I have a 91FX on a Goldberg Ultimate that runs erratic in the midrange around half throttle.
The engine is well broke in with about 25 tankfulls of fuel, the engine is mounted so that the cylinder is at an 8 O'clock position and I have to run the Idle mixture a bit lean, about 5 turns out.
The main needle valve is at 1 3/4 turns out. At full throttle I tune it for max rpm then back off about 4 clicks, at this setting you can back it down to half throttle and hear it missing, it sounds better if I back off the needle another 1/4 turn.

At these settings, (5 turns on idle and 1 3/4 on high needle)
Idle and low speed is fine, great transition to full throttle, smooth running and great power at full throttle. Has good vertical and dont miss it being tuned to max rpm. It just sounds lousy at the midrange. Using stock muffler, have tried different fuels, using os 8 plug and 13x8 APC. I can back out the high speed needle up to another half turn and it still makes no difference in the midrange.

everything souds just fine maybe at half the engine is fourstroking that all

Now that Im thinking more about it, it used to run clean throughout the range. I changed the fuel lines at one point to a type that fit tighter on the nipples and maybe its just enough smaller diameter to make a difference of how much fuel its getting. Maybe it needs more flow at that range and it isnt getting it?
I have to go out of town for a week and when I get back Im gonna change back to the other fuel line and see if that makes the difference.

I searched, but could not find the post. Some guy claimed he solved the problem by going with large fuel tubing. Everyone said that the line would not cause the problem, but he swore it fixed it.

bigger fuel tubing fore sure fixed our fx problems from lean running to easy tuning no more dead stiks etc. so please look at your tube size.

Hi bipeflyr, what tube i.d. were you using originally and what are you using now ?

Are ya'll talking about the fuel line between the needle valve adjustor and carb, or about all the fuel lines? I just put a new 91FX on a plane, and after about a minute of flying, the engine seems to fade and then quits. Is this the infamous O ring thing maybe, or a fuel line fix?

tank to needle to carb

Hi!
Inner diameter on fuellines has no effect on engine performance or behavior if you just keep using popular sizes (1,5-2,5mm for engines from COX .049 to big OS .300 twins!
Most of the time when an engine quits .....it's because you the owner has made a misstake and set the highspeed needle (or idleneedle) to lean!!!

Regards!
Jan K
Sweden

Okay. RC Report mentioned something a month ago or so about fixing the OS .91FX by replacing the needle valve adjustor with a K & B needle valve adjustor assembly because with the OS assembly, the engine got hot and vaporized the fuel before it got to the carb. OS says that I need to buy an O ring set to replace what they originally put in it, and a guy at my flying field said that he fixed his problem by wrapping the standard needle valve assembly threads with teflon tape.
So, is it:

A) new O rings ($7)
B) bigger fuel lines (pennies)
C) teflon tape (pennies)
D) a K & B needle valve replacement ($18)
E) I'm leaning the thing out too much (free)

or, is it any of the above?

Hi!
Probably E...!!!


Regards!
Jan K

do the tank to needle believe me after months of unhappy running fx this fixed all, offcoarse you have to tune and adjust properly anyways no.
ps we also did the o ring before the tubing.

Pressure loss across a straight tube is proportional to the length, and to the 4th power of inner diameter.
It's worse if there's a bend.

I am not surprised that some people fixed the problem by going to a larger ID fuel tube. The whole concept of remote needle lends itself to lower fuel pressure at the carb. So larger ID fuel tube can help compensate for the pressure loss on *this particular* engine.

Several guys have reported solving the leaning out problem by using a detached needle valve assembly. The K&B needle is only about $15. The attached needle appears to get too hot, causing the fuel to vaporize. They did not need to go to larger fuel tubing.

I wonder about the fuel tubing size issue. The orifice sizes through the needle valve and then the spray bar assembly are much smaller than the fuel tubing. It would seem that most of the fuel restriction would be taking place through these orifices and that larger fuel tubing would have very little benefit. Also, it is curious why the engine can get enough fuel at full throttle, but not enough at mid throttle.

Someone should try a different carb. on this engine, one with a built in needle valve. Let us know what you find out.

