I asked Hobbico to send me a free replacment o-ring for the needle valve. They sent a new, black, slightly harder o-ring that, I presume, is supposed to fix the bubble problem. I installed the o-ring and went flying today. It initially worked great - no bubbles. After a minute or two of flying, the engine went lean. I landed. The engine was hot and there were massive amounts of bubbles appearing after the fuel went through the needle valve, not coming from the tank.

I let the engine cool down and restarted it. Same thing - no bubbles for a minute or so, then lean, hot engine and massive bubbles abain.

I noticed the needle valve was pretty hot. I riched the needle to the point just before the engine starts 4 stoking and flew it. The engine performed flawlessly with no bubbles. I think that there may be enough heat transfer from the case into the needle valve body to actually cause the methanol in the fuel to vaporize, forming bubbles. It takes about 140 degrees to vaporize the methanol. When the needle is part of the carb., it stays cooler since it is further from the head and has air rushing through it to keep it cool.

That seems to be the only scenario that fits the bubble problem. When you isolate the needle valve from the crankcase, by cutting it off, or using a different needle valve, the problem goes away. I plan to try reduce heat transfer from the case into the needle valve body, by drilling holes through the web that connects the needle valve to the back plate. Or I may cut off the needle valve area and reattach it with an insulator plate to stop heat transfer.

Perhaps someone else could experiment with lean/rich needle settings and see if they have the same results.

RCMAN

Great! Great! observation. I think it's the thread that ties all we've seen together, with a running engine. I'm going to post some more on this tomorrow.

This engine seems to run hot, which aggravates the problem of heat transfer into the needle valve body. OS has even changed the prop recommendations for this engine to smaller props, which will allow it to run at higher RPMs and presumably cooler temperatures. I noticed that the engine runs hotter with the baffle in the muffler. If you are having bubble problems, you may want to remove the baffle. The engine will be a little louder.

You may be right that with the needle in the carb it might tend to be cooler because of the forward airflow (not preheated by the cylinder) but also in this (the old ) configuration vaporization is taking place right at the spray bar, where it should, even if caused by heat. Also all that vaporization at that point tends to cool the shaft, shaft housing and bearings and carb. The needle hanging off the back doesn't get this benefit.
I tend to run my engines a little over rich and as a result may have not had problems running in the air although I've certainly had bubbles. Also the few degrees difference in running temperatures caused by tight or loose parts fit could be the straw that causes a cascade of riseing temperature and boiling alcohol.
This could be why some people seem to be having problems with this engine going lean and quiting after a couple of minutes of flight. Smaller props and higher rpm will also help with this kind of problem. I'm currently running an ultrathrust muffler and when under proped just slightly it tends to go rich in the air.

It could probably be noted that most engines with remote needles don't seem to have this problem. But most engines use a bolt on needle assembly, I think, rather than one that is an integral part of the back plate casting. This means there is one more thermal junction impeding the heat getting to the needle housing. On the OS91 that housing also looks like a nice little pot for boiling alcohol. It's also suspended out there where vibration can be amplified for agitation.
Of course none of this is supported by good scientific testing supported by high dollar test equipment and well documented numerical results that have been scrutinized by other disinterested experts, and are therefore not conclusive.

It is conclusive enough for me to cut the suckers loose and mount them truly remote though.

I don't have a .91 FX, but a friend of mine said that he found a fix just this morning. He took an old K&B .40 needle valve assembly, placed the line from the tank on to the assemblies nipple. He then placed the line from the .91 FX Carb, on to the K&B spray bar. It's a simple fuel metering device, and NO BUBBLES ! He says that it runs great now.

It's still a shame that a person, after paying over $200 for an engine, has to endure such poor performance. Why doesn't O.S. just do a total recall on this model, and put the carb back on the front ? I would think that their reputation has been damaged enough.

"AS GOES OS, SO GOES THE WORLD" They are the leader. They are the one everyone else copies. They set the standards for everyone else to strive for. Excellence in engineering, Value, and ease of operation. They are simply the KING. There is none no better!

Sounds like they are running on a bunch of tradition?


Jim

I put a hopper tank on my Shoestring with the 91fx and have never had a problem since. I run a APC 13x8 and have unlimited verticle.

I am posting on this thread to refresh it becuase of the continuing interest in the subject.


I note that YS is going to a remote regulator on the YS63 in 2004 because of engine heat adversely affecting the regulator.

Maybe they have got a handle on bubbles and vapor lock.

I hope OS does

OOOOHHHH !!! the pain
why did you rsucutate the thread.. leti it slip away .. (along with the 91FX) so we might go on....

