Hi

The problems experienced with the 91fx were without the nv extension.
I do not think that having that amount of weight that can vibrate attached to the nv is a good idea but unfortunatly
it is not the source of the problem.

This motor like to rev not lug heavy props.

Hi all,

About lugging this engine down : OS recommends a 15/8 prop in the manual, which was surprisingly much prop to me ( I had bought a 14/8 together with the engine, before reading the instructions). Everybody on this thread seems to use a lot less prop, and still the engine overheats ... ?

Because everybody blames the remote needle valve, I have bought a GMS 2000 remote needle valve assembly, and I am planning to use the 14/8 once the engine goes into my 70" Extra. I will also use a soft mount, although most people say that the bubbles are not caused by the vibration. Everybody uses low nitro content in my club ( about 0-5%), so I will stick with my normal 5% mix. OS#8 of course.

Break-in seems to be critical, so I will be careful and patient there.

Maybe I will be one of the lucky ones, where it all goes well

Cheers,

Kris

Ha Tim...I read of some people spinning APC 15/10 props on the 91FX...I find that almost to hard to swallow...but mine is still strong and I know the pipe really woke the engine up...I would not sell mine as others are doing...once broke in behavior is good as is power.

Ha El Capitan...when you gonna have it ready>>?? I am interested to read your report.

Hi

The heavy props definatly cause the motor to get hotter. **If** you dont lean your motor too much you can run the bigger props successfully but most people
are not able to resist the temptation to go for the extra rpm.
My prop of choice is 14x6.

If you get bubbles it will lean the motor out no matter what prop and cause it to get hot.

I have 2 and this is what I have found:
They do take a while to run in
They run hotter while they are new
They have a loose big-end bearing fit with a pronounced clack at TDC when turned by hand

When run in and no bubbles (perhaps with other remote NV assembly) they give good power, are reliable and mine are only started by hand.





The specs have been revised by Hobbico see : http://www.osengines.com/engines/fx.html

From the link
.91 FX
(OSMG0591)
rpm 2,000-16,000
bhp at rpm 2.8 @ 15,000
weight 19.3
recommended props 13x8-9 <---------------------------------------------------





From the OS faq

.91 FX FAQ

My engine seems to be bogging, running 'heavy' or hot, and does not hold a needle setting well. What's wrong?
--------------------------------------------------------------------------------
My engine seems to be bogging, running 'heavy' or hot, and does not hold a needle setting well. What's wrong?
Assuming the engine is fully broken in, you are using good quality fuel with at least 18% oil, and that there is no fuel foaming occurring, it is possible you are overpropping the engine. With use here in the U.S. at our altitudes, fuels, etc, we have found that this engine prefers slightly smaller props - 13x8 or 13x9 specifically - and turning at higher RPMs. If your engine's max RPM is under 10,500 then the engine is working too hard and not making peak performance.

Remember, this is a 2-stroke engine. It cannot be expected to turn the same enormous load of prop as a far more expensive, pumped or super-charged 91 4-stroke engine.

.................................................. .................................................. .................................................. .................................................. ............

As far as i am concerned the problem is caused by vibration. The needle is vibrating and causing the fuel to foam as it passes. I have had this issue with a clunk as well in the past.

I run a 15x7, it goes well at about 9000 rpm. Did try a 15x8 but it was too much. I would like to use a smaller prop but it is too noisy for my club field.[]

Well I got my muffler the other day and I picked up a couple of props.... a 14x4w APC and a 14x5 Zinger Pro... I am going to be paying EXTREME attention to this engine onthe first several tanks... I plan on running 1 or 2 tanks on the bench with a high volume fan blowing across the head. I am also going to go ahead and start out with a remote needle and a PERRY PUMP.... The perry pump is due to the fact that I will be mounting the tank on the CG and the remote needle is just because it sounds like a typical fix.

I may start the bench running in a few days, but the plane won't be ready for a month yet.... Short on time right now.

I think the pump and R/N will work so well you will not see any of the past probs...had I had it all to do over again I would run at least 1 gallon thrurich as you would normally do a ringed engineand see if the low end vibes are there but the heat will go down and after a gal it should be more than ready to fly...this motor like to rev..at least mine does. I am sure it has some good low end but I never proped it for that. Have fun and keep us posted.

RE the OS 91 bubbles in the fuel line.

My thoughts on this topic are that engine heat is being conducted through the remote NVA and causing the methanol to vapourise. The bubles are not air but methanol vapour. This, combined with the (relatively) lower pressure on the carb side of the NVA caused by engine suction assists the vapourisation - the pressure drop is likely to be the final factor needed to vapourise an already warm fuel supply. This could account for the reports of high engine temperatures - if the engine heats up and runs on partial vapour the resulting weaker mixture would cause higher engine temperatures.

The reason a pump helps is because it will tend to reduce pressure fluctuations across the NVA: it addresses the symptoms, however, and is not the cure. All engines have a pressure drop across the needle valve - not all engines have a warm fuel supply AND a remote NVA. Move the NVA to a bulkhead or change the carb for an on-carb needle valve assembly and the problem will not occur.

With an on-carb NVA it makes little difference if the fuel reaching it is hot and ready to vapourise - the carb is about to do that anyway.

OS scored a bit of an own goal with this one...

Mick.

