OS35FP running problem

To Len (since he worked on the engine) or anyone else,

I'm haing a weird problem with the OS35FP that is in my classic plane. Len stuffed the boost port and set it up. The thing was running perfectly last weekend. Then, without warning, the engine quit. Inverted, at that. (the damage was minor and already repaired). Prior to this, I had three flights that were great then it weirded out.

After getting the plane home and repairing the damage, I cleaned the engine, needlevalve assembly and flushed the tank. Put it all back together with new fuel line and filter. Great.

Ran it today and had endless problems. The engine would start fine and seemed to be running OK, but as soon as I took off, it quit. Tried changing the plug and there was no difference. We tried changine fuel (from Wildcat 10% nitro, 23% oil - 15% synth, 8% castor to Powermaster 10/22). The engine seemed to run then, but was quite rough. Needle was really sensitive. Even when I got the plane in the air, the engine sputtered in maneuvers and just didn't seem very happy. Thought it was going to quit a couple of times. It would gain and lose rpm in level flight (tried it with and without muffler pressure---same story). Just wouldn't hold a needle setting. Compression seems fine and it starts very easily.

Would an idle bar plug help? More nitro? Less? Could excessive vibration cause this? I'm going to try re-balancing the spinner and such. But that doesn't really seem right. I mean, it the same setup worked perfectly last weekend then all of the sudden, wham!

Any ideas. I'm about ready to chuck the thing and try something else.

Randy

Randy Powell

I would take a hard look at the fuel tank. You have already taken care of the spray-bar and not much can go wrong with an engine that you cant easily detect.

If it has the stock OS needle, check and be sure the needle is tight in the threaded housing. I have seen the needles come loose from the housing and create all kinds of run problems.

Tank, NVA, or glowplug would be my guesses, in no particular order. What glowplug are you using? Leonard uses Fox plugs, but I have not had good results with them. They can run ok and then run very strangely. T-Bolt idlebar plugs work, and lately I have used Zinger plugs with similar results. Check out the tank for leaks and junk rattling around inside. Copper tubes, right? Bob might be right about the NV, tho I have not used OS NVA's much, so haven't had this problem. But you might replace it with a ST or PA unit and see if that solves the problem, even if it chokes down the venturi. You could fix that before or after. What kind of filter? Could that be leaking? Also, don't forget Leonard's lecture about the stuck wristpin.
Sammy

LAST EDITED ON Jul-06-03 AT 02:15 AM (CDT)
 
Thanks for the responses. It has an ST NVA. I checked the tank for leaks and it was fine. What gets me is that is was running fine before. I'll try an idle bar plug and check the tubes again. The tank is one I made and yes, it has copper tubes. But I agree, it's likely something wrong with the tank. I may just put in a clunker to see how that works.

What stuck wrist pin discussion?

The wrist pin is something that after extended running (about two years for us, maybe a life time for others) the wrist pin can get carboned up and stuck inside the piston. This causes uneven expansion and the engine can go suddenly sour. Cure here is to make sure the wrist in is a loose fit in the piston, that's all. If it is stuck, unstick it (drive it out with a dowel). That is probably NOT the problem and I would assume the problem is not the engine at all. Also, if your engine worked fine with your present fuel, prop, glow plug, etc., that is not your problem (although a glow plug can go bad and you can change it, you do not need to change types.)

When something goes bad on a previous good set-up, check to see what is going bad. Matt's Nat's ship went sour a week before heading out to the Nats. He switched engine/pipe combos from the other plane and it worked fine. That eliminated the plane and put him back in action. A look at the engine showed that the header had carboned up along with the inlet to the pipe. We cleaned all this and should be back in business with this engine now serving back up. But, what I am getting at is if something suddenly changes, don't change your set up. Look for the cause. There is always a cause.

