Hey guys,
I just flew my avistar again and there seems to be a big problem with the engine, i always fly for about 5 min and then the motor is going slower and the motor is going up and down (slow and fast) for 20 seconds and then it just quits, my tank is about 3/4 full and I had the problem on all my flghts today (Always made a nice dead stick landing)
Where could the problem be? I checked the tank yesterday and everythink semmend fine! Mabe the needle value?

-Nik

It sure sounds to me like you have one of two problems. Either you have something obstructing the fuel intake ... clogged filter, or gunk in the carb, or there is some type of air leak in there. Double or triple check all of your connections, make sure your filters are all clean, make sure there are no pin holes in any lines.

I have an Irvine .40 that drove me crazy with this type of problem and I found that there was an air leak between the carb and the manifold. Some high temp RTV sealant around the carb and all problems cleared up.

You may have more help if you give the details of the engine.

Just my 2 cents ... Bo

Air leaks in the fuel system, or around the needle valve are good possibilities. I'd also make sure that you're not running it too lean.
Dennis-

Well if it there are leaks in the fuel system why does it run for 5 min without any problems? Also how can i make sure the engine is not too lean?

-Nik

Try the obvious, and just open up the needle valve a few clicks and see if the problem goes away.

If that doesn't work, it could be an air leak inside the tank, in the clunk line. When the tank is full, the engire clunk line is in the fuel, but as the fuel level drops, the leak is now exposed to air and sucks in bubbles. Depending on the tank, this can eaisly happen where the clunk line attaches to the tubing that goes though the stopper.

Also check to make sure the clunk is not stuck against the front of the tank.

Make sure you don't have the fuel lines reversed. Usually you get shorter than 5min flights that way, but it's worth checking.

Easiest way to check is to see if you can pump the tank empty. If you can't empty the tank with your fuel pump, then you know you have a problem.

It could also be fuel foaming.

Next time it happens, take the plane back to the pits, and re-start the engine with out adding more fuel to the tank. Take a close look at the fuel line. Do you see air bubbles? Pick up the plane, and hold it nose up. Does the engine keep running with either a slight RPM increase or no change? If you notice the RPM drop off, or the engine quits, you are too lean on the needle or have some other fuel draw problem such as the ones above. Usually it's too lean on the needle valve.

Two easy tests to check an engine to see if it's too lean.

Both are best done with the tank 1/2 full of fuel, though they work ok with a full tank as well.

Both are done with the engine running at full throttle.

First, pinch the fuel line briefly with your fingers or a clamp. You should hear (or observe with a tach, but I do it by ear most times) a slight RPM increase. No increase means you are right on the edge, best back off a click or 3. An RPM decrease means you are too lean. Of course, if you hold the pinch long enough, the engine will drop off and die as you cut off the fuel. But you should still hear an RPM increase before it drops off.

Second, lift the airplane and hold it vertically nose up for several seconds. Again, you should hear a slight RPM increase. Any drop in RPM means you are too lean.

Some guys who have 3 hands do both, hold the airplane vertically, then do a pinch.

How much foam do you have wrapped around the fuel tank, and is this foam still uncompressed? An/or how well-balanced is your prop?

I'm thinking along the lines of fuel foaming.

It is probably overheating because there is some obstruction in the muffler, or the muffler itself is too restrictive. The other possibility is of course, too lean. What happens is it will run erratically and then finally freeze up. Try running it at a lover throttle setting, with the needle valve opened up a couple of clicks. Tell me what happens

Allright guys thanks for the help!
Now i have one more question: When you go under the RTF Avistar accesories page they say you need a preciosion filter: http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXK134
Do i really need it? (It doesn't say anything in the manual) Also where should i put it?

-Nik

That filter just goes between the tank and the carb. I don't use them. I didn't catch what engine you are running. I have a couple OS engines that are doing what you are describing and it seems to be the needle valve leaking. I wish they would put them back in the carb where they belong. I am almost to the point of selling them and getting a different brand. A few more tests will tell. I took off my fueler and will see if maybe that is the problem. I am hoping it is. I have the same problem on 3 planes. What a pain.

i got a os .40 la

Ok i think i could fix my problem with installing a fuel filter but where should i install it? At line 1 or 2? (See Picture)

Put it in Line 1 so it's before the needle valve. This is what you must protect against getting any dirt in. The opening is so fine that the smallest speck can badly affect the mixture making it go lean. Lean is what kills an engine.

I rather doubt this has anything to do with particles in your fuel. It's either too lean (my guess) or an air leak in the tank.

Check presure line from muffler to tank.

My guess is fuel foaming. While the Avistar is a great plane, the inherent design flaw is the fuel tank. It is mounted directly attached to the firewall and very prone to vibrations thus causing the foaming.

