OS Needle Valve Problems

Interesting problem happend to me. The run on my Fox 35 keep getting progressivly worse. Part of it I thought was just my general lack of experaince with needling the engine and experimenting with different props. Well yesterday I took my trusty Fox to the field and could only get it to run in short (3-4 seconds) very lean bursts. Flushed everything out to no avail. I returned home and let it sit overnight and went back to it the next day. Gave me time to think about the problem. The first thing I checked was the NV. This is an OS35 NV that has been in this engine since day 1. It has had a couple of no power inverted landings but everything ran fine afterwards. I found that the Needle was loose on the knurle or knucle of the stem. Would move back and forth pretty good so my problem was an bad needle. I put the original Fox NV back on and test run the engine, ran like a champ. I've had needles break due to unintentional landings but I haven't seen one do this. From now on I take extra needle valves to the field with me.

Since this is an Fox needle valve should I cut down the needle stem and put a LG retainer on it to help protect it from an inverted landing? If so how short should I cut it down?

Fly straight in circles,

Mr. Finch,

Although I applaud your changing from a stock Fox needle, The OS needle valve would not have been my first choice. The OS needles have a tendency to work loose because of the way they are constructed. Needle is press fit into knurled adjusting nut and can either slide forward or back and may also wiggle sideways causing very difficult settings. I would suggest you use a ST NVA with the short needle. Should solve all the problems your mentioned. Later, John

>Although I applaud your changing from a stock Fox needle,
>The OS needle valve would not have been my first choice.
>The OS needles have a tendency to work loose because of the
>way they are constructed. Needle is press fit into knurled
>adjusting nut and can either slide forward or back and may
>also wiggle sideways causing very difficult settings. I
>would suggest you use a ST NVA with the short needle.
>Should solve all the problems your mentioned. Later,
>John

I just happen to have an ST NVA on hand. It has a long stem on it comes to about the same height as the Fox. I could shorten the stem on the ST but what would be an acceptable length? I have the Fox NVA on it right now and I understand the problems with breakage and with maintaining a setting. The ST NVA would hold a setting better but it seems that the stem would be much more fragile. The OS is the first needle valve that I have went though on this engine and performed well until the last couple of flying sessions. If this is a common problem with them I would certainly think twice about using them again, but they do seem to be rather popular.

The old (original) OS needles were soldered on. The new ones are not. I have not seen the problem you experienced with the new ones.

On the needle question, you can get a round needle for the Fox assembly and that works well, also (I still think the threads are too coarse, however.)

We made a wire loop that we either glued into the nose of the plane or soldered to a brass strip that was bolted to the motor mount on the engine to "protect" the needle during wheels up landings. (Ah, the memories of the good old days). Either method works well, however, to protect the needle valve. Or, you could even do as I did on one plane and put wheels in both directions. Then you don't care which way is up and you can even practice flying the "wrong way" if you want to. (Not recommended.)

Common problem with the older OS needles. The fix was to take the spring off, solder the needle to the housing and put the spring back on.

I was wondering if I could do that. There is nothing wrong with the NVA execpt the needle being loose. I will have to take a closer look at the assembly and if it works it works if not I'm not out anything.

Finch,

I would certainly agree with the advice that you have been given with one reservation. Soldering the needle will certainly solve the problem of "In and out". However, unless you have a way to make sure that the needle is perfectly centered when you solder it, then what happens is that as you adjust (turn) the needle it may close or open the passage of fuel over the NVA assembley hole to the crank. This gives you a problem just as if you were holding your finger over a hole in the soy sauce bottle at the local Chinese resturant. The closer your finger is to the air vent then the less sauce you are going to get on your rice. Back off and you may get more than you want. Depending on how far from "perfect" centering your solder job is, this can make for some real interesting needle adjustments. In my experience, nearly impossible.

ST Needles come it two lengths that I am aware of. A short and a long. You apparently have the long needle. We flew the short on Combat planes back in the 70s for the very reasons you mention. If we couldn't get a "short" then we heated the long and bent it too the (proper?) length. As an aside, if you stick a combat plane straight in, it doesn't matter what length the ST needle valve is. You are going to bust it. My two cents: Later, John

Come on John!! Surely you jest!! The ground around the Lubbock, Odessa, Amarillo area is soft enough to let a plane survive a "straight in" with little or no damage!! Isn't it??? WELLLLL, maybe a wee bit of damage.
Bigiron

PS It's softer than pigiron or perhaps grey cast.

How ya doing buddy?
B

> PS It's softer than pigiron or perhaps grey cast.

I dunno, Marvin. That's giving a large benefit of the doubt.....I grew up in Lubbock, and have dug postholes in that very soil.

So here's what I'd do: Put the needle inside the valve body and screw it in all the way until it's seated. Then solder the needle. Then replace the spring. Not being an expert, I can't say for sure it'd work, but it seems like it makes sense.

Regards,

>So here's what I'd do: Put the needle inside the valve body
>and screw it in all the way until it's seated. Then solder
>the needle. Then replace the spring. Not being an expert,
>I can't say for sure it'd work, but it seems like it makes
>sense.

You hit it exactly! Well, almost. Screw the body almost all the way until it is seated. Then make sure the needle is seated all the way and solder. This way ou give yourself a little lee way.

And if you are experiencing this problem and soldering the needle, then it definitely is an old one. But the method you (and I) describe is the way to fix them.

>Finch,
>
>I would certainly agree with the advice that you have been
>given with one reservation. Soldering the needle will
>certainly solve the problem of "In and out". However,
>unless you have a way to make sure that the needle is
>perfectly centered when you solder it, then what happens is
>that as you adjust (turn) the needle it may close or open
>the passage of fuel over the NVA assembley hole to the
>crank. This gives you a problem just as if you were holding
>your finger over a hole in the soy sauce bottle at the local
>Chinese resturant. The closer your finger is to the air
>vent then the less sauce you are going to get on your rice.
>Back off and you may get more than you want. Depending on
>how far from "perfect" centering your solder job is, this
>can make for some real interesting needle adjustments. In
>my experience, nearly impossible.

Actually, in reality, the OS needle is very difficult to get far off center. The constant-diameter section of the needle is a very close fit to the barrel of the spraybar, so it will bee in the center even without the threaded collar. That's why they are preferred to ST needles on "shaker" motors, since they are so well supported. the worse thing that can happen if you get it crooked is that it will be hard to thread on, or bind. It won't get off-center enough to affect the functionality in the way you describe.

The support afforded by the tight fit is actually much better for vibration than the ST and ST-style needles, since they are cantilivered over most of their length.

LAST EDITED ON Aug-15-03 AT 07:05 PM (CDT)
 
All I know is I never had any problems with the ones I fixed with solder