I owned two of them a couple years ago. The only way I got them to run better was to return them and install a different size. I am an OS fan. I have had most of them and all are fine but the 91fx--JUNK

MickJC

My position exactly and I've been pressing it on this forum for about 2 years now. The problem with this theory is after while you notice when you're walking down the street that people point at you and laugh and whisper (flat earth person).

Denis

I have been around here for a couple years now and no other engine on the market has been slammed as hard as the 91fx. It either runs good or it doesn't. Most of the time it doesn't. All of the OS line up is great with no problems except the 91fx.

I agree that most of the problems fall into the carb....with the remote needle
mounted where heat and vibration could be problematic, not to mention the
bubbles due to leaking parts or vaporazation. I wonder if the ASP carb would
cure alot of the problems.

[sm=thumbdown.gif] Too bad the Lads at OS weren't clever enough to
make the carb "convertible"....like the obvious brighter Lads at the Tower
works....

....I believe the Towers .75 carb has a 15mm flange as well.

Dave.

I bought a used one ... read all the (-) posts ... and got worried.

I have had absolutely NO problems with the engine. It turns a 14X6 APC with authority, never dead-sticks and seems surprisingly tolerant of sloppy needle valve settings. It starts by hand every time and is the most quiet engine I've ever owned (I use the stock muffler). I also get good fuel economy ... I can fly for 50 minutes at mixed throttle settings on a 24 oz tank (plane is a SUPERSTUNTS 60).

I find that 15% fuel works the best ... I noticed significant power reduction on 5%. I use a OS #8 glow plug, and have not replaced it yet (> 100 flights).

Maybe I got lucky, or maybe the previous owner worked out all the bugs, but it's my favorite engine.

You certainly are lucky twice................getting the engine to run with out mods..................AND, be able to stand in one place for 50 minutes!

Meet me in Vegas and we will clean up.

Dave,

If the theory is correct then the ASP carb/NVA should cure all the problems - Just Engines can supply them for around £20 or $30.

If the ASP carb does cure the poor running it would be a bit of a poke in the eye for OS...

I would think any combined carb/nva would do the trick, though.

Mick.

Denis,

Don't fear paranoia: the enemy IS real.

Mick.

I have to say that I have a number of OS engines over the years and never had a problem. I had an OS 45 FSR which was a stonker of a motor until it was abruptly terminated at the hard end of a spin.

I think OS were a bit unlucky with the 91 (not to mention their customers). Where they made their biggest mistake was in not offering the customer support a company of that stature could surely afford.

Mick.

Well I guess you win some...and you loose some...even O.S can make a boo boo sometimes...all is not lost...I am sure it will all be forgotten in time and we will all look back and say...ha ...remember the 91 FX....sure glad the came out the the .91"XX" series...they seem to do that a lot. Like I say..the only problems I have is the vibration at idle...but bigger displacement means more vibes...this is true in most all the .90 and up motors. I have started using electrics now and there is little to no vibration and with the brushless motors and li-poly batteries power is now at and beyond the same glow output, but thta's off topic so I will ease back to the topic at hand.

Boss248,

Yeah: It's a wonderful thing, technology. I've never got into electrics, though: I think I would miss getting my fingers rapped, having engine problems (the ones I can sort out, that is) and going home stinking of fuel. It's all part of the enjoyment for me.

I love IC engines: electric is soooooooooo clean.

Mick.

If it's not greasy, noisy and dirty it's not fun.

WHY SHOULD YOU HAVE TO RESORT TO SPENDING MONEY ON A ENGINE THAT SHOULD RUN CORRECTLY IN THE FIRST PLACE? IVE NOTICED WITH EVERYONE ELSE THE NEEDLE VALVE O RING IS HORRIBLE THERE IS NO SEAL WHATS SO EVER. OS HAS SOME FIGURING OUT TO DO I HAVENT GIVEN UP IT WILL BE FIXED WITHOUT SPENDING MONEY.

Hi all,

Well, I just wanted to jump in here and say that I love my 91FX. I put it new on a GP
Big Stick 60 after running one tank of fuel through it like the manual said. I then flew
it rich for a good half an dozen flights and it told me when it wanted to be leaned out
a bit. After initial setup, I have not had to touch this engine! Week after week I take
it to the field and just touch the starter to the spinner and she fires and idles very
nicely. When pointed down the runway and throttle is advanced, she is very smooth
and responsive. One of our club members commented that the plane looked it had
been shot down the runway like an arrow. The climb-out is great and there is never a
hesitation when in flight. I have done nothing to this engine since I first set it up.
I am using Power Master fuel with 15%nitro and 18%oil content. It is swinging
a Master Airscrew 13x8 prop.

Just my two cents...............but I love the engine and just bought another.

Regards,
Andy

rcandy, My experience has been the same as yours. I have a 91FX in a Goldberg Chipmunk and I could not ask for a better running engine. Mine is also completely stock and I'm running an APC 14/8 on Powermaster 10% fuel. Maybe we got lucky????RS

I have been following this thread and other places about the vice of the 91FX needle valve assy, but I decided to get one anyway 2 weeks ago. The motor sat in its shipping box for a week unopened until the wee hours of this morning, when I opened the box while I was on a conference call and discovered that OS changed the design of the needle valve assy. See attached pictures. I have planned all along to install the Perry regulating pump. Now, with this needle valve assy, I would imagine that I don't need the pump any more, right? Thanks in advance for your response.