Make sure your prop is balanced as perfectly as you can get it since an unbalanced prop will cause the most vibration. If that does not work you my have to make some modifications. If you pull the tank out of the hole in the firewall you will have to seal that hole up somehow to keep fuel from leaking inside. Wrap the tank in foam and secure it.

Good Luck!

of two os engines are doing that, you aredoing something wrong. Os engines are ONE of the best names in miniature glow engines. As i have stated in other threads, i have not had much luck with the la series, but i am 100% impressed with their four strokes, and their higher-end engines such as the fx, sx, and i have heard only miracles about their fuel injected engines aswell.

How do you fix a needle valve leak? my Os46 Fx has the same problem. Runs fine for 2-3 mins and then quits. Checked for all leaks,pinch test, foaming etc. I am suspecting it is a needle valve leak. The problem is that engine reaches peak rpm when the main needle valve is open only half turn from fully closed. It does all sorts of fluctuating and coughing and then quits. I'll never buy a Os engine again.

As spaceclam said the higher end engines are great, but like NIK I also have an os 46la that does the same thing, I have plenty of experience with smaal engines, I set it, fly it & it will run great for 2-3 minutes. Then it will start slowing down, loosing power until it is a dead stik. I have tightened all the engine screws thinking I had crankcase pressure leaking or head leaking, made sure the carb was sealed good to the crank case. This engine has maybe 20 flights on it & I have tried every thing I can think of to make it run right to no avail. It ran good for about 12 flights & has been nothing but trouble since. I have quit trying & wrote it off as junk!!
Any Ideas???
Gil

I've got the same problem with a 40LA, I was told it could be dust inside the cylinder as well, my be worth to check that as well.... I started haveing this problem after a crash so I'll take it appart and try all the advices in this thread as well

I have the same problem with a K&B .61, and it's fuel foaming in my case. Plenty pressure from the muffler combined with an RTV-mounted tank. I'm going to try adding a few drops of Armor All.

Try this:
Get the engine started on the ground, and set the needle right at peak. Then push the needle in varous directions, but don't turn it. If you hear RPM changes, the o-ring isn't sealing. Put a bit of fuel tubing over the needle to help seal out the air. It's a "standard" thing I do to my Magnum engines to fix mixture funnies. OS engines usually don't need this, but it can't hurt.

When the engine is running, do you see air bubbles in the line before the needle valve or between the remote needle and the carb?

You might have an air leak in the carb. When you installed the carb, did you really push down on the carb as much as possible before tigtening up the screws? You really want to compress that o-ring.

a leak, or crud, thats my guess. I always fly with a filter, but you have to be careful with them also, cheapy filters can develop leaks. I like the types where I can screw them together real tight. I even have a filter in my bottle as I draw fuel out of it and into my tank. Double filtered before it even gets to the needle valve. Be very careful with your fuel bottle. Don't leave it open for extended periods and don't let dirt like grass particles get into it. Leaks can be anywhere, check all fuel lines, and bolts on the motor. If you have a loose backplate or head, you're definitely going to have those problems listed. Sometimes the best thing you can do is take motor out and put it on a test stand and just start eliminating problems, one by one. Frustrating as hell, but it should run correctly.

You know these are all good suggestions ,!!!!!!

but the most obvious has been left out , send the engine back to os and tell them to make it right or replace it .


I will work only so long to make one run as it should then it becomes a manufacturer problem.

Highlander

Highlander,
Well, that's not a bad approach if you've really had several guys who know engines look at it personally. The best way to "fix" a problem engine is to get a local engine gruru to go over it in person and make it work.

Oh, here's a good one I just remembered. One of my students had an OS .46FX, and it was cutting out whenever we went to full throttle. Not always, but most of the time. He had a K&B glowplug in it (1L or HP, not sure which). One of the local guys (not me) suggested replaceing it with an OS #8. I figured there was no way it was the cause of the abrupt quitting, but we tried it anyway. What do you know, fixed it right up. Now, I run K&B plugs in all my engines, including OS .25FXs, with no trouble. And I run mini-pipes (mousse cans) and 15% fuel.

We pulled an OS #8, and a K&B 1L and held them side by side. They are both listed as "Long" plugs, but wouldn't you know it, the K&B is noticeably longer than the OS. (Ill avoid any jokes about who's definition of "long" means what here)

Apparently that was driving up the compression and causing all kinds of troble. If we had tried two or 3 glow plug gaskets, I'm sure it would have fixed it as well. Or if the engine had been a bit older, when the compression isn't quite as high, I bet it would work fine. The smaller engines usually like a higher compression anyway, so that's why it doesn't bother the .25FXs that I have.

I don't think this is the problem in this specific case, but you never know.