GREG1

I have a 91 FX that is really starting to get me down. There is a constant flow of air bubbles in the fuel line from the needle valve assembly to the carb. I have tested "everything" for leaks and found nothing. The only thing I can see, is possibly the vibration is causing the problem, (I've used numerous balanced props with no difference.) This engine vibrates more than any other I have ever seen. Is anyone else having these problems, and has anyone fixed them???

Thanks Greg

TerrellFlyer

Hi GREG1,
The needle valve o ring is to small,causing it to leak air at midrange and low speed,it has a gray o ring,hobbico,in the us,will send you a new o ring if you'll drop them a line,it will be black and a little larger,also you can buy the new o ring at the local hobby shop,make sure it's the black o ring,some guys have changed to another brand of needle valve assembly which works.
Have a goodun,John

w8ye

My Feedback: (16)

You can use the K&B or the Tower engine needle valve.

Jim

Robotech

My Feedback: (9)

I bought two of these engines within 3 weeks of each other. One is as fine an engine as you will ever come across. The other is a POS. Even with the new O-ring it sucks air and well ...........just sucks. As far as I could glean from the distributor's rep., there are some that they can never get to run right or figure out. I now have a $200.00 POS on the shelf.

Before this I was OS's most loyal customer. Now, ......not so much. Maybe there is something to be said for TT and the rest.

Ed_Moorman

My Feedback: (1)

I don't know about the O-rings, but a K&B .61 remote needle valve assembly, $15, made mine run great. I learned this from another guy in the club who also had leaning out problems. I like the K&B needle so much I hack sawed the OS one off the back plate.

Robotech

My Feedback: (9)

Very cool news. I just happen to have acquired one in a buyout of a former modeler and was wandering what I would do with it.


Thanks!

djlyon

Both mine have bubbles and work great. I think the bubles are vaporized ahlcohol and not air from an air leak.

Robotech

My Feedback: (9)

I've read your posts on the "vaporized alcohol" before. How can you explain this being the only engine that developes this phenom? My other 91FX also has bubbles and works great but this engine has been gone over a thousand times by many "xperts" and still runs rich on the ground and lean on the air with frequent and numerous dead sticks.

Like what was said in another forum, if you pay top dollar ( and you do pay extra for OS ) for an engine, you shouldn't have to bury it in a coffee can under a full moon, sprinkle it with eye of newt and then fly it only on the third Tuesday of the odd monthes. Other engines, all other OS's in fact, don't have these problems.

Whew. That was like opening an old wound. I had almost forgotton my $200 trophey on the shelf until I read this thread.

TerrellFlyer

Robotech,
so far this year I have seen one OS46FX and and one OS61FX ,on the 46 the flyer used a tower hobby needle valve to solve the problem,on the 61 the flyer emailed hobbico and they sent him the larger ,black,0 ring and that solved his problem,yes I too am an OS flyer, but with this problem in the needle valve I'm looking at other engines,when I'm at the field I want to fly,not work on a 3 or 4 hundred dollar engine,there's to many lot cheaper engines I can do that with!!!ARE you listing OS?
Have a goodun,John.

Robotech

My Feedback: (9)

I installed the new "o" ring awhile back. No change. I think I'll try the K&B Needle valve since I already have one. The problem is definitely in the Needle valve. There are NO bubbles coming from the tank. There must be some other variable somewhere though because I have a second engine that adjusts out and runs fine even with the bubbles. In fact I always brag that I'll bet a paycheck It'll start on the first flip. Of course I get no takers but it ALLWAYS starts on the first flip.

We have an abundance of OS's at our field and I've only seen a problem on the 91fx's. If someone gets their plane in a tight situation and the engine carries them out we say " Thanks OS!" Kind of like the "Thanks AC" commercials that used to run on TV a while back. I guess every family has to have it's black sheep.

TerrellFlyer

The tower hobbies needle valve works great ,it has two o rings.
Have a goodun,John

djlyon

ROBOTECH
This is what I previously posted. I'm still convinced that with these remote needles you will have ahlcohol bubbles without an air leak. That's not to say though that many are not seeing air leaks and that's not also to say that alcohol bubbles are not bad. I've also had to replace a worn out needle on the OS91 because I started getting erratic mixture control such that If I just touched the needle the mixture would change dramatically, but it wasn't leaks.
The Leak test I ran any one can run. The vaporization test any one can run. The confirmation test of alcohol or air is exposive and I didn't run that in the garage because my wife frowns on it. If I'm right no explosion, if I'm wrong BANG.


