I've got a slightly used O.S 91 FX and I've been trying to work the bugs out of it.....maybe someone can help me out......It starts very easy and idles great...but when I get to about half throttle going to max throttle it starts to die and the only way to keep it running is to move the throttle back and forth from idle to max then it starts to kick into full power slowly.....I've adjusted to needle valve and the results were not much better. I'm swiningin or attempting to swing a 14x6 prop. Can anyone help...
Thanks
Shogun[&o]

The 2nd series of the 91FX has given people fits with air bubbles from the needle valve area to the carb. This was cured with a different remote needle valve assembly.

The 3rd version of the 91 FX had a black plastic body on the remote needle valve. These have been OK so Far.

If the temperature has been in the 40 to 70 degree range, You could possibly be getting carb ice though it would be more common around the low 40's? I say this as you lead me to believe that it will run at high speed OK at times?

If the engine wouldn't run fast ever, I would say your LS needle was too lean?

I would try opening your LS mixture screw in the middle of the throttle arm pivot about 1/4 of a turn and see what results you get?

The 14-6 is correct.

Enjoy,

Jim

This is what I did, see if it makes any seese to you.....I played with both high and low needles and here's what I came up with. I opened the low end first 1/4 turn and opened/closed the high a bit and ran like trash....next I closed the low end about 2 1/2 turns and also closed the high end about 3 1/2 turns and the engine ran like a champ on the ground and also when I picked it up vertical, it idles nice about 2,400 and at max between 11,200-10,900. The only thing I worried about is overheating with all the needle adjustments. After getting the motor to run good I did move the high needle valve forward a couple of clicks just until I could hear it slightly drop rpm( richen a bit). It just seems to me that the needles are closed to much but if it runs good should I just leave it alone?
Thanks
Shogun

I'm glad you achieved some results.

It's not easy to trouble shoot a problem over the internet.

People don't always say what they mean and sometimes, my reading comprehension falters.

But maybe if a person is given the area to look at or how to diagnose the problem, or some cause and effect relationships...?

With the help of several people, the owner, most often, can do the work himself?

Enjoy.

Jim

I believe I have one of the 2nd series .91 FX. My problem is keeping full throttle rpm's constant, especially when going vertical. I tried adjusting both needles as much as I could and just when I thought I had the perfect setting, it would overload or simply quit. I installed a Perry pump and that seemed to improve performance but, it's still a bit unpredictable. Sometimes it may run perfect for 2 or 3 flights just to have the same problem reappear for no reason whatsoever, same fuel, needle setting, etc. This may be OS's problem child.

Well, I thought I had it figured out and the plane ran flawlessly for one flight then I refueld and cranked it up and attempted a take off and the dam thing just quit, I did minor adjustments to the high end first which didn't fix it so I adjusted to low end and it still didn't help, I can get it to run just fine at idle and max then after about 3 minutes or so it just quits.......i don't know what to do now.....I'm considereing just getting a brand new engine.[:@]

One more thing how can you tell if the motor is the 1st, 2nd, or newest type of OS 91 FX..????

There are several threads here on the problems with the OS 91 engines, a search will bring them up for you.

Most seem to have been solved with the addition of the Perry pump and/or the use of the K&B remote needle valve.

Bill.

Shogun,
I think your engine is transfering heat to the needle valve and causing bubbles in the fuel line.If you are getting bubbles in your fuel line cut off the needle valve and let it dangle.The newer needle valves have a plastic piece that stops the heat from transfering from the block to the needle valve.

Even the Tower Hobbies remote needle assembly has worked good. Its cheeper. The new OS 91 FX remote needle valve can be purchased and installed? http://www2.towerhobbies.com/cgi-bin...?&I=LXMW56&P=Z but you'll need the bracket # OSMG9152 I did mine and sawed the remains of the origninal bracket of the backplate. I didn't have to buy a new back plate. This needle valve is from the LA series of OS engines.

After having the bubble problem, people have been mounting their needle valves on the firewall. Try to keep the needle as close as you can to the spray bar without it being too close to the prop?

I was around one Saturday that had a K&B needle mounted on the firewall. It ran perfect. The old needle was still on the back of the engine but not hooked up.

Enjoy,

Jim

Thanks all for the help, here is what I did, I had an old 46 LA laying aroung in my junk box and mounted that needle valve to the fire wall and just left the ****ty one on the 91 FX, then I changed out the fuel tubeing using a larger diameter tube. I have conducted several runs on the ground and seems to be working a whole lot better that before. Hope that this cures the problem, I'll head to the feild just as soon as I get some more fuel, used 1/2 gallon trouble shooting........I think OS should cover my next gallon for me.LOL!!!!!!
Thanks again all, and I'll send another message after my next day at the feild.
Shogun

[8D]

That sounds great, but the 91FX is a 2 stroke engine and has no valve adjustments.

You are correct. I misread the original post. [X(]

FINALLY!!!! Well all I finally got this thing running right.....After changing out the needle valve and replacing it with my O.S 46LA one then replaceing all of my fuel tubing with a LARGE DIAMETER one.....Went to the fly field and made 4 flights with no problems what so ever....(DIDN'T NEED A PERRY PUMP after all)

Thanks again all for all the help!!!!