I have 2 91fx motors .one in a midwest little cap the other in a hawker sea fury and as of today I have 2 less airplanes to fly Cap totaly destroyed the sea fury may be fixed dead stick**dead9 stick**deadstick** I bought one about a year ago the other about 2 years ago.one had grey O ring the newer one had black ring..I just started flying these two planes last month..( os sent new black O rings and said it would stop running problems but it didnot ) I ran both of these motors on test stand for about 8 tanks each (14%) 14x6 ms prop omega 10 fuel...motors ran very good on test stand with black rings no air bubbles in fuel lines grey ring had small amount of air bubbles but still ran good..motor put into little cap 2 tank runs one with cowl one without (14%)..the motor ran very good on the ground no air bubbles in lines ( 14x6 ms omega 15 fuel slim line pitts muff. dubro vib mounts ) about 10600 rpm pinch test on fuel line was good idle was a little rich mid range trans. was a little rough but still good..first flight 3 min. dead stick .. 3 other flights with richer settings all endedin dead stick before 7 min....New fuel tank.new larger fuel lines..3 line dot fuel setup. new o rings back to the fieled more dead sticks...crashes remove cowl .dead sticks try differant prop 13x6 ms 11200 rpm..dead stick crash destroys slim line pitts try bisson (91 large) worst flight ever stalls evon on idle.. get stock muffler will fit know with out cowl no better then slim line 10600 rpm dead stick crash etc new slimline new needle valve assembly( black O ring )motor check for dirt .new fuel line again..(os claims they have very little problems with 91fx ) run motor in plane 4 tanks inverted upside down-straight up etc..NO PROBLEMS this motor runs great on the ground just gets AIR SICK WHEN IT GOES UP***in 30 flights only 6 was the motor still running on landing..the only way to stop it was to land after 3-4 min.turn needle one turn to super rich then complet the flight and hope it dosnot stop on idle..on last flight started off super rich only lasted 4 min.dead stick again-Why did I park the tomato harvester their--clipped it whent in to spin nothing left**SEA FURY with the newer 91fx only lasted 2 flights first one lasted about 6 min. last one about 10 min...flew very fast with 13x6 ms..had GP motor mount no air bubbles in linse ran very good on ground pinch test straight up etc..ran it four 3 tanks before first flight and it RAN VERY GOOD ON GROUND..over estimated glide ratio landed in corn field wing destroded and another slimline muff.. THIS motor has the same problemes as the other one... BOTH PLANES LOST BECAUSE OF 91FX MOTOR PROBLEMES>>>CAN THESE MOTORS BE FIXED AND MADE TO RUN RELIABLE>>> one at club said he fix his by a remout needle valve change ..8 flights latter he now has a hanger9 mustang to fix dead stick again>>>>HAS ANY ONE GOT THE REAL FIX FOR THESE MOTORS>> I would like to fix these motors ase I like the idea of the same weight as 61fx but a lot more power>>>> I have a WM super CHIPMUNK built ready for a motor if i cant get these to run reliable ill have to use the good old 61fx<<<< ANY ONE OUT THEIR HAVE A FIXED 91FX**********

Well, I've followed these threads with interest.

I have a 91FX in a Hog Bipe... been a year now. Only one deadstick in all that time because I set it too lean. Otherwise a perfectly good engine, and she does pull that Hog vertical! Running a 13.5 x 8 Graupner.

I almost feel guilty with this fine engine, honestly... no problems! I think I've had the same glow plug since it was new.

Engines do not cause crashes. Unprepared pilots do.

Bad mouthing an engine that has been used with carbs or mufflers different than that provided by the manufacturer is bad form and is not fair to the manufacturer of either device.

I would purchase an Ugly Stick type of model that is capable of utilizing your engines comfortably and then test fly them with their stock mufflers. This would give you an opportunity to become familiar with the engines under better controlled circumstances. Only then will you be able to determine if the engines are truly having difficulty.

Insufficient cooling air entrance or exit can cause an engine to overheat and quit.

Insufficient vibration isolation for the fuel tank can cause an engine's mixture to vary so much that it will quit.

It sounds as though you have invested considerable effort and thought into solving your problem. Don't give up yet. Try to isolate the problem by narrowing the possibilities. Good luck.

I agree with ED.

Everytime a plane crashes it's an engine problem or a radio glitch.

There's no way to know for sure but I would bet that 99.999% are caused by pilot error. Either in setup or operation.