I ran into some problems with my 91FX at first but it was not bubbles in the line it was how hot the motor ran in an uncowled plane and the vibration at idle was very bad...the worst I have ever seen in an OS. I finally got the motor to run real good but I had to prop it down to a MS 13/6. I am burning Omega 15% and I think the motor may be alright except for the violent shaking at idle,,,plus it has some fairly bad top end vibration also. The low speed vibration was so bad it backed out 2 of the bolts that secure it to the plastic motor mount. When I tried to tighten the bolts they just stipped the threars in the plastic mount because the plastic was so hot.
The really strange thing is my buddy has the same motor with stock muffler as do I and he is swinging an APC 14/10 and it pulls real strong and no bubbles but his also runs hot and has a lot of vibration also. I know that there is going to be more heat than the 61FX will have because of the larger displacement in the same size motor but this heat and vibration thing is really going to be a problem. I am going to balance the crank on mine and see if I can get the vibration down...I did this on a Super Tiger 90 that had a bad vibration and I was able to get the vibes down to near nothing or as good as it can get for a single cylinder.

I purchased one, ran it two weeks, could never get it to run right, (erratic idle/top end RPM, air bubbles sucked in from the RNV,...the usual stuff in the thread) sent it back to OS, got a brand new replacement under warrenty claim, took it out of the box and have the same problems. This one won't even run well enough to conduct break-in procedures. 2 for 2 in the bad column. I guess I will have to join the ranks of throwing OS's comeptitors stuff (K&B RNV, Perry carb, Super tiger parts) on this thing. I really dont feel like I need to keep paying postage to swap out a bad engine for a bad engine

I know what you mean Brett...I just said to hell with it and installed an old but very smooothSuper Tigre .61 on a macs tuned pipe and it is so much fun to have a good running motor . I did not mess around with my OS 91 for very long because it behaved almost as predicted so I just decided I wanted to get some air time and have some fun so on went the ST .61.
I am going to balance the crankshaft on the 91 to see if it will smooth out some plus I have a carb off an old 61 SF to try . I hate to give up on it totally but I am not buying a pump or a needle valve for an engine that vibrates so bad it was shaking my plane to pieces.
While I am unhappy with the .91 I am not going to say I will never buy another O.S. but I will take a harder look at the other motors on the market from now on.

Heres my story of all events that have taken place during my troublesome adventure with this engine.

Out of the box this engine was doing the same thing that everyone else has described. What was done to solve this problem was change out the muffler with a Magnum, replace the needle valve and fine tune both needle valve settings. This engine is a very picky engine, the slightest debris will cause a fluctuation in the rpm. After a few days I was able to get the engine running as it should and placed it on my WM SS60 which flew and ran great. I then moved the engine to its final destination which it was intended for (GP Shoestring). Since it went from open air to a cowl along with a pits muffler it started doing the very same thing it did when it was new out of the box. 10 in the air lean, power down.

I pulled the engine off and sent it into HS to have it serviced. I just got the engine back with the following replaced parts, piston, piston ring, cyl liner, cover plate assy and rear bearing. No where in this invoice did it list the cause of the lean run, nore did they replace anything that had to do with the fuel system. I wrote them back via email stating this and am now waiting for there reply.

Am I destined to place a pump on this engine in order to solve all issues? If so then I would have to shame OS.


Blackie

Blackie:

I would remove the current needle valve parts to save weight. Then install a remotely mounted needle valve, probably to the firewall. I have heard the K$B needle valve for about $15 works fine. The engine is then supposed to run okay. It's less trouble than having a damaging lean run and dead stick.

Make sure your Pitts muffler is for a .91. The .61 sized Pitts also fits this engine, but its displacement is too small and this causes problems.

Also, make sure there is a brass gasket ring between the head and cylinder. Without this ring, the compression ratio is higher and would cause the engine to run hotter. Let us know how you make out.

Where do I get the K&B at?

The pits is a slimline for the 91 FX but no gasket I was using a thin layer of high temp RTV for gasket material. Will this work?

Thanks
Blackie

Tower Hobbies has a Fox needle valve for $12, a Tower version and a Perry version for a few more dollars. The K&B needle valve should be available from your local hobby shop and many other online hobby shops. Any of them should be okay.

The gasket you want to verify is between the head (where the glow plug screws in) and the cylinder liner beneath it. It is a thin brass gasket. If it is missing, you can order one for a couple of dollars. When you replace the head, tighten the screws each a little at a time, working back and forth across the bolt pattern, not sequentially around the bolt pattern. I don't think you want to use RTV between the head and cylinder liner. RTV is fine between the muffler and engine, although I usually don't bother sealing this joint. Any minute leakage here has no effect on engine performance.

A perry remote nv and a BCM pitts muffler works great for mine.

Tim

rcman. ok I did not read your post clearly enough I had the muffler and engine in mind.

Thanks for the info.

Hi,
For over 60 flights of fun-fly powered by OS91FX I fought over poor fuel flow to carby. I by-passed the original needle valve and took one from OS160FX and screwed it to engine mount. This valve is bigger, so 1/2 to 3/4 opening is enugh for good engine operation. Works well for last 50 flights, and only occassionally requires minor adjustment, like two-three clicks. Because of this my next 90 2-stroke will not be OS. Probably MVVS which including tuned muffler costs about 3/4 of OS (exported from Germany).