You seem to have elininated engine and needle. I would go back to the line to the tank (filter?) and the tank itself. Yes, we have had tanks go bad. Matt has one that just quite halfway through the pattern. There are no leaks. It works fine otherwise. If we took it apart (may do that some day) it would probably have a split tube inside. Brass tubes can split (or disappear) if left soaking in fuel. Try dropping a brass tube in a fuel jug some fall and then look for it in the spring. Copper tubes can corrode inside. Nice green stuff. They are not immune to problems. Any tank can suddenly go bad. It would guess that is your problem.

Len,

That's what I think, although I've been trying to eliminate other possibilities. Unfortunately, the engine takes about 4.5oz of fuel for a pattern so the perfect 4oz clunk tank I wanted to use isn't quite big enough and a 6oz won't fit in the hole. I'm going to have to look for some more exotic shape that might fit.

I just don't wnat to make another tank for it. I can't imagine what happened. I made this tank for the plane and it worked fine before so I can't imagine the copper clogged or something happened already. It only has about 20 flights on it. I tried flushing it out, running pipe cleaners down the tubes and flushing again. Hmmm, just have to try something else.

Randy,

I no nothing about OS engines but hearing your motor run is different from other motors. It sounds to me like it is burbeling or blowing bubbles under water like it is continually sputtering. Sometimes it sounds ok but other times not. I would assume this experience would disappear on a test stand situation. This may be reaching for straws but could it be that the fuel is not atomizing properly going into the crankcase? Is there an unique turbulance effect around the nose of your airframe that affects the airflow into the venturi? Thoughts to consider. Is your OS a bushing or bearing engine and is there the possibility of an air leak there?

You mentioned about changing the needle valve and thought the spraybar is ok. I would opt to change the spraybar as well on a test situation. A spraybar can wear out since it is a softer metal (looks like bronze) than the steel needle valve. It might be flaired thus rendering only one perfect needle setting for an optimum run.

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler

> ... Is your OS a bushing or bearing engine and is there the
>possibility of an air leak there?
>
All FP model OS Maxes are bushed shaft engines. They do vary between iron/steel and ABN innards, though. And I have had at least one "real" ABC FP-25.
>
>You mentioned about changing the needle valve and thought the spraybar
>is ok. I would opt to change the spraybar as well on a test
>situation. A spraybar can wear out since it is a softer metal (looks
>like bronze) than the steel needle valve.

Yes, that's entirely possible. And if it was a OS needle, it could develop an air leak that it never had before. Two of the more unusual needles ever used in models, both from Fox, were designed to NOT be able to get into a harmonic vibration inside the spraybar. There is a potential of that happening in a single cylinder engine. The spade tip needle was supported at the far side of the spraybar outside of the needle seat by the two spade ends. The odd half-round needle was designed the way it was, and the two spraybar holes added, for the same reason.

It's an ST NVA, so the spraybar is brass, I believe, instead of bronze - still softer than the steel in the needle. I'm most suspicious of something in his tank having come undone somehow, though.

.
Kiwi

If it was "running" perfect, then I am likely to eliminate the set-up and the plane. The only question would be heat. Did it get substantially hotter than earlier. This could lead one to consider cooling within the plane. If your outside temperatures are remaining substantially the same, then that is not it.

Is your tank removeable? Is the fuel foaming? Shake the jug? If it looks like a fresh pour of Budweiser and the foam does not dissipate almost immediately, then I would suggest adding defoamer. Then, also, make sure your tank is wedged in solid. If the tank came loose enough to shake "just a little" that could lead to foaming. My test stand it that way. It is a plastic tank (Hate those things) and it foams--even with defoamer in the fuel. All I have to do is put my hand on the tank and settle it down. We have had situations where we just had to wedge the tank in a little tighter and that solved it (with a metal tank).

Just remember, something changed. You don't need to redesign what you had if it was running fine. You need to fix only what changed.

Len,

The tank if very tight. I'm certain that's not the problem. Maybe I'll wihip up another tank. I agree that's it's probably the tank.

I don't think it's too hot. It has the right formula. 2x the amount of exhaust as intake. The head hangs out in the air.