Ken

I had nothing to do on the farm today so I took EDs advise and put the 91fx out of the little cap into a plane that was good I have about 12 planes at the farm but I chose the easy one..HANGAR 9 super stick 60 had it for about 3 years kids use it as a trainer...61FX for power so could change motor to 91 in less then 5 min...flew the plane once to make sure everthing was goodplane has more then 200 flights on it and 61 has never dead stick or given any problems......has std fuel tank with 3 line setup....after flight changed to 91FX...13x6ms prop..new oring for carb..all other gaskits new..head removed no peeling on cylinder wall...os#8 glowplug..omega 15 fuel..motor mounted upright so could use stock 91 muffler with bafful..ran plane on ground for one tank.ran good but not as good as the 61..one or two clicks of the needle make diff.to it but the 91 you could move it a half a turn with little diff.....set high speed needle to very rich seting as OS said to do....running not to bad so took off...very rich good smoke trail..this 91 sure polls this plane a lot faster then the 61.. IM sure if you let it the 91 would fly it to pieces plane seem to fly great good smoke trail then in about 3 min. the motor started to loss power ( still had good smoke trail) and run very rough landed plane still running had to turn needle valve back almost 3/4 turn to get it to run as good as begining***plane ran good on ground so took off again good smoke trail but maybe less..plane fllew good for about 2 min. then motor stoped dead stick##(still had smoke trail) which is no big deal with this plane ase a blind man could almost land it###filled tank had to open needle valve to get motor to start tried two more flights all ending in dead stick## maby needle valve was bad so changed to commeplet new back plate assbly I purchased last week##motor still ran the same..sometimes not even lasting 2min. in the air but RUNNING GOOD ON THE GROUND###* 8 flights all ended in dead stick unless you landed and adju...needle sometimes twice####motor would only run super rich in the air if you lean it down motor would not last 30 sec. in the air but run fine on the ground###please remeber that this plane flys great with the 61FX and ther is no air bubbles in line from needle to carb....I put the 61 back on the plane and had a great 15min flight landed it and put it back in the barn before the kids come I have 6 46FXs 2 50sx and 3 61fxs and all these OS motors have never given any proplems????????? I will try tomor to run the 91fx out of the sea fury but it dosent look good###this 91 was doing the same thing in the midwest little CAP and a differant plane did not help###OS STILL CLAIMS THEY DO NOT HAVE PROPLEM WITH 91FX......IVE DONE everthing they said##needle valve change..stock muffler...13x6 prop...fuel tank within 1/2 inc. of centerline...larg fuel line..lone break in must have at lest 2 gallons of fuel throught it by know########and the motor is stillno good....this motor is out of warrinty but the other one is still good so willsend that one back####....LOTS OF PEOPLE ON THIS SITE WITH NEARLY THE SAME PROPLEM HAS ANYONE BEEN ABLE TO FIX OS91FX OR IS IT JUST A GOOD PAPER WEIGHT ?????? or should i look for a SAITO 100 for my chipmunk it would be my first 4 stroke :drowning:

I have had 3 of these engines and al have run very well. But they all have had weird needle settings.

All three needed to have the high speed needle out about 3 to 4 turns to run good. I still have one on a Kangke cap 232 and it runs very well. I have the high speed needle out at 3 1/2 turns.

All three of my 91 fx engines have had this problem. It seems to me that 3 to 4 turns is way too much but they dure run good at that setting.

Anything less than 3 turns and they all will dead stick just as you described.

Givve it a try at 3 to 4 turns it will seem a little rich but once in the air they run great.

Let us know what happens and good luck!!!

Well, no one can say that you are not giving the problem plenty of effort.

I wonder what would happen if you were to replace their carb and needle set up with a different carb?

Are these ringed or non ringed engines? Thanks for going along with me on this.

If you haven't already, I would suggest looking at the thread "OS91FX problems" on the second page of this forum and then trying a realy remote needle assembly.

Ed, the .91FX is a ringed engine.

In other threads, people have said they have good luck with the Perry Remote Needle Valve, but they are the ones seeing air bubbles between the needle valve and carburetor.

Try tightening the head bolts.

What I noticed on my 1.60FX when it was new was that it took nearly a gallon of running on the ground before it would run reliably in the air. One night I deadsticked it six times in a row. The next day I tethered it and ran three tanks through it gurgling rich before trying to fly it again.

It's a fact that ringed engines require more breakin than ABC types, and that breaking them in 4-stroking rich is the way to go.

