OS 91 FX problem
 

I have an os 91 fx which isnt running steady. Is easy to start , but when it runs, the rpm is unsteady. This is first time Ive experienced this type of "unsteadiness" (maybe bad english - Im from Norway :) ). Its difficult to explain, but I will try:

The sound from the engine isnt steady, one way to tell itis that itis 2 different rpm-levels at almost any throttle-level, not at full throttle. When the engine runs, the engine is jumping between thesetwo rpm-levelsin a steady frequence. The highest rpm-periodin the frequenceis always longer than the lowest. Both the rpms is "following" each other propotionially when I change the throttle.

Maybe difficult to understand what I mean

Last summer, I crashed the plane with that engine, an oldpatternplane, using the standard muffler included with the engine, in the next plane, a goldberg cub, im using a pitts muffler. Maybe that will help :)

This could be somedirt in the needle valve seat, but I have checked this.

Someone havesomeknowledge about this type of engine problem?

Maybe O-rings and air-leakage?

RE: OS 91 FX problem
 

The metal needle valve housing was replaced by the newer plastic one due to difficulties with the metal one. Do you perchance have the older metal one?

RE: OS 91 FX problem
 

I have the older one made from metal. Im gonna google it tomorrow, maybe its some replacement needle housing to buy.

RE: OS 91 FX problem
 

May be a piece of dirt in the fuel path causing a rich lean pulsating effect.

RE: OS 91 FX problem
 

It can be a lot of things, but I'd recommend eliminating the needle valve issue first. This happens on the ground, or in the air, or both?

RE: OS 91 FX problem
 

Could also be loose linkage

RE: OS 91 FX problem
 

Since you changed to a pitts muffler from the stock OS muffler, it could also be the problem if not a larger volume pitts for the 91FX. You might also have to block one of the pipes. Try running the engine on an engine stand with your stock muffler to see if the engine runs normal as on the first plane. My 91FX has a SlimLine large volume pitts muffler with built in restrictors in the exhaust tubes.

RE: OS 91 FX problem
 

It happens on the ground and in the air.

The problem was a reality whenI used the stock muffler. Has never used a pitts muffler on that engine, thats what im gonna do. Off course, that will probably not solve the problem.

I dont think it is loose linkage. The planewho I crashedis anyway gone.The fuel path has been replaced once or more. The needle housing has been checked for dirt many times :)

Later this winther Im gonna bench-test the engine before I mount it in the cub.

On the engine, the carb is turned 180 degrees from original position, thefuel inlet on the carbis at the opposite side of the needle. The fuel line between the needle and carbis then a bit longer, compared with the original position. Could this be a possible reason?

RE: OS 91 FX problem
 

As someone suggested, if your needle housing is the original combined backplate with the needle housing built in, a remote needle assembly should fix the problem. Mine has that type backplate. I like the front needle carb so I installed a Magnum / ASP 91 carb on the engine to fix mine.

RE: OS 91 FX problem
 

Hello Kjell. I have one of those early 91 FX`s too. I had alot of problems with flame outs and reliability before I changed the carb to a different one. In reality it is not the carb itself causing the problems, but the integrated needlevalve on the back plate. You could cut it loose from the backplate with a Dremmel cutting disc and fix it to the firewall or something. If you have other engines lying around in similar size there`s a good chance that you could swap the carbs around and see if that cures your problems. It could also be an air leak causing the problems. I like to put a tiny amount of some flange compound on the backplate and the neck of the carb to ensure that no false air can pass on those places.

If you are unable to sort it out you can ship the engine to me in (living in Florø) and let me take a shot at it, I am fixing engines at no charge for my buddies already:)

RE: OS 91 FX problem
 

Ok, sothe integrated needlevalve could be the reason

Someone knows why? Vibrations? Air leaks?

Well, before I eventually send the engine to you Asmund, im gonna try by my self :)

I have 2 engines of this type, the other one, who I bought used, I haventran it yet</p>

RE: OS 91 FX problem
 

Check for air leaks, especially around the backplate. If the backplate screws are loose, it will give you a wacky running engine as you have described.


