my dad bought himself a Nexstar ARF plane. it came with an OS 46. the first time we flew it we had experienced pilots tune the engine. so iknow its tuned correctly. Ok so we rev it wide open on the ground, we see air bubbles in the tank to engine line, but its not affecting th engine so we fly it. In the air the engine was racing every once in a while like its overspeeding, its like the bubbles got into the engine and leaned it out. and then we had it die and we had to dead stick it. but wehave no ideawhats causing these bubbles. the fuel tank looks perfect, all the lines are perfect. and we have no modified it in any way, the tank is in a lot of foam. Anyone know what the possible problem is here?

there might be cracks in the lines inside the tank. also check to see if the stopper in the tank is in tight. lines from the tank might have a small nick, just because it is new does not mean it is not cracked

You might want to check this out:

[link=http://www.rcuniverse.com/forum/m_2120620/anchors_2120620/mpage_1/key_nexstar%252Cbubbles/anchor/tm.htm#2120620]http://www.rcuniverse.com/forum/m_2120620/anchors_2120620/mpage_1/key_nexstar%252Cbubbles/anchor/tm.htm#2120620[/link]

-Mike

thanks, that threads a lot of help

never had the bubbles but I did have my stopper come loose and leak fuel all over the place after just a few flights, also make sure the clunk line in the tank is on the bottom and not tangled up inside the tank 9 out of 10 times bubbles are from a air leak.
my NEXTAR has tons of flights on it and does not have any foam around the tank. the tanks supplied with this plane are not the best so its possible there is a hole . try this, take some new fuel line put it on the fill tube and plug the others put the tank in some water and blow into the line if it is leaking you will find out fast , also double check to see if the lines are hooked up properly inside the tank the one I have is solid white so you can't tell unless you unscrew the stopper even if it seems like its O.K. best to make sure, mistakes do happen during assembly and hoses get switched it has happened to a few on RCU .I did this to myself switched the lines on my tank after fixing the stopper the engine would run because there was plenty of fuel but would quit after a minute or so realized I had hooked up the fill line to the engine after mixing up the different colored hoses . that was a long time ago LOL

Yeah check for air leaks. Its possible that you may have an air leak from the fuel tank or in the tank-to-engine line.

did you balance your prop?

In my RC career, I have balanced exactly 0 props. Unless its been busted, a prop on a 46 sized motor isnt going to be out of balance enough to cause an issue. Big planes or Helis...balance them...little throw-around birds like the nexstar...dont worry about it.

I had the same problem with my EVO .46. I changed out all the lines, with the exception of the lines inside the tank. That fixed it for me. I inspected the lines I removed for hours and couldn't find any cracks or anything. Go figure[sm=75_75.gif]

Pinholes in fuel tubing are really hard to spot most times...

You said the first time you flew it you had experienced pilots tune it, so you know its tuned properly. This is an incorrect statement. The high speed needle should be checked every day at the beginning of the day. Atmospheric conditions change on a daily basis that affect the high speed needle. If you tune it on a hot, humid day, the air is thin and full of water, requiring a leaner mixture. Then when you go to fly on a nice cool, dry day, the needle will be set too lean, and the engine will overheat and deadstick.

High speed tuning is simple, takes a few seconds before the first flight of the day. Its easiest with a tachometer, but can be done by ear. After starting the engine, go to full throttle. Now turn the high speed needle a few clicks in the richer direction. You should hear a slight drop in rpm. If you hear an increase in rpm, you are lean. Continue richening until you hear a slight drop in RPM. Now your ready to go. If you hear a slight drop in RPM when you initially richen the mixture, then start leaning out the mixture until the rpms no longer increase. Now go back in the richening direction until you hear a slight drop in RPM. Go fly.

Bubbles in the fuel line shouldn't be a problem unless your seeing them in the line between the needle valve and the carb.

You also may need to adjust the low speed mixture as the engine breaks in. If the engine is deadsticking when you try to go from idle to full throttle, you may need to adjust the low speed needle. This takes more time and patience, and should be reserved until you've run several tanks of fuel through the engine. The good news is that the low speed needle setting should never have to be readjusted once you get it properly set.

Brad

This is where alot of pilots disagree.

A good engine shouldnt need retuned every day. If the weather is DRASTICALLY different, then yeah, But I might touch my needles 3 times throuout the year on my 2-strokes, even less on the 4-strokes.

I've seen so many guys waste time from messing with their engine. You may move 100 RPMS either way with normal changes in weather, but thats all. Now, the difference from winter to summer is DRASTIC so yeah, it needs to be tweaked a bit there...but not much.