I agree with Mathew on ringed engines needing a lot of break-in time. Even OS engines with rings need the time.

I know that people don't want to hear that kind of talk these days, but that's the way it is generally.

I wish the original purchaser good luck in straightening out the problem. There's not much worse in our hobby than spending money on proven name equipment and then having what appears to be insurmountable problems with said equipment.

This is just a guess mind you but you seem to be running small props on this engine and I'm wondering if you are leaning out in the air when the prop unloads which might cause you to deadstick from overheating?

I find it curious that both your engines have the exact same problem which seems to indicate
its somethng you are doing wrong with both.

Try moving to a higher pitch larger diameter prop like a 14X10 and also check all the bolts (head and backplate) for tightness. The only bad running 91FX I have ever seen out of many was missing 2 headbolts for some odd reason.

Other problems with this engine have been "O" ring related or even the size of the casting for the needle being too large - both allowed air bubbles which you seem not to have.

My last suggestion is to send one of your engines to someone here that you can trust to check it out for you and see if they can find a simple solution OR send them back to Hobby Services with a note about what is happenening.

This has been in other OS .91 threads, but I have one and so does a buddy. Both of ours run great NOW. Get yourself a K&B .61 remore needle valve assemble, $15.95, and replace the OS one. The K&B screws to the firewall or where ever you want it and doesn't leak.

Just for information, I had mine on a Joss Stick, engine out in the breeze. Ran crappy-always leaned out. I normally set it for a smoke trail so I didn't have any dead sticks, but I noticed it leaning out. I finally took it off the Joss in favor of a Magnum 1.08. Good running engine and so is their new .91XLS.

Get the K&B NVA and your engine will work.

.. FARM work and bad weather have pervented me from doing the inportent things in life. ..I did geat time at knight to put 91 from cap back on the test stand....13x8 ms prop 15 omega fuel os#8 glow stock muffler with baffful..this is a test stand mounted on bench fuel tank center line ( 16%) is about 1 inc. below carb. center...motor would run good with needle out 2.5 to 2.75 rich setting much past 3.75 motor would stop because of to rich setting## I tried the bisson pitts muffler (new slimline comming next week ) I noticed with the bisson the needle had to be out .5to.75 more to get same run ( richer )..tryed to tack motor but geebee would not work with barn lights##you could almost turn it out 4.75 turns before it would stop##motor rpm seemed faster then stock muffler.....but would not idel as well...I put stock muffler back on an decieded to try something runing at a rich setting needle 2.75 out and max thut..Ipulled muffler presser off the motor stopped int..##you could not get this motor to run good without muff. pressure evon with needle out 6 or more turns#### I have had other OS motors on this test stand that ran good without muff. pressure simple by turning needle out aturn or more###and some of them were not evon brokin in yet###IS THE FUEL DRAW LOW ON THESE MOTORS and this is the real design flow or problem with them##

I tryed something from the old race car days pressurize the fuel system>>>>I took an old spray can from a model car spray kit and hooked it up to the muffler line of the tank giving it about 1 to 2 lbs.of air ....started the motor and boy did it come to life just screamed I wish the tack would have work..must have been near 1000 rpm more needle vale was out about 1.75 turns and still running rich and one or two clicks would make a diffferance###Idle was better ..mid range trans.was great this idea brought the motor two life
IS THIS WHAT A PERRY PUMP WOULD DO?????? and if so dosent this prove that their is a design flaw in the fuel system of the OS91FX#######

HAS ANYONE USED A PERRY PUMP ON 91FX WITH STOCK CARB.AND NEEDLE SYSTEM??????? I have exstra back plate I can use but have to order perry nearest hobby store 100 mi,l.e but dose not stock it......are they ease to install and is there any problems with useing them????????

THANKS FOR YOUR TIME AND I HOPE IVE FOUND THE ANSWER

Hey guys!!!! Stop Bashing the os 91fx!!!!! I have one and after a little work and setting it up properlly it runs great. If your using any other type of muffler besides the stock muffler .....MAKE SURE YOU HAVE THE RIGHT AMOUNT OF BACK PRESSURE OR SHE WILL RUN LEAN AND YOU WILL DEAD STICK!!!!!!. p-51 STANG.14-6 PROP 15%NITRO,20% OIL....SLIMLINE PITTS MUFF. iF YOU LOOK AT THE SLIMLINE PITTS MUFF FOR THE 91FX CAREFULLY YOU WILL NOTICE THE OUTLETS ARE ONLY ABOUT 1/4'' INSIDE DIAMETER. tHIS IS FOR A REASON....BACK PRESSURE.....CK IT OUT..tONY gONZO

Mine ran great for a little over 3 1/2 hours then the needle valve went to leaking so bad I can't use it. GP service sent me out 2 packs of 2 orings. Guess that will be enough to get at least 12 more hours out of the engine. Hope to have a remote needle on it by then. Trying out the first replacement oring this afternoon.

