Hello I have problems with the 91FX, in my club there are three friends with the same motor, but they don't have any problem (with differents prop, even one has a Bisson Pitts muffler)
But with mine I don't obtain any result, in earth it works well but in the air it begins to fail.
With some adjustments of the high needle, at the little time of flight he stops suddenly, with other adjustments at full throttle, begins to fail and to work erratically.
I have carried out the following changes:
-First I use the same brand fuel and nitro percentage that my friends used
-Change airplane (from a cap232 to an extra300)
-Try with different adjustments of the needle (the high and low)
-I have check and change the fuel tank.
-I have changed all the fuel tubes
-I have revised the height of fuel tank and try different positions
-I have revised the O-ring of the high needle, a friend also lends me a needle from his motor.
-I have put a fuel filter
-Try with several O.S #8 plugs, all new ones
-Try with different prop 14x6 14x8 15x8 all MA
I have read in this forum several topics on similar problems but I cannot reach any conclusion.

I CANNOT SOLVE THE PROBLEM!!!
ANY IDEA?
Thank you

Please do a search on this engine, you'll find at least 27 pages of related problems on this same engine, AND IT'S WORTH READINNG!!!

Dear Mr. Crazycap,

Try using a 13X6 prop. It appears that your engine may be over loading. I had the same problem with my 91FX, and I read all of the zillion posts looking for a solution. By chance I began using a 13X6 prop and this seemed to fix the problem. You may have similar luck.

thanks ben flyn and Guerra

I read, read and read all the tread about the 91fx problems
Here are some of the modification that members have done:
-cut the needle valve assembly and put it on the firewall (no way!!!)
-buy a new K&B needle valve
-buy a perry pump
-change the o-ring (mine is a black one, even a friend in the club lend me his needle (gray) to check)
-Bigger fuel tube (help in some case)
- etc.etc.
So let me see...............after I spend $200 on a high quality engine, pay the extra price for
the reputation (trouble free right out of the box) and the quality...........I have to spend an extra $25-$30 for the pump and $15 for the needle to get this engine to work well? or like the O.S.people say,"With use here in the U.S. at our altitudes, fuels, etc, we have found that this engine prefers slightly smaller props - 13x8 or 13x9 specifically" (my 46FX and TT46pro moves a 12.25x 3.75 without problems!!!!)

What a shame, O.S. does it again, do you remember the 46FX peeling problem?

Conclusion : I have bought this brand for more than 20 years, I am really a fan of this brand, but now they really disappoint me.
I am going to definitively switch to another brand.
This 91FX really is a piece of..............!!! [>:][>:][>:]

Unfortunatly, your joining the ranks with most of us....being disapointed.[&o] I put on a OS 108 remote needle valve, and a head shim for around 30 bucks It really is a good engine. I just wish OS would own up to there problems!

Check with OS service. There is a backplate/needle valve replacement for the 91FX.

Greg

I had the excact same problem. The problem is in the needle valve assy. The valve will suck air in the air, but not on the ground. I know this sounds silly, but trust me, it's true. I have a .61FX doing the same thing, I have just ordered a remote needle valve to solve the problem. The motor really is a good motor, the needle valve assy needs more than just improving. It needs re-designed.

Hmmmmmmm, all your problems sound very familiar......and ya wanna know how I fixed them?? Huh, do ya? Well....do ya?

I bought a Jett 90!

Man, I feel your frustration...good luck.

Gary

i use an apc 15x4 up to 15x8 prop. cool power 10% fuel bisson pitts muffler on my 91 fx. my altitude is 2500 and this thing runs great. this is my second 91fx and the first one was just as good. just dont use fueling valves. i have wasted my money on these twice. just use fuel dots.

I was running a stock muffler on mine until recently. I bought my motor used. And it had a OS40FP remotor needle valve on mine. running 14x6MA fat blade wood prop. byron 10%nitro 20%oil runs great with setup. 9600RPM ready to fly

I have just now figured out a moosecan for this motor with some help of other people. but I swapped over to a magnum XLS91 remote needle valve and it works great too. same fuel but taching 10,500 ready to fly.

If you have a extra remote needle valve lying around try it.

also as someone recommended, don't use fueler valve, or filters in your sytems. Just use fuels dots.

I was looking at tower's site and the replacement needle valve seems to be the needle valve assembly from the 40 to 65LA with a retrofit bracket like the one on the 1.60FX. This needle valve looks to seal better than the old version. Check it out.

Check this link out: Its for the magnum 91xls is a better version of the OS91FX.
www.hobbypeople.net/gallery/281158.asp

right now hobbypeople has the magnum XLS .91 2 stroke on sale for $129.99. thats not a bad deal. If my OS 91FX wasn't running great I would try one of these.

Thank you all guys
Ok I put a 13x8 prop as some of you have suggested me (and O.S. too) and he run much better , but still have problems after a couple of minutes of flight.
Like I said before, I already spend $200 on this high quality engine[>:], and I won't spend any more of my money on this engine, even on a simple needle valve at $19, it is supposed that it should runs well right out of the box. []

I will listen to the advice of AirGar, I will sell this motor and I will buy another of any other brand.

Again Thanks

BTW: before yesterday I was in my LHS and I saw the new 91FX, O.S HAD SOLVED THE PROBLEM!!!
The new 91FX has a NEW needle support, the same as that of the new 46AX

have you tried replacing the little clear fuel line from the needle valve to the carb. replace it with some larger diameter fuel tubing.. Mine sounded like it was knocking untill I replace the stock fuel line in between the carb and remote needle vavle.

Why'd it take them so stinkin' long? I purchased mine when they first released that motor (3 to 4 years ago?) I knew about the problem after hours of frustration back then. Why doesn't a company that makes engines for a living figure out the problem and fix it right away? Did they acutally fly this motor befor it was released?

m.gramling , yes I replace the fuel line from the needle valve to the carb, same problem[]



Ick, They don't fly the motor, they sleep on their reputation!!!

Thanks

I installed my new needle valve today, it worked great. I ordered a GMS .61, part number 613130, around $12.00 from Tower Hobbies. I took the back plate off and rotated it 1/2 turn so that the old needle valve holder is opposite it original location. I then mounted the new valve, works great, no air.

Thank you Ick

I will give another chance to O.S and I will order a new needle.

Are you for sure the needle of GMS works well?

I emailed OS about the same problem and they seemed to know right about it. Told me that they would send me a new cover plate, remote needle valve assy, and a bracket which all sounds like the way they are selling them now. I took a look at some new OS motors in the store and they all look like that now. They did ask for a receipt for proof of purchase but they are sending it out free of charge. The only bummer is that they are out of stock on the cover plate till mid October. I will be waiting since mine is no longer flyable till then it quits for no reason at will.