reyn3545

I have a FX91 that starts immediately, idles great, but at about half throttle, it starts to sputter a little. This continues all the way through high end. I've changed the glow plug, changed fuel, etc.. but can't resolve the problem... The engine doesn't die, and it will fly, but there's an obvious loss of power and it sounds like sh(&. Any suggestions, short of rebuild?

NikolayTT

Have you noticed bubbles "travelling" in the fuel line ? Have you changed all tubbings with new ones and tighten them at the ends ?
Any loose bolts and broken gaskets ? I have similar problems and managed somehow by adding small second fuel tank in series
of the main one to "clean" the fuel from bubbles. But I had also this problem with both .61FX and .91FX and I would like to ask
the same question as you to the engine support. Time to time the run is erratic, power is far from max one while there should
be no detonations with fuel of 8% nitro and 18% Aerosave Oil, I tried with glow Plugs OS-8 and A5, still sounds like detonations
time to time, and always only(!) with older well broken in engines, newer with a new one.
What else could be some other reason ?
Nick

Bax

My Feedback: (11)

If the midrange is excessively-rich, then it may be due to a too-rich idle mixture. To get the carburetor properly-set, you have to get the full-throttle setting first, and then work on the idle mixture.

RCWI

HI, IM having the same problems, My 91 has a total loss of power at full thrott. I have had to replace the high speed needle valve do to a high speed nose landing, so i ordered a new one and they said the valve that is for the 61 workes with the 91, Is this to small for my 91? before the change of valves I was getting over 13,000 rpms with 13x8, and now just over 12,000. I also had to repare the muffler but I think it is as good as before. also at high speed in the air, there are puffs of smoke, not a steady flow just small white puffs as she sputters along. Please can anyone help me. Thanks

reyn3545

I sold my problem today. Now the boys at the club will use the engine/plane as a target for their other beaters.

RCWI

Oh sorry i for got to mention that the high speed valve is located about 4 inches behind the engine, remotely. So there is a a longer hose than normal but she use to run fine before even with the longer fuel tubing.

Russell Bear

Are these high mileage engines? Sounds like bearings to me or too small a fuel line.

RCWI

No not high milage, maybe 4 gallons or so. I do have a little air in the fuel line but i didn't think it was very much. The tank is wraped in foam also the front of the tank that touches the fire wall.

reyn3545

Mine was definately high mileage. city miles. never an oil change.

Bax

My Feedback: (11)

When any engine has problems with fuel draw at full throttle, it's usually something to do with how the fuel is getting to the carburetor. If an engine ran well for quite a while before the problems started, you have to determine what changed. With larger engines, many times the muffler used to deliver pressure to the fuel tank just won't deliver enough pressure to insure good fuel pressure to the carburetor.

This is a lot more common than many people think. Muffler makers don't design their muffler specific to the engines. They have a standard can size and outlet stack size and simply change the dimensions of the mounting flange to fit different engines. This results in some mufflers delivering more pressure than the engine needs, and some delivering less, even though the muffler is stated to be for that particular engine...that just means it will fit and not necessarily that it's best for performance.

The surest way to find out if there's a problem with an engine is to mount it on a test fixture with a good fuel system setup and run it. Get the needles initially set to how the instruction book tells you, and then go from there. If you can duplicate the problem on the test stand, then you have a problem to investigate. If the engine runs perfectly-well on the test stand, then you have to look somewhere else...your difficulty is in the airframe/mount/engine/fuel system combination, and will take some more detective work.

It's not always easy to locate a difficulty, and we're not really able to offer a cure via a forum. Working with the engine is the only sure way.

As always, you can send the engine to us for examination. We're glad to see how we can help you.

RCWI

Thank you very much for your help. i will see what i can find

ArthurD

Bax,
I have four OS91FX engines. The older one runs great. The last three run sputtering from about 3/4 throttle to just short of full throttle, then will clear up and produce power. All three of these last engines are the newer version and have the two groves cut into the piston just below the ring. I suppose the groves are to hold lube oil. ONLY the newer engines are having trouble. One of the three was particularlly bad; it would even die rather than transition. I sent it back to Hobby Services and they sent me a new engine which is much better in the exact same installation. Adjusting the HS needle leaner will clear up the problem but now the engine is too lean at high rpm. I understand your instruction for carefully adjusting the HS needle valve and then the low, but no combination seems to make the problem go away. Seems to be a problem with intermidate throttle mixture control. Do you have any idea what can be done for a more lineiar mixture control?

ArthurD

Forgot to mention the muffler is stock OS, baffle installed, no extension, no deflector. Total stock.

Bax

My Feedback: (11)

The "sputtering" you mention sounds like your engines are just too rich in the midrange. You may need to adjust the idle mixture a bit leaner. If you have a check-valve in the pressure line from the muffler to the tank, get rid of it...check valves usually cause over-pressurization.

One thing to realize is that you will not get linear throttle response from almost any model engine. The carburetors are too simple for linear response. They all tend to be rich in the midrange, and you adjust it by leaning the idle mixture a bit and upping the idle RPM a bit...unless the carb has a specific midrange adjustment.

ArthurD

Bill,
This is a follow-up about sputtering high end of several of my OS91FX engines. It was the fuel. Even though the problem persisted through several jugs of new fuel, there was contamination. To prove it, beginning with an empty tank, and an unopened jug of fuel, the plane was fueled up and run. Immediately I noticed the HS needle was tuning much different. More than a turn of the HS needle to get the engine up to speed. Then it ran perfectly and throttle response was excellent.

It seems the practice of pumping the plane's tank back into the jug at the end of the day, together with our very high humidity affected the fuel.
Morgan says water (a product of combustion) is introduced into the fuel from the exhaust and unusued fuel should never be returned to the jug.

The strange thing is that the oldest .91 engine, which does not have the two groves below the piston ring, isn't affected and runs fine on fuel that won't run well in the newer engine. There's something different about the new OS91FX engine.

Bax

My Feedback: (11)

If you're running muffler pressure, then the fuel you return to the main can from the fuel tank at the end of the day may also be contaminated with exhaust products...oil, water, acids, and so on. They can contribute to erratic running if they are in large-enough quantities.

ArthurD

My point exactly. Such a simple problem but it wasn't easy to diagnose. For weeks I resisted a fellow flyer who kept telling me he had the same problem with a new 46AX and the cure was new fuel. I finally tried it just to shut him up and my problem immediately went away. It's amazing just how quickly a new jug can get contaminated. One unused partial tank of fuel pumped back into the jug is all it takes. The odd fact is that it only affected the newer engines. Maybe they have a little more compression or a differently shaped combustion chamber. Who knows, but there is something different with the newer engines.

Piet Le Roux

Hi all OS91FX “fans”. I had had similar problems with my 91 that is installed in a sportsman pattern plane with a tune pipe. The 91 is the biggest two-stroke engine that we can use in the sportsman class and because I don’t like four-strokes I had to get this engine to work. The first thing you must make sure of is that your needle has a good and airtight seat in the housing. The next point that you must be aware of is that if the bottom end is set too rich the engine will still start and idle well, the transition will be good ( especially when the engine is cold) if you then set the top end on the rich side, like it should be, the mid range will be too rich and it will splatter. When it starts to splatter, it will in some cases, not be possible to reached your peak and it will seem like its splattering from mid to peak. I have found that even a ¼ turn leaner on the bottom end can solve the splattering. I have also found that different bathes of fuel and weather conditions effect the engine, so check your mixture setting once a day when you fly.

Cheers Piet