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Still need help with tuning

Old 05-24-2008, 10:11 PM
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Jetdesign
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Default Still need help with tuning

I'm not having any real problems, but I really don't feel like I've got my OS 46AX dialed in right yet. I've got about 2 gallons of fuel through it over 7 or 8 days of flying.

I do the pinch test on a regular basis; I pinch the line going to the carb, and the RPM's increase. Sometimes it won't even let me get to the point of where it would decrease before it will stall. When I tune my engine, the grass is SOAKED under the exhaust port, and people have commented about how much liquid is coming out of the exhaust. My flight times are also very short (going through lots of fuel). Again, when I try to lean out the needle, the engine wants to quit. Also if it's leaned out, it makes almost a rattling vibration sound at mid to high rpm, like the seal is not tight enough. I don't know if it's my imagination, but a few clicks rich seems to get rid of this noise. The plane does want to JUMP when I hit the throttle, so there seems to be plenty of power when advancing from idle.

My trainer seems to think it sounds fine. It idles well, and there's slight hesitation or no hesitation when the throttle is then opened to full, so he says to leave it. Ever since I switched from 10% to 15% nitro it just doesn't seem to be running as PERFECT, again, nothing really wrong, just not appearing to be as dialed in as it was with the 10% fuel, and it's tearing through tanks of gas.

I am going to get a tachometer. Is this the most accurate way to tune an engine? Is it agreed that you lean the needle to get max rpm's, then richen the needle to lower the rpm's by 300?

Thanks. I know this has been covered many times, but I don't think I'm missing anything...
Old 05-24-2008, 11:05 PM
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Campgems
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Default RE: Still need help with tuning

Kat, a tach is really a must for peak tuning.

Two strokes pump a lot of unburnt fuel out of the exhaust.

Go back to the 10% Few of the 2 strokes need 15%

Last, find the adjustment and then don't screw with it. Every time you pinch the line, you stand a chance of knicking it. Every time you turn the needle valve, you wear it down some.

Here is what I suggest. Make sure the carb is seated well, the O ring is compressed. Make sure the O ring on both the high speed and mix control are good. Set both to inital settings. Adjust the high speed for maximum rpm less about 500 rpm (once you have the tach). Set the throttle as slow as you can and keep the engine running. At this setting adjust the idle lean for the maximum RPM. Note that the idle changes need to be made very slowly so as to allow time for the fuel in the crank case to clear. Once you have the idle set, go back and re-adjust the top end, Go back and adjust the idle one more time. Now do you nose up test and then your pinch test. Go fly and don't screw with the needles until you really see a change in performance. Even then, it should be one click to two max on the high speed.

We have guys at the field that constantly "tune" their engines. We have others that get them out of the car, fuel up start it up and fly. They don't need a lot of adjustments. They will need a click one way or the other as the temperature changes a lot, or the humidity goes up or down a lot, but basicly they are a set and forget carbs now.

Last question, You say you are going through a lot of fuel. How big is your tank and how long does it take to pump it empty by running the engine at full throttle? My 52 four stroke will give me a full 15 minutes on a 8 oz tank. Your 46 two stroke should probably go for at least 10 minutes. If you are getting less time, check for tank leaks. I had one split at the bung hole, an ARF tank. You have to be careful of over-tightening them because they will split.

2 Gallons in eight days, man you are living at the field. Great fun isn't it.

Don
Old 05-24-2008, 11:32 PM
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Jetdesign
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Default RE: Still need help with tuning

Thanks a lot, Don...will do all of the above. I believe you when you say there's no need to fully tune an engine once it's been set. Mine was running great and I didn't want to touch it, but I had to switch fuels, so retuning was in order. Hopefully will get back to good setting and leave it alone.
Have a good holiday!

Oh, and yes, I like to fly my plane
Old 05-25-2008, 07:49 AM
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da Rock
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Default RE: Still need help with tuning

When the engine is warmed up and running with the throttle wide open, simply open the needle valve until you're absolutely sure it's rich. Then slowly turn it in a click at a time. It will slowly speed up until it's screaming. And as you continue to lean it out it will reach it's highest rpm and then start to slow. At the point of highest rpm, the fuel mixture is perfect for running right there on the ground. Of course, you want it perfect in the air, not on the ground.

As you're clicking in on the needle, listen for when the rpm passes that perfect mixture point and the rpms start to lessen. As soon as you are sure the engine is sagging off or going too lean, back the needle back out to that perfect setting. Then go another 3-4 clicks richer.