Ed Cregger

RE: OS 91 FX problem
 

I would pull the engine from the airplane, and put it on a engine test stand and try running it there and see if it has problems or not. If it runs good on the test stand then you have a problem with the fuel tank or fuel tank locaction.
If the engine is still running poorly, look at the fuel line going to and from the remote needle valve, do you see little air bubbles in it or not? If it has excessive bubbles then it is either causing the fuel to bubble (like in a vapor lock situation) or it is sucking air in through the needle valve unit.
You can try a external remote needle valve unit, before you try cutting the OEM one loose from the engine to see if it works better or not.

RE: OS 91 FX problem
 

Hi!
Find it hard to belive that just moving the needle from the back of the engine could change anything.
I have several engines where a remote needle works just as good as as having the needle on the carb.
Most of the time when We modelers have trouble with an engine it's our own fault! Find it very hard to believe that OS with so many years of producing glow engines made a mistake with just the .91 engine.

RE: OS 91 FX problem
 

Jaka,
While what you say may be true, the original OS 91 FX had a well documented problem with the cast on design of the rear needle valve transfering heat to the fuel and causing lean running.

This caused OS to redesign the rear needle valve with a less conductive design.

RE: OS 91 FX problem
 

The 91 carb problem is well documented.

RE: OS 91 FX problem
 

Do you agree with the two other posts to re-position the remote needle valve? I have two of these and both are giving me "in flight" issues of dying even with pre-flight tuning that sounds perfect. Do you think this is a problem that is resolved with getting the valve away from the carb or are these valves defective and need to be replaced entirely?

RE: OS 91 FX problem
 

Initially, my first OS.91fx ran erratically because it came from the factory with a loose "banjo" fitting on the needle valve. If you can rotate that fitting (see picture) using moderate pressure with your finger tips it will be a source of air leakage. To stop the leak just tighten the nut against the fitting. The nut only has two flats on it and may not look like a nut at first glance.

All of my OS 50sx's, 91fx's and 160fx's have back-plate mounted needle valves and all run perfectly.
No dead sticks, no over-heating, no finicky needle valve adjustments, no anything except strong reliable operation. "Lucky me". I wish everyone the same luck. :)

RE: OS 91 FX problem
 

This is the style needle valve that gave all the trouble on the OS 91 FX. The assembly looks well made but was trouble. Remote mounting solved a lot of the heat transfer/bubbles trouble.

http://images.rcuniverse.com/forum/u...16/If10926.jpg

The newer OS 91 FX with the black plastic needle valve body mounted on a steel bracket didn't have the trouble

RE: OS 91 FX problem
 

EXACTLY, this matter has been up in here several times before and is a well know "defect" on the early 91 FX`s. There`s a reason why OS changed the design after a while;)
I have personal experience to show for in this case, I bet this is the well known "remote needle syndrome on the 91 FX only" that strikes again

RE: OS 91 FX problem
 

Bench-testing first, if the engine still runs unsteady, cutting off the needle valve assemblyand mount it in the front of the fuselage would be actual

If I dont remember wrong, the engine ran almost identical unsteady when I had it in my big stick 60, the first plane it was used in.

I have been looking for faults many times , but havent found any fault. Changing plugs, replacing fuel path, tighting screws, remove dirt from the needle valve seatdidnt solve the problem.Ive been flying model airplanes in over 20 years, so I have some Knowledge

So, theneedle valve assembly/heatingproblem looks logical for me :)</p>

RE: OS 91 FX problem
 

There was one down here in my club that did about the same things. About a year later he admitted his low speed needle valve was what was giving him headaches. I saw him make probably 2 dozen dead stick landings and not all of them were good.

RE: OS 91 FX problem
 

The low speed needle valve has been adjusted many times, which didnt solve the problem

RE: OS 91 FX problem
 

If you have buddies that fly helicopters maybe one of them have a worn out/broken Hyper .50 engine? That carb (os 60L) is what I use on my own 91 FX and it cured all my problems (the very same as you describe) It is a truly fantastic carb and makes the engine dead reliable, but any carb with a 15mm throat will work. Get the Dremmel out and cut that needle off and I bet your engine will behave it self;)

RE: OS 91 FX problem
 

I have a 7D carb, but I guess the 60F, the original carb,will work good if I find the fault, probably by cutting of the needle valve assambly :)