Blackie

I have read threads where most everyone having the problem I am having with the OS 91 FX leaning out in the air. I understand the fix to be either a O ring replacement or the needle valve Assembly for the OS 160 replacement. Well I placed an order for the needle valve but the part was on back order, I waited several weeks, still no part. I contact T/H referring to the part and also explained what the problem was. T/H tech support told me that they were solving the problem with a fuel inlet valve replacement at a cost of $6 and some change. I went ahead and ordered the part receiving it yesterday. My question is will this solve the problem?

Thanks
Blackie

adrian-RCU

please let us know, sounds interesting anyway

nashtm

Hi

Whats a "fuel inlet valve " ?

thanks
Tim

Blackie

After comparing it with the inlet valve that was already on the engine it looked to be the same. The opening was the same size as the new valve. Other changes I made were the tubing. I resized all tubing through out the fuel system.

I truly don't believe its going to make a difference. I will wait and see what happens.

The inlet valve is that round nipple that your fuel line connects to as it leaves the fuel tank. Its part of the needle valve Assembly

edge 3644

I still have same problem after 50 flights. OS91fx is regularely leaning out after few minutes of flight with 14oz tank. Air bubbles are clearly visible in the fuel line, especially on high revs. I have tried additional o-ring on needle valve, bigger o-ring, needle from OS120III. Nothing helps. I even pressurised the whole fuel supply system under water to find the leak and couldn't find any.
Real mystery. I thought about inastalling perry pump, but how can it prevent from air getting into the fuel system?
I wonder if anybody is out there without this problem. For time being I quit and go back to super tigre, which may have less HP, but at least is reliable and runs without any trouble for at least
120 hours if its life span.

nashtm

Hi

I wonder if it isnt a vibration issue - many people have used a k&B
remote needle which solved the problem - apart from being a new
NV assembly it is also mounted in a different possition,
presumably on the fire wall.

I also have a 91fx and I see some bubbles as it transitions from low to high revs but then they dissapear, maybe in my case there is resonance at a point.

It would be interesting to see if soft mounting the motor will make a difference.

Tim
South Africa

Matt Kirsch

My Feedback: (21)

If there are bubbles, there is an air leak. You're probably right about the resonance; it's shaking at just the right frequency to cause something to move enough to let air in.

Has anyone tried stuffing a piece of fuel tubing over where the needle valve and needle valve seat meet? On a couple of my old Magnums, you could push on the needle valve a bit and cause the engine to lean out. Air was getting in around the needle valve. Sealing the needle valve with a piece of fuel tubing solves the problem, every time.

rajul

My Feedback: (58)

Some time back I remember there was a finding that the needle valve temperature was too high causing the fuel to "boil". The os120 bracket solves the problem due to the smaller heat conduction . Just passing on what I can recall.............

Spicoli

I had the air bubble problem from the 91fx and I changed the needle valve to a Perry remote one and it fixed the air bubble problem.Then I had a fuel draw problem and fixed that with a pump.Now It runs great .I also had a heating problem with a incowl muffler and went back to the stock one.I really like this motor now it is setup right.It hauls my TF Texan like a Reno Racer.

Blackie

I plan on running mine on the GP shoestring but first its going to set on my WM SS 60 so I can work the bugs out of it.

Ed_Moorman

My Feedback: (1)

Everyone around my club calls the things "banjos" because the sort of look like a banjo.

I took mine apart and sealed it with hi-temp silicone and it didn't help. I finally spens $15 for a K&B remote needle assembly and it works. The engine runs fine now.

PilotLight

My Feedback: (1)

I too received the fuel inlet and orings and tried them, but there was no change. Still air in the fuel line to the engine. I was in the process of removing the needle valve when the rear cover plate broke at the needle valve assembly. Didn't take much to break it. I will purchase a new rear cover plate, mount the broken needle valve away from the motor, and try a pump.

Let me know if anyone has success with this motor and their solutions if you could, before I go throwing $$ at it. Do I need the pump? Do I need to mount the needle valve away from the engine?

Thanks

rogwabbit

FYI the bubbles you are experiancing... Sometimes the needle valve gasket is the cause. It doesn't seal well and allows air to pass through.. the gas doesn't come out of it but air does go in it. I've had this happen many times on diffrent engines. replace the o-ring on the needle with a larger one if you can and cover it with a little tubing also.

Flypaper 2

Why not just cut it from the backplate and screw it to the firewall or let it hang on the fuel lines. It will solve two problems. If it's engine vibration, it will be much lowered. If it's a heat problem, it won't be there. But maybe thats to easy.

nashtm

Hi

I think you should mount the nv on the firewall.
My 91fx developed bubble problems after a while - I replaced the nv with a perry one and the motor runs great.