Now point the nose straight up. If the engine sags (rpms drop off) you're still too lean. It should pick up rpm just a bit. You can also do the pinch test, but the nose up deal almost exactly duplicates what happens in the air compared to the pinch test. Nothing pinches the fuel line in the air, but airplanes sure do climb often.

Now, take off and do a circuit or two. Look and listen for the way the engine runs. Plan to just do a circuit or two and land. But if the engine doesn't sound too rich or too lean in the air, go for it. If it looks, sounds or feels too rich or too lean, then land it and give the needle a couple of clicks. And take off right away to see how that worked. It won't take but a couple of tests and you're good for the rest of the day.

Once you've gotten that needle setting, you won't need to mess with the needle the rest of that session. After awhile you'll learn how many clicks affect the mixture for that engine.

It's always a good idea to check your needle the first flight of every flying session. Start the engine and let it warm up. Then richen the needle until the rpms drop. That insures you're starting from the safely rich side of the mixture setting. Then click it in to find the peak, where you'll back it off whatever you've learned is about right. With most 46AXs it's around 3-5 clicks richer. And plan to observe how the engine runs on that first takeoff of the day and the first couple of circuits.

We don't have to pay takeoff fees. So it's no problem to land right away to give the needle a click or two.
Old 05-25-2008, 08:04 AM
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da Rock
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Default RE: Still need help with tuning

BTW, after you've done this simple and short little drill a couple of times, you're going to learn how that engine responds to the needle. And you won't need a tach. Truth is, unless you know beforehand what rpms to look for, you still have to make a judgement about rich/lean and the tach simply tells you what that means in rpms. And until you learn the sound, you'll need the tach again and again.

The "adjust it to rich, lean it to max rpm, back it out 4 clicks" routine is simple and effective. And if you do that the first flight of every flying session, you'll be insuring that the mixture is set for whatever conditions are each new flying session. Temperatures often change in the Fall and Spring from session to session. And change enough that the mixture will be wrong. Pressure changes. We fly at different fields with different altitudes. The "too rich, too lean, just right" routine covers it all.

Also, if you're flying 4cycles, you probably will need a tach. But they're not really used much by 2cycle flyers and for good reason.
Old 05-25-2008, 08:08 AM
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da Rock
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Default RE: Still need help with tuning

The pinch test is an excellent one for setting the lowspeed needle. Truth is, the best test for low speed is how the engine handles a quick idle to WOT stick movement. If it doesn't stumble, forget the pinch test. After all, the test that matters was just passed.

But pinch tests aren't so good for your primary needle setting test. Because the nose up more closely tells what needs to be told.
Old 05-25-2008, 08:57 AM
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Default RE: Still need help with tuning

All of the above information is right on!

One item that I would like to add regarding setting the idle mixture....
I call it the idle test. I let the engine sit at an idle for 30 seconds, I then advance the throttle to full and check for any burbling or hesitation.

Ryan
Old 05-25-2008, 09:13 AM
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Jetdesign
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Default RE: Still need help with tuning

Thanks guys. The idle test could be a key component here. My trainer usually has me idle for 10 or 15 seconds, then advance to WOT, and this gives a slight hesitation. 30 seconds I imagine will give a real hesitation. Will check it out when I get home.
Old 05-25-2008, 10:27 AM
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ChuckW
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Default RE: Still need help with tuning

If it hesitates bad or dies, then the idle mixture is likely rich. Every engine I have ever had was at least a little rich on the low end from the factory. smalll adjusments are key on the idle mixture. Go 1/8-1/4 turn at the most each time.
Old 05-25-2008, 01:46 PM
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Default RE: Still need help with tuning

The high speed needle on most engines I've run only takes over completely after the throttle is open 75% or greater. A lot of your inflight symptoms can be coming from the low speed circuit in the carb.
It is always good to be sure that the basics are up to snuff before spending time tweaking the needles...backplate, carb neck and head is sealed, glow plug is in good shape and the fuel tank and lines are leak proof from the exhaust pressure tap to the tip of the feed line. You also want to see no air bubbles coming from the tank. Meet all these conditions first, then move on to fine tuning.
I always set the high speed needle with the model pointed straight up, this is as close as you can get to simulating the worst case running position.
Old 05-25-2008, 04:16 PM
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da Rock
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Default RE: Still need help with tuning

The idle check is a deal all to itself.

You want the engine warmed up completely. And you want the engine cleared out when you start the idle run, no matter how long you plan to let it idle. So.................