My YS53 has bubbles in the fuel line between the regulator and the carburetor when running, from idle to half throttle. At full throttle the line seems to be clear. I don't do anything about it because the engine runs great through out the throttle range and produces great power and besides it is a common condition, which the experts have blessed as normal.

On a RC online thread awhile back it was discussed at length and I presented a far fitched (imo) theory that perhaps the bubbles were vaporized alcohol, the vaporization taking place as the fuel exited the pressure regulator. I didn't really believe this. I thought there was an air leak in the system some where. No body on the thread commented on the theory and I lost interest because my engine and others that I knew of ran just fine.

Now to the OS 91FX, a fine running motor that I've been running for a long time. Good idle, good throttle response and makes really good power. However posts on RC online and magazine reviews have complained about air leaks in the remote needle and discussed what to do to fix them. A few weeks ago I noticed that at low and mid range my engine had bubbles between the needle valve and the carburetor. I ignored it because the engine was running well. Last week I even spotted fine bubbles at high throttle. I suspect the bubbles were always there and I just hadn't noticed. I generally don't try to find out what's wrong with a good running engine.

A couple of days ago I put in a new needle "O" ring anyway because I was sure there was a leak. It still had the bubbles and it still ran really good. After the flying session I started to drain the remaining fuel from the tank and noticed I was getting almost solid foam. I had hooked the fuel pump to the line between the carburetor and the needle instead of the needle and the tank. As a result the pump was sucking the fuel out of the tank through the needle valve. Then I noticed that the fuel coming out of the tank had no bubbles. It was solid fuel. Conclusion, big leak in the needle valve requiring fixing, broke or not. Latter that day I put a clamp on the line from the needle valve to the tank and drew a vacuum on the line to the carb. It held the vacuum, NO LEAK. I filled the tank with fuel and then began sucking it out through the needle valve with the pump. Sure enough, foam again, but still the line to the tank was clear fuel with no bubbles. I left the pump run and clamped off the line to the tank. All flow of fuel, bubbles, foam, air or anything else stopped coming out of the needle valve. Again NO LEAK.

Now I'm really beginning to believe that what I and maybe many others have been seeing in the OS remote needles and YS 4 strokes is alcohol vapor bubbles and not air bubbles caused by leaks. I not sure it makes a difference if it's alcohol and not air. Uneven fuel flow is still uneven fuel flow.

Any comments? Any fluid dynamics persons out there that can give some insight? Fluid properties folks? I'm an aerodynamics major but that's just a small segment of fluid dynamics and doesn't help me here a bit.

rcman-RCU

My Feedback: (31)

I just bought one of these engines used. Air bubbles were appearing after the fuel passed through the needle valve. I cut a short piece of fuel tubing (about 3/32"" long) and put it onto the needle valve so it would be squeezed between the needle valve and the needle valve housing when the needle is screwed in to the correct location. It sealed this area and the bubbles stopped. The clicker spring has a bend in it that passes over the fuel tubing, so the clicker still works. The fuel tubing also keeps the needle valve from vibrating and loosening up its threads.

This is an easy fix and only takes a couple of minutes. Put the fuel tubing over the end of a needle nose pliers to stretch it open. Then slide the needle into it and pull out the pliers.

djlyon

Another thing to consider, and I'm not saying that people aren't curing the bubbles problem with tubing ,"O" rings and different needle assemblies, how does air get in even if the "O" ring does leak? The fuel in the tank is pressurized above ambient if you are using exhaust pressure, not much but still higher than ambient. That means the fuel going into the needle assembly is pressurized above ambient. This being the case fuel should leak out past the "O" ring rather than air leak in I would think.

rcman-RCU

My Feedback: (31)

I would guess that there is some pressure between the needle valve and the tank, the muffler pressure providing the "lift" to get the fuel to the needle valve. Between the needle valve and the carb. fuel outlet, there must be suction, to draw the fuel throught the needle restriction and on to the carb. outlet. This means that there is a pressure drop across the needle.

