W8YE and Hobbsy. Oh well.
No you weren't. Your username is registered in 2004. I made my original post 9 months before you even JOINED this forum.

Oh whatever. As if you and w8ye were the only ones who could tune a glow engine back in the day.

I'm surprised you waited that long to take another dig at me. You're wrong, of course.

Last edited by Rcpilot; 01-30-2020 at 02:52 PM.
Old 01-30-2020, 06:47 PM
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something else a newbie should know, prop size can alter where an engine's needles tunes and also alter how well an engine runs. as I have seen you surly do not put a super light Zinger 9X4 on a GMS 61, after 3 hours of tune-fiddling and changing plugs I told them to swap the prop to a 12 inch'er, problem solved in the 2 minutes it took to swap that prop. I have seen the same problem on a 4St 160.
a longer prop creates more "Flywheel Effect", more "Flywheel Effect" produces a better/smother idle too. so I like to suggest to newbies to use a larger diameter prop than the smallest one that the manufacturer recommends.

good luck !

Jim

Last edited by the Wasp; 01-30-2020 at 06:49 PM.

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