I have been struggling loads with my OS91 engine... each time I get it in the air it cuts, well sounds like its dying or struggling and then dies... I am using a 15x6 prop... haven't flown the plane I use it with once, just need to repair the damages every time, the plane doesn't even get 10m of the ground so I don't have height or speed to land it softly... on the ground the engine is fine... loads of power and response is good... I pick it up rotate it sideways revving it tilt it inverted and also have the nose straight up like if I were hovering... everything is awesome... but as soon as it gets into the air there is trouble... is it just too much for the engine then or what? I am using it in a 60size plane, so its way overpowered... any ideas? it gets too about 9300 RPM with BCM pitts I have on it at full throttle on the ground...

Try a 14x7 prop

Might want to check your fuel lines.... I had a plane that would run great for a flight and then I could start it but not get the carb off seat for the rest of the day. I finally got mad and pulled the engine apart. Guess what I found-a piece of silicone tubing the size of a pin head in the needle valve intake. After that piece was out, I never had another problem.
dan

the 15x6 has worked before on my maiden flight... but the problem then was my pickup... it got stuck in the corner... at least I was in the air for a while but then it cut, had a safe landing back then cause I was very high and had speed... after fixing the problem, making the line a little bit shorter in the tank and also cleaning the HSN valve... taking out the whole carb to clean the LSN... took it apart complete... the tune sounded good at least... so I don't know... is there a specific size or type of fuel line to use? I use Omega 10% fuel... I don't want to try my luck anymore and take off... too scared, cause the same happens each time... I have a 13x8 prop on there now, but I have no idea what will happen... why is everything good on ground? good response, all power is there no bogging... but when it gets in the air it bogs and dies?

Also, make sure it isn't overheating in the air. Try richening it 2-3 clicks.

Is it .91 2-stroke 4-stroke? On either one make sure the head screws are tight, back plate is tight, bearings are good, etc. On the 4-stroke, check the valve adjustment.

its not getting into the air thats the problem... its a 2stroke... everything seems good on the ground so I do a take off it climbs barely and starts bogging and dies... I tune it on the ground to give me at full throttle no power loss and constant RPM... and then I richen even a little bit more... the response on LSN is good... it gets good speed on the ground no bogging just get a good sound of power... but as soon as it starts to climb... I really cant understand this...

I would really look at the carb again, bearings, and make sure everything is tight on the engine. I'd try another glow plug too. Also, maybe try it a slightly richer. Those .91's generally run flawlessly.

Two other things to consider. Do you have a cowl over the engine - the exit for the air could be too small so the engine could overheat. Also some of the pitts mufflers do not create enough back pressure to pressurize the fuel tank - try it with the stock muffler or block one of the exhaust tubes to check that out.

yeah, I have checked all that... cause those were my first suspicions... i have a new glowplug... the engine head is open... it cant overheat that easily could it? i just get speed and then take off and it bogs... cant be enough time to overheat... and I run it full throttle on the ground for over a minute... and it doesn't overheat... the prop cant be too big either, people run 16 props on there and also OS has this prop on recommendation list... so I am stomped... how should I tune it to know that I am rich enough? I did it how OS told me to in the manual... they said to be at full throttle and then lean slowly until RPMS pick up and once the RPM starts dropping it loses power cause its too lean then I need to richen it back to the setting where the RPM is constant and not dropping...

How about removing the cowl for your test flights? This will allow you to see any fuel line bubbles while on the bench - between the tank and the high speed needle assembly and then to the carb. It will eliminate "overheating" as a source. Guessing that there's a remote fuel filler - might remove this variable while the cowl is off to simplify issues. After thought > removing cowl may be a problem if there is "nose weight" glued to the inside of the cowl.

How old is the OS 91 FX? There are several o-rings that might have been nicked during (re)assembly. The needle valve may not be seating accurately. It may be worn. The cowl may shift and put pressure on the needle when in flight. Just tossing out some rambling thoughts ...

the engine is brand new... I bought the plane from a guy... he has run it in and flown about 15flights... I saw him fly the plane which is why I bought it... I always tune without the cowl also to do fault finding... and because everything seemed fine I put it back on...