Now...if you run some of the "cheap" engines...then you will be doing more work, but the OS that the nexstar has in it is just fine.

For the record...I own a nexstar. I've had it for 2 years. It was tuned on the first day...I havent touched the needles since...I can tell a power difference in it sometimes, but its not a pattern ship or a speed ship, so im not that worried about it.

I've noticed the connection though, at my field, the guys who are always playing with their needles are the same ones who are always yelling "deadstick"

Are they the same ones that always tune too lean? That's probably what is happening here IMO.

OMG, gotta get that last revolution out of the engine, after all I paid for it. It's a trainer for gosh sakes, what's the hurry? It shouldn't be at full throttle anyway. It makes me sick to watch the needle tweakers at the field. They can't start ANY engine without tweaking it. They keep coming over and want to tweak mine and I have to beat them away. [:@]

Bruce, I get that too. Im still kinda "new" 2 years, but maybe 20 visits to the field..life keeps happening, though lately ive been on a roll

but since alot of people dont know me, and see the trainer (i love to fly the nexstar, i love making it do things a trainer shouldnt do!!!) they come over and want to "help" me tune it...

Case in point...about a month ago I started back heavy...hadnt flown since about april. April is cool and dry, august is hot and wet. No after run oil (i forgot it, or i would have) and no touching my needles..get out the starter, my 12v batt. is dead...started with a stick, it took one try and it was purring.

The best spot on the needles isnt where you get the most rpm, its where it runs the best, and not for one flight or one day, but for long periods of time.

I am a tweaker
OTOH, I never have deadsticks

My engines get tuned when being broke in, then get dialed in a little better after they are broken in & that's it. After that I pretty much don't touch them. Since it aint broke & it runs great, I aint trying to fix it! No Tweaking here.[sm=75_75.gif]

Well, I'm going to have to disagree with you RJM1982. First of all, I'm not a "tweaker". And I'm not trying to wring out every extra RPM that I can out of an engine. But there are days that I get to the field and I may have to move the needle one or two clicks to deal with the weather that day. Here in Oklahoma we also deal with 40°+ temperature differences in a day. It can be 60° on my first flight of the day, and 100°+ on the last flight. The needle usually has to be adjusted sometime between those flights (usually to make it richer so the extra fuel will keep the engine cooler). You can't just set your engine and forget about it, you have to deal with the conditions on the day you are flying. I'm not saying you need to tune it before every flight (those kind of people drive me crazy), but it should be checked at the start of the day. Those that live in milder climates usually can get away with not checking, but those of us that live in more "interesting" climates have to deal with changes like this.

Ken

I also disagree with the "set it and forget it" approach. As I stated, it only takes a few seconds to check it at the beginning of the day. Sometimes its a few clicks different, other times it isn't. But its part of my preflight routine to make sure everything is working OK.

BTW, I've had three deadsticks in two years, and those were because of glow-plug failures, not tuning issues.

To each his own.

Brad

Hey guys!!
It might sound stupid but Im not sure of one thing.. What do you mean by low speed needle valve and high speed needle valve. There is only one valve you adjust on the engine?? Or does it mean Adjusting the engine at idle and adjusting at full throthle ??

Sorry for stupid question but thats whats bothering me for some time.
Thanks!! Patryk..

Oh and my engine is the O.S 70 FS so If you people had an experience with it you may give me a hand ..
Thanks !! Patryk.

All engines have a high speed needle. Thats the large one with the knurled knob. The low speed mixture is controlled by one of two methods. One method is an air bleed hole that meters the flow of air into the carb when the barrel is approaching fully closed. The second method is a low speed needle valve. The low speed mixture adjustment is typically located inside the throttle control arm attachment to the barrel of the carb. Is usually a small screw, and should only be adjusted in very small increments (1/8 turn) and should only be adjusted when the engine is not running.

Check your engine manual.

Brad

Thank you Bkdavy!!
You were really helpfull. Now I know approximately whats going on with the low and high speed needles on the engine.
I can see a small screw just next to the High speed needle knob. Ill post a picture later on to show if it is the on ok .. The manual does not say anything about the low and high speed needles.

Thanks!
Patryk.

I looked at the FS70 Ultimate manual (http://www.osengines.com) and there is a section in that manual about the "idle mixture adjustment". The exploded view shows the "mixture control valve assembly". That is the same as "low speed needle". The manual describes how to adjust it as well.

Brad