One could do an easy experiment, by putting a tee in the line between the remote needle valve and the carb. Attach a two foot length of fuel tubing to the tee, hold the fuel tubing up in the air and start the engine. If fuel rises in the tubing, there is pressure. If the tube allows the carb. to draw air, there is some amount of vacuum or suction. If no one comes through with a definite answer, I'll try the experiment.

djlyon

That's a good experiment, and I think you'll see a vacumn, pressure below ambient. But you are on the down stream side of the needle and I don't think the pressure at that point is valid for upstream of the needle where the "O" ring is.

djlyon

It's the drop in pressure across the needle that I think might cause the alcohol to vaporize.
Again need a fluid dynamics guy.

rcman-RCU

My Feedback: (31)

djlyon is right about the 91 FX needle valve. The side of the needle with the o-ring is fed from the tank. The fuel then passes throught the needle valve opening, traveling away from the o-ring end, and then goes to the carb. On the other hand, the OS 61 FX needle valve seems to feed in the opposite direction, with the o-ring exposed to suction from the carb. It seems strange that OS reversed the fuel flow direction on these two engines.

The fuel draw on the 91 at high throttle settings may cause suction all the way through the needle valve and to the tank. This could explain why there appears to be air being drawn into the needle valve around the o-ring.

Another possibility is that there are bubbles coming from the tank. The bubbles would be smaller before coming to the needle valve and would then get bigger after passing through the needle valve where the pressure is lower.

I will try to find the vapor pressure for methanol. Liquid vapor pressures go up with temperature. Vapor pressure is simply the combination of temperature and pressure where the liquid will begin boiling or converting to vapor. For example, water reaches atmospheric vapor pressure (about 15 psi) at 212 degrees F. If you reduce the pressure, for example at an altitude of 7000 feet. water boils at a lower temperature, perhaps around 190 degrees. This might put this idea to rest one way or another.

I emailed Hobbico and they are sending me a free replacement o-ring.

rcman-RCU

My Feedback: (31)

I found the vapor pressure of methanol at 80 degrees F. It is only 2.5 psi. Since atomopheric pressure is about 15 psi. it means that the carb. would have to be drawing down the fuel pressure by 12.5 psi, which is impossible. I estimate that a carb. can pull fuel up at most 12" which equals a suction of less than 1 psi. So it would seem that the bubbles are definitely not being caused by the methanol vaporizing.

Since engines get hot, I checked the vapor pressure at 100 degrees F and it is still only 4.63 psi, still far too low to cause methanol to vaporize. At a pressure drop of 1 psi, it would take a temperature of approx. 140 degrees to cause vapor bubbles.

djlyon

Consider the velocity of the fuel as it goes thru the tiny oriface presented by the needle and the metering tube. At that point you might see the necessary drop in pressure in that venture. That's why I think was getting foam when I used a pump to pull the fuel thru. But it is clear that the pump was dropping the pressure far more than a running engine would.

rcman-RCU

My Feedback: (31)

There have been some threads about using Armorall to prevent bubbles from forming in fuel tanks. In describing how it works, they talk about "excitation points" or miniscule sharp projections as being the initiation points for bubble formation. Somehow, Armorall does something (coats these points to remove the sharpness, lowers fuel viscosity, or ?) to stop the bubbles from forming.

This could be what is happening inside the needle valve body. Someone needs to try some Armorall to see if it solves this problem with the 91 FX. It only takes a few drops of Armorall per gallon. Robotech's $200 paperweight would be a good experiment, since it seems to have a severe case of the bubbles. How about it Robotech?

The other thought would be to remove sharp corners from inside the needle valve assembly by polishing. This could explain why some engines have the bubble problem and others don't.

Matt Kirsch

My Feedback: (21)

Hey Robotech, why are you resigning yourself to a $200 paperweight? Send an email to [email protected] explaining your problem. Great Planes is the exclusive US distributor of OS engines, and their service is second to none. They'll at least try to fix it for you.

Try slipping a piece of fuel tubing over the joint between the needle valve and the seat. If that doesn't work, the leak is probably in the joint between the needle valve seat and the backplate. Crank that down good and tight, too.

GREG1

I cut the needle valve assembly off the backplate, and mounted it on the firewall. And hey presto, no more bubbles, the engine gives consistant runs and there is very little vibration. The only thing is does that I don't like, is it hunts when you cut the throttle back to idle when flying. It doesn't apear to do it on the ground. Could this be something to do with the idle mixture?

Thanks Greg

djlyon

That's interesting. The fact that remounting the needle curing the bubbles and now the throttle hunts at low speed suggests you may have a worn needle valve. If the threads are worn or the tip of the needle has unevenness on it you can get the problem you describe. Try just touching the needle with the engine running and see if you get RPM changes. If you do you need a new needle. If the needle seems OK just richen a little. That should cure the hunting.

downunder-RCU

Seeing that remounting the needle valve cured the problem made me wonder. Does that needle valve hanging off an arm mounted solidly to the engine amplify the natural vibrations and maybe get into a harmonic at certain revs?