Here's just an idea--can you run the engine in a test stand (use a plastic clunk tank)? At least you can make sure whether it is the engine, tank, or cooling.

If it won't run on the bench, one possibility is that the liner might have peeled--unlikely, but I have seen one (FP40) some time ago that peeled.

Well, I pulled the back off the tank tonight to inspect. Everything looked fine. Hmm: I pressure checked each copper tube and they also were intacted and solid. Didn't appear to be anything at all wrong. I sealed it back up. After that I ran the engine on the test stand with attached clunk tank (I have a 10oz clunker semi-permanently attached to the stand). It ran aweful. Choking and sputtering. Would not hold an rpm setting. My test stand is set up so the engine can be tilted back to about 60 degrees while the engine is running. It really didn't like that and cut out as soon as it was tilted back. This running a pretty sloppy 2 stroke. So it's starting to narrow down. I have to get another 35 sized ST NVA (all mine are 46 and 60 sized). I may fake one up just to test, but I suspect that something is terribly wrong with that NVA. I pulled apart and inspected the engine. The wrist pin is fine as Len said it would probably be. the cylinder looks really good. No scores or problems and the piston is fine. The crank pin also looked fine. Once re-assembled, the compression is very good. Hmm again. It's just got to be the NVA somehow.

If you have a 46 size ST NVA, that will work fine. The hole does not have to be centered in the venturi--just make sure it is within the bore of the venturi somehow.

Are you sure the needle valve assembly does not have "junk" in it. Take a paper clip and run it through backwards with the needle out. Squirt some fuel through it backwards. Make sure the little hold that is supposed to let fuel into the venturi is not blocked. (The body can be open but that hole can be plugged. Put the needle back in and make sure that it will seal off (blow through an attached tube) and that it is open when you open the needle slightly.)

Also, make sure the venturi is sealing on the o-ring. That shouldn't cause the big problems you are having, but you never know. Make sure the back plate is sealed and that the head is sealed (You said it had good compression) Just watch that you do not overtighten the bolts.

We will get this thing solved.

WE've seen backplate leaks do this also. I use black engine RTV sealant there to be sure and blue locktite on the screws. IT can work fine until the engine gets warm and then it just quits.

FWIW

Now that Curt mentioned it, I had a Fox 35 act that way a couple of years ago and found that the backplate had come loose. It was a tight-fitting Stuffer plate and all three screws were completely out (one of them gone) and all that was holding the backplate in was the firewall. Apparently, the last time I had tha engine out and apart, I didn't tighten the backplare bolts adequately.

Bigiron

THe ones we saw had all the screws in place but not real tight.

When the engine came up to temp, the backplate leaked. Generally, a thin smear of black silicone on each side of the gasket will actually hold the plate in and prevent leaking even if a screw comes out.

Randy,
One of the other things is to check your fuel. We had one doing this same thing and after trying some other fuel the problem went away...
Went back to the old fuel and the problem re-appeared.....
Gil:+

LAST EDITED ON Jul-08-03 AT 03:59 PM (CDT)
 
Well, I ran the engine on the test stand again last night. I had taken the needle valve out and completely cleaned it, running a wire through it and flushing as completely as possible. Put it on the test stand with clunk tank and original fuel and it ran fine. I have to think at this point that it is either a bad fuel tank or particularly heinous demons. I'm going to put a new tank in it tonight and fly it later this week. If it still doesn't go, I'll hire an exorcist.

>Well, I ran the engine on the test stand again last night. I
>had taken the needle valve out and completely cleaned it,
>running a wire through it and flushing as completely as
>possible. Put it on the test stand with clunk tank and
>original fuel and it ran fine. I have to think at this point
>that it is either a bad fuel tank or particularly heinous
>demons. I'm going to put a new tank in it tonight and fly it
>later this week. If it still doesn't go, I'll hire an
>exorcist.
>
>

Randy Powell

You may, indeed, have "fixed" it. Just a little "junk" in the needle valve will do strange things. We use three filters and it sometimes still happens.