I have 2 FX91's, both had midrange problems. They also ran hot at full power. I shimmed the head like the helicopter guys do and they both run fine. I think it's a great engine but not right out of the box, very unusual of an OS product. BOB.

That's a new one. How do you shim the head? Is it a part I can order or is it gasket material? Keep in mind that I'm a guy who takes his car to the Quick Lube for an oil change, so if this is difficult, I cant do it. Thanks!!

You just add an additional head gasket. The engine should already have one. You can order this part for the engine. It may come as part of a gasket set. As I recall, it is only a few bucks. This will lower the compression ratio and the engine will have a little less power, but will run cooler.

I changed the fuel lines back, from 2mm to the original 2.5 mm. The midrange is running a little better now but not like it should, its missing about half the time its running in the midrange.
As for the post from "rcman", yes, it would be strange that its lean on midrange but not in the high range.
I tend not to believe its related to the "common 91 FX needle valve problem" as it doesnt deadstick or show the caractaristics of the other posts related to the o ring problem, but I may be wrong.

Another factor involved is that the feed line goes from the tank then down a couple inches to a GP fueler valve then back up to the needle valve. The middle of the fuel tank is located 1/4 inch above the spraybar.
Hope this sheds some light on any ideas you may have.

Just guessing here, but there could be an air leaks in the carb, either around the barrel or where the carb. mounts to the engine. A coating of grease is a temporary fix to diagnose the problem. I have also heard of problems where the spray bar is not positioned correctly, in or not in far enough or rotated in the wrong position. Are you using the standard FX muffler. Some of the Pitts mufflers have too much outlet area and don't adequately pressurize the tank. The wrong prop can also cause problems. Hope this gives you some new ideas.

Hi!
I have never in my 27 years of R/C flying seen or heard of a bad OS engine......only heard of ones ...from you in the states....

First of all ...Try to eliminate every possible fault, take away everything that might causing you trouble ...like the GP fueler ...I never use these things myself ! It is much safer ..and better to have just two fuellines hanging down under the engine ...one for fueling and the other for overflow...both closed by a short 3mm bolt!
Second what fuel do you use??
Over here (Sweden )most guys blend there ovw fuel...that way you know what you get....
Use 5-15% nitro and 20% castor oil ..that's the old stand by for nearly every engine on the market! Better yet is all the syntetic oil lfrom Graupner in Germany : Aerosynt 2 and Aerosave or the French brand : Motul "micro" - these oils are videly used in Europe in 10%-15% blends.
As for glowplugs : The best plugs for general sport flying in twostrokes are is Enya 3 or OS 8.....!!! Of coures there are others but these will make most engines work perfectly ...all the time.
What plug do you use?
Propeller : 14x5-14x6 for general sport flying at sea level!
Tank size: 14-16 OZ
Yes it could be that your tank is too high that is causing you trouble?? Why do you have it mounted so high????
What tank (brand) do you have?

Regards!
Jan K
Sweden

Jaka,
Thanks for your input on the subject here. My fuel tank is mounted as low as it can be without modifying or cutting into the cowl, it has been lowered from stock when building it, the engine mounted at 8 O'clock cylinder position has lowered the carb on this model. plug is OS 8, prop recomended as per OS is an APC 13x8, fuel is Omega 10 to 15%.

I am thinking most of my problem is the way the engine is mounted, seems when the cylinder is pointed downward the idle mixtures need to be leaner to help prevent it from loading up.

Yesterday the slightly larger fuel line helped some, today when I flew I used Omega 10% rather than the 15%. The engine ran great today. Im sure that the small changes in nitro and fuel line didnt do much but it was enough to clear up the midrange symptom. Im sure that the heart of the problem is still there and finding that, may give me better power in the midrange.
It was convenient to install the fuel filler with the cowled set up but I will try the 3 line method as you mentioned. I have done that with my Sportster and it works fine, you're right about eliminating all that unnecesary stuff, sometimes it just causes more problems.

glad to hear that the bigger fuel line helped a bit but what just came to mind is that don't forget to drill out your clunk in the tank had many under size before and we use only 5% omega with 5cc Castor added per gallon.