After fighting this engine for a whole day somebody at the airfield brought up some heli "powermaster" fuel 30% nitro. After having very dismal performance out of the 15% Byron Nitro and what seemed like every prop at the field, we thought,"sure...why not...can't work any worse." So we fueled it up and cranked it over...Couldn't believe the difference. Most importantly the Needle Valve had a much broader range and all the air bubbles went away. Instead of four-stroke rich, two clicks, dangerously lean you could actually adjust it. We ran a tank through it, then filled it up again and flew the model through two more tanks. Upon further inspection the engine was much cooler than ever before. We switched back to the Byron 15% for the next tank and it is still running much better. Nobody could exactly explain the phenomena, but it worked. The best explanation the most experienced guy had (professional model airplane person), was that the piston was in way too much contact with cylinder wall to function properly and while trying to break itself in it was generating way too much heat causing a chain reaction of other issues and potential damage. His advice was to break these engines in with very-cool, high nitro fuel to get the ring broke in and then you should be ok. Sounds good, and I certainly hope he is on the money...(because its my money in the air ;-) ). I am still a little trepedatios about this engine what with my past experience with OS 91 FX #1 that got sent back and then seeing my friend's 91 work well for three months and then go kaput in one flight exhibiting all the characteristics that were mentioned in all the other threads (he has placed a super tiger 70 carb on it and it seems to be working much better but he has noticed the decline in power since the modification). Anyway, I hope this may helps some of you disappointed 91 fx owners, but I will still get and, keep a K&B needle valve at the ready.

Brett
In your case you need to be careful of a poorly fit main needle. I won't say worn because it is a new engine. When the engine is running touch the main needle, like put a little side pressure on it. If the engine changes rpm you have a loose needle that is causing most of your problems. You will have to fix that before you can make since of anything else. The loose needle doesn't cause an air leak it just allows the fuel metering to wander all over. I have noted that the needles on my OS91s seem to wear a little fast. A piece of tubing over the needle reduces this wear. Often folks put tubing over the needle to stop air leaks. Often there is no leak, the tubing just stops the needle from wiggling.


Denis

Well lt face it the OS 91 FX is not up to OS standard, I have try quite a few thing on this engine recommendd by BAX and still doesn't perform the way it should, now mine doesn't quite in flight but being installed in a UCD, I cannot enjoy the airplane since it get lean when I start hovering.....to paraphrase OS answer it's normal that the engines goes lean with extended nose high.....well I guess they really screw up on the other engines that doesn't get lean with hovering ie:my 2 46FX, 50SX, and my 3 140RX....to name a few.

Very disapointed, why should I have to by a K&B needle valve, or a carburator, I did pay over 300.00 cdn for this............engines, I was expecting to bolt it in do some adjustment...and that is it.

I agree with you Daniel...I was very disapointed with my 91. I am not sure this was such a good idea for OS to put this engine into production. I was able to get mine to run fairly good but the vibration at idle that it shook my plane so bad the nose wheel was a blur and it full throttle it was vibrating so bad it backed out 2 or the motor mount bolts(8-32 bolts) and the motor ran so hot that when I tryed to tighten the bolts the mount was soft from being so hot that it stripped the plastic threads. I guess I got spoiled from OS engines of old where you brake it in for 2 tanks, adjust and go fly.
I am going to balance my crankshaft to see if I can get the low speed vibration down, I have done this before and I had to remove a bunch of steel to get the counter balance on the conrod end of the crankshaft to sit straight across from the counter balance with the piston, conrod, and wrist pin lighter than the counter balance. It worked real good as I was able to eliminate most of the vibration. After I balanced the crankshaft it was the smoothest motor I have ever had, plus I gained 500 rpm's.
I am not real sure what I am going to do to get the motor to run cooler.
I only messed with it for 1 day and when I got home I cured my problem by installing a Super Tiger .61 with a tuned pipe and now I am flying and not fighting. My buddy has a .91FX that runs great and he is swinging an APC 14/10...go figure.

I can't say just yet whether it pays to send your engine in for warranty repair, as I have yet to run mine through the break in period once more, I'll be doing that this evening.

I did, send mine in and all major internal parts were replaced, all except anything that had to do with the fueling system. I sent an email to HS in reference to this asking what's to keep it from leaning again? I have yet to receive a reply. :angry:

I'm going to put it back on my GP Showstring and if the same thing continues, I'm not going to give up, I'll just keep sending it in until I get either a new engine (one of those that claims to be working) or the darn thing fixed right. :angry:

Idea: :idea:
Why don't we all just flood OS, with our engines, perhaps they'll get the hint. pirate:

Blackie

More suggestions:

This engine needs to run cool. Try using smaller props and low nitro fuel: 5% or even 0%.

Some guys say their 91s run great. The only thing I can deduce is that their engines run cooler for some reason. Of course, you would always want to run this engine with the carb set as rich as possible.

This engine may be just a bad design: too much displacement in too small of an engine. I switched from a 61 to the 91. It produced more power, but not nearly as much more as I expected with a 50% increase in displacement. The 91 also really likes to drink fuel. I would look for OS to do something about this engine, like a new version with bigger head, diferent carb, bigger frame and crank, etc. The 1.08 has been a great engine, but just very large and heavy.