Let it run awhile when you got her cranked. If you crank her with the throttle just a couple-three clicks opened, then let her run like that as you pull the glow igniter and move around behind the prop. While you're doing that, you'll very probably hear the rpm pick up on it's own just a bit as the old oil gets pulled out of the bearings and nooks and crannies. By the time you're around back, you can advance the throttle to wide open. Let her run for a bit like that and........

you're cleared out any residual fuel that might cause your idle-up test to stumble
you've now got a warmed up engine
when you pull the throttle down to idle the engine will be coming down like it does in the air, cleared out and warm

NOW let her idle for your test. When you now hit the throttle to see if she'll stumble, you'll actually be testing from a condition that's more like flight conditions.
Old 05-25-2008, 10:58 PM
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Default RE: Still need help with tuning

How do you check for air leaks by the way?
Old 05-26-2008, 09:38 AM
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Default RE: Still need help with tuning

I know a guy at a mower shop who quickly diagnoses weed eater engines by sealing off the exhaust port, and he introduces a low pressure air line into the intake. The engine is then dunked into water and any leaks are obvious as he turns the motor by hand. I've tried this on model engines and have found pesky problems. Usually, you don't have to come to this point until after you've examined the engine either while it is running or with the crankcase flooded and turning it over by hand. The other test to be done by hand is with the engine primed, bring the piston up to TDC and time how long the cylinder stays pumped up. A good engine will hold a seal for several seconds or more. If it only stays pumped for a couple of seconds it is marginal.
Turn the prop slowly by hand as you side load it and feel for axial play in the shaft, there should be none. If so, there will be air and fuel leaking out the front end.
Old 05-26-2008, 09:44 AM
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Default RE: Still need help with tuning

Well my plane has not been flown or started since Wednesday (5 days now) and when I just removed the line between the carb and the fuel tank it sucked in a bunch of air. I'm going to take this as a sign of no air leaks!
Old 05-29-2008, 07:46 AM
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Jetdesign
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Default RE: Still need help with tuning

So I borrowed a tach at the field yesterday and adjusted the high speed needle. Someone even commented who showed up a few hours later about how good my engine is sounding. (He got to the field and asked if I'd leaned it up yet, I said I think so, and at the end of the night he said it sounded great.) So thanks for the help with that.

I still have to work on the low end. When I start the engine, I have to slowly advance to WOT or it will stumble or even stall. But, once I start it, and get it up to WOT, then back to idle for 30 seconds, it will go back to WOT pretty much no problem. Is this normal, or a sign of a rich low end needle?

Again, thanks for the help, she really ran beautifully yesterday. I can't say enough how much I love my OS 46AX. Every time I order something online, I think about ordering a starter and battery. But I can't, I just can't justify spending the $50 when my engine starts on 3 flips for the first starting, and ONE FLIP after it's been warmed up. Every time.
Old 05-29-2008, 08:57 AM
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Default RE: Still need help with tuning

I've got the O.S. 46fx and it always needs a little warm-up before it will give a smooth transition from idle to WOT. Sounds like you've got it dialed in pretty close now. Yeah, electric starters are nice but not absolutely required. They really come in handy,though, when you're starting a cold engine in the winter.
Old 05-29-2008, 09:03 AM
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Default RE: Still need help with tuning

When you first start the engine, leave the glow igniter attached until you have run the engine up to full throttle. Give it 5-10 seconds at full throttle, then reduce throttle and remove the igniter. This allows the engine to clear out the excess of fuel that often builds up during the starting process.

Brad
Old 05-29-2008, 09:06 AM
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Jetdesign
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Default RE: Still need help with tuning


ORIGINAL: bkdavy

When you first start the engine, leave the glow igniter attached until you have run the engine up to full throttle. Give it 5-10 seconds at full throttle, then reduce throttle and remove the igniter. This allows the engine to clear out the excess of fuel that often builds up during the starting process.

Brad
Oh. Didn't know that. Definitely been taking off the glow ignitor just after it starts up, at just over idle speed. Thanks.
Old 05-29-2008, 10:42 AM
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Default RE: Still need help with tuning

Yup, what you're experiencing is entirely normal. It's a result of the priming you did when you started the engine. The first blast at WOT burns it out, and then things are normal.

Leaving the driver on till it's revved up a bit helps alot, as noted.

It's a great little engine, isn't it? When you get hungry for a touch more power, if your field allows, remove the baffle from the muffler for a free 400rpm, and a meaner sound... If noise is a potential problem at your field though, resist this temptation....

But for now, you sound like you've got it figured out. Enjoy!

J

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