Dont give up !!! 91fx bashing has gotten out of hand and now
many people just write of the motor without giving it much attention. I will put my 91fx up against the ST91 and the MVVS91
contrary to what people say.

Good luck
Tim

Blackie

We just had one show up at the field this past Sunday in which was broke in, in the air on a tiger 60. 10 min into flight it leaned out and shut down. Before he was air born I told him what it was going to do before it ever did it. Did I mention it had not been ran yet? Now that's sad when you can predict what the engine is going to do before it even does it. Whats the bashing? no bashing, just facts.

The pilot did get the engine running as it should be removing the baffle inside the muffler and running 30% through the engine, it ran like it was supposed to then, I wonder just how long though.

I sent mine in and am waiting for the turnout. I do how ever have a perry pump in my inventory just in case all else fails. If you read my previous post on this issue you will find out all that I have done.

My question is why should we have to go through all this trouble?

Blackie

djlyon

I don't know if I love or hate to say this anymore. It ain't air bubbles, it's vaporized alcohol. It's the result of heat and the fact that the remote needle is integral with the backplate instead of a bolt on. The bolt on is one more thermal junction (barrier). The engine is a 90 in a 60 size case, not good for dissipating heat.

You guys that have had old cars, I mean old, remember the term vapor lock. This is a similar condition.

The above is based on some non scientific experiments that are not conclusive but point to the above being true.

rajul

My Feedback: (58)

Some have solved the problem by using the OS120 bracket which conducts less heat to the needle valve assembly.

nashtm

Hi

I am using a perry nv but I want to try the original nv again - I am thinking about drilling some holes in the base of the nv assembly to reduce the heat transfer. If that weakens the structure to much i might cut off the nv assembly and re-attach it to the original position using 2 small metal landing gear straps. This will definately provide a heat barrier ( I find the position it is in very convenient).

Tim

rajul

My Feedback: (58)

Nashtm, let us know how it works out. Sounds good.......

PilotLight

My Feedback: (1)

I've finally got the engine running good, but it was on a engine test stand. I received a new rear cover plate. The old needle valve was broke off the engine so I let it hang by some blue medium fuel line. The engine ran OK, but the important thing was there was NO air in the fuel line. Next we changed the glow plug from a Fox with an idle bar to a 0 S #8. (more improvement) Finally, we noticed the fuel tank had a lot of fuel splashing inside so we decided to balance the prop. We found the prop was perfectly balanced on a magnetically suspended balancer, so we installed the propeller so that when the piston was at top dead center the propeller was horizontal not vertical. This produced a lot less vibration and the engine ran perfectly.

To sumarize:

* New fuel tubing between needle valve and carb
* remove needle vavle from rear of engine (eliminate vibration)
* run OS plugs
* mount prop horizontally to piston top dead center (this made big difference)

If anyone else has success with any of these changes, please post technique and results.

I will next try the engine on airframe mounted inverted and will post results...

rajul

My Feedback: (58)

Hi pilotlight, this is new to me and would like to find out more. Do you have any reference that explains how this works ? Thx........

av8r4aa

My Feedback: (82)

Well after reading all the horror stories I decided to look a little closer at mine.
It is NIB awaiting the airframe to be finished.
On the rear needle valve I notices something interesting.
On the top side of the needle valve assy. is a small 5/16 bolt.
Naturally being nosey I took it off to see what is inside.
Well there is nothing inside. But the INTERESTING thing that I noticed is that there is NO GASKET . That means metal to metal contact.
After re reading all the posts about this engine I found that most fixes can when changing the needle valve to a remote location. I assume that this means getting a new needle valve also.
I found no posts about removing and sealing this bolt.

I removed it and used some (very little amount) gasket sealer. I then screwed it down and let it dry.
This may all be smoke and mirrors but as a aircraft mechanic I felt that a gasket is required.
Anybody else on this same boat?
I am curious if this may be the much needed fix

martyg

My Feedback: (42)

Had the same issue with needle valave "leakage" on a 25 FX.
Replaced O-ring, sealed with plumber's tape, sealed joint with tubing.
Nothing worked - Clean fuel going in, bubbles coming out.

After pulling my hair out for weeks, concluded this was heat and not air leak related.
The 100-degree heat out here in Texas probably didn't help any.
Installed a secondary needle and mounted it on the airframe - Problem solved.

Here is a good part to build a 25-size remote needle from: (link)

PilotLight

My Feedback: (1)

av8r4aa:

I already tried that. Look here at post 93.

http://www.rcuniverse.com/showthread...=&pagenumber=5

I also tried a copper crush washer from a glow plug. There are no leaks from here on mine. I applied 15psi pressure and 22 psi vacuum and couldn't find any leak. I'm still convinced its vibration or heat related. Needle vavle broke off and was hanging by the fuel lines and the bubbles were gone!!