Ed M.

It sounds like these 91FX engines are lacking in fuel draw. Then again, putting the tank a full inch below the carburetor center line will make your engine run much leaner. It's best to have the tank centered on the carb centerline.

A Perry oscillating pump should make a whale of a difference in this case.

Dear Everyone,........IF YOU ARE USING ANY KIND OF PITTS MUFFLER MAKE SURE YOUR GETTING ENOUGH BACK PRESSURE!!!!!!!!!!!! BEEN THERE DONE THAT......NO PUMPS NEEDED!!!! STOP AND THINK......Tony Gonzo

SCREW os!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
GO WITH MVVS MOTERS,THEY HAVE THE BEST THROTLE TRANSITION EVER,HANDS DOUN!!!!!

I had the same problem with fuel draw.I put a Perry Oscillating pump on it and it fixed my problems

Porsche911 I keep it simple and to the point I have a os91fx and it runs perfect several hundred flights and no dead sticks, But I have had the same exact problems on a Supertiger 51 ringed It's scary how you threads and my experence with the motor was similar.

After all the problems I faced it turned out the ST HAD A BROKEN RING SHEEEESH!!!!! HAD 40 DEADSTICKS WITH THAT ONE just like you the needle never seemed to be set right and it changed on a min by min basis.

I think it may be time to pull the piston on that 91 and check the rings.

Hope this helps

Talk about grabbing an old thread to post to! I get it, recycle everything, so as an update I am now out of o'rings. Put a Perry VP30 pump on it. Kinda hard to suck in air when its under fuel pressure at the o'ring. So far so good, but not enough time on it to tell yet.

BTW, each time it would start acting up (sucking air at the needle valve) after a few hours running time I would change the o'ring and everything was fine again.

Ed M.

THIS is one of my old threads I started last year.......but I guess as BENTGEAR said its "recycle time" ..so I will bring you all up to date........

I NO LONGER HAVE THE TWO OS91FXs engines ..I traded
them for a new LANIER STINGER 60 ARF and parts........

I replaced the crashed LITTLE CAP with a SUPERKRAFT EXTRA 300S arf ( THIS IS A VERY GOOD ARF )..and the SEAFURY with a WORLDMODELS super CHIPMUNK arf ( this to is a good one )....

KNOW for the big part...I replaced the two 91fxs with two new SAITO 100s...ALL I can say is WOW I should have done this from the very begining...when I relized.the 91s were not running that great on the test stand and other people at my club having proplems with os91fx as well.....................

THE SAITO 100s both ran perfict rightb out of the box...no modifficatoins needed...no perry pumps..no differant needle valves..no speacil exhaust...etc.....just run it stock right out of the box just like it should be
THE only thing I did was to take motor a part and pre oil everything with after run oil as I was told some saitos have bearing to dry in the begining........

IT LOOKS LIKE OS HAS LOST ANOTHER ONE.....it seems as though there is a new grop of people willing to give the 91fx a chance.. I HOPE THEY HAVE BETTER LUCK THEN I DID...maybe os hase fixed the proplem on the newer ones..............

I HAVE NOT TOTALY GONE AWAY FROM 2 cycle as I have two new motors for other projects....both are WEBRAs
webra 50GT for a cessna ag wagon ......webra 61F for the stinger.........but those motor are for a new thread..???????

thanks for your time and good luck to 91fxs owners...####

I have an OS .91 too. It seems that it must have been buit by Magnum, because some guys at my field have no problems with them and others like myself, do. I dont believe it's operator error either...its the motor, and the luck of the draw.

yup we had been there, but after changing to big tubing and DRILLED CLUNK we now even run our 91fx in our razzle fun flier, we hover and flat spin upside down and never had a dead stik.
yes we tune with a tach and run omega5% and use os a3 plugs.
and only the stock muffler.
GREAT ENGINE!! pic to show the fx and razzle and if you insist i will post a hover pic

The Magnum can't be an OS .91 copy because my Magnum ran fantastic right out of the box What's a deadstick? Never had one?

Good luck with your .91's