The exhaust area for the air going into the cowl needs to be 2-3 times the inlet openings - flying without the cowl as SeamusG suggests would rule out insufficient cooling if the problem still exists without the cowl on. Also when tuning after coming back to max power on your needle adjustment you still need to richen the high end needle about 3 clicks, as the engine unloads in flight.

The exhaust area [for the cooling air] needs to be at the back of the cowl usually down by the muffler exit.

Stumbles and coughs on takeoff. What's different between static bench test and accelerating then pull up? Is the tank clunk stuck in the front of the tank? When shaking the plane around before starting do you hear the clunk sliding along the tank bottom? If memory serves me MinnFlyer recommends adding a piece of metal fuel tubing between the "cork" and the clunk to ensure that the soft in-tank fuel line doesn't allow the clunk to fall to the front during a down line.

are you saying that the clunk(we call it pickup) is getting stuck? cause I have made sure of that previously already... I have even shortened it a bit to ensure its always in the fuel...

Did the problem go away when flying without the cowl on as in my previous post? Pressure changes in the cowl in flight. I had to work through this with my OS91FX on a chipmunk.

I suppose you're using fresh fuel, right?

Is the carb's fuel inlet LOWER (more than about 1/4" or so) than the fuel tank outlet? If so, this will cause problems like you wont believe.

I had a similar issue with the same engine. I re-positioned the tank/tank outlet as necessary (a major alteration)to be slightly lower than the carb's inlet, and it's been sweet running since then!

Richen the high end a bit. If it starts "stuttering" when you accelerate, then lean the low end just a bit.

Switch has stated that the engine & plane performed very nicely for 15 flights before he bought it. Now throttle transition from idle to max results in engine failure. I'm not inclined to suggest that he move the fuel tank around.

What has changed since he bought the plane?

Fuel is very reasonable as it is unique to Switch. Old fuel with water, significantly different nitro content, ???
Junk in the fuel tank as a result of removing the tank and cutting the internal fuel lines is a candidate.
Junk from the tank being pushed into the high speed needle is a candidate.
Compromised fuel lines (kinks perhaps) is a candidate as the fuel tank has been removed and replaced.
If the high speed needle has been removed it is possible that the o-ring has been compromised.
Junk may have entered into the carburetor throat that might compromise fuel / air flow.
A glow plug may not be optimal and may "cool off" resulting in no spark thus engine "flame out".

Try an os four stroke glow plug, Also use a smaller Prop, I think it calls for a 13X8 or so.

Crusty - you're spot on "13x8, 13x9" per the os engine spec page. Em, isn't that an electric spec prop in the picture? Perhaps they make both.

that prop is way to big, if your getting 9300 on a 2 stroke you are well below the powerband, you should be in the 12000-15000 range. try a 14x6, 13x8 that will make all the difference in the world.
the os manual is for japan, which regulates noise from the engine, in the USA we dont really care (as long as it isnt crazy loud) so you want your rpm's much higher. your engine will make a lot more power in the 12-15000 range

whats most likely happening is your engine is overheating, or your way to rich cause your prop is way to big (im guessing you have the needle at least 4 turns out)

ok guys... will keep all thoughts in consideration to try again the weekend... I also feel that its either a tank problem or fuel line problem... but I have changed nothing, thats the biggest head scratcher... am using new fuel, Omega 10% (castor) like mentioned... I have cleaned all the valves... just don't have anymore thoughts on this... but thanks to all your help...

Anybody up for a road trip to Port Elizabeth?

a 2 stoke doesnt have valves, the engine on your plane is a 2 stroke (or at least the pic you posted)'

try a smaller prop it will make a huge difference, weither this is the problem or not changing the prop will make a huge difference in performance.

the guy is in south aferica so good luck driving their from co