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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Engine Manufacturers Direct Support Forum >> O.S. Engines Support >> HEATED owner of an OVERHEATED 91FX
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HEATED owner of an OVERHEATED 91FX - 3/15/2003 6:45:11 AM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		I have been having trouble with my 91FX ever since I bought it. I bought it from a local hobby store but have since moved from the area. I’m not sure what to do. The engine is very unreliable and has put me in many bad situations. The manual said the high speed needle should be at 2 – 2 ½ turns out. But if I do that the engine runs WAY to rich. For peak power and RPM the needle valve has to be at ¾ turn but runs WAY TO HOT. I have to back the needle out another ¼ to ½ turn out and only run it at half throttle to get any flying time out of it. Because I have to make it SO rich to keep it cool enough to run above idle for any period of time the engine just doesn’t run right. Transition from idle to full throttle is very sluggish at best and when I return to idle the engine takes about 4 or 5 seconds then drops in rpm again to a lower idle. I know this is because it is so rich but the engine over heats if I don’t have it really rich. Also, I can’t run more than half throttle for more than a minute or so or the engine will over heat. Local hobby store said to try and remove the baffle to reduce the pressure in the gas tank so I can open the needle valve to where it should be. I tried that for a little bit but it is still the same only I just have to open the needle valve more to get the same results. I read about an air leak at the carb with the “gray” o-ring and that I should have the “black” o-ring. I checked and I do have the black o-ring. I have changed fuel brand from powermaster to omega to coolpower, nitro from 15% to 10% to 5%, and from castrol to castrol/synthetic to all synthetic. Going down in nitro seemed to help a little bit. I have switched glow plugs from O.S. 8 to A3 to a duratraxx gold plug(high, medium, and low heat range). I also went from 14X8 to a 13X8 prop to be sure not to over-prop the engine. None of witch has helped enough to make the engine run right. I had it off the plane and mounted on a work bench and ran about 2 gallons of fuel through it really rich going from full throttle for about a minute to reach normal operating temp then reduce to idle to cool down then back to full throttle again varying from peak rpm to rich at full throttle. No help. Well maybe a little but still not even close to a reliable FLYABLE engine. I have it mounted inverted on a Hangar9 .60 p-51 ARF with NO cowl. When it was on the bench it was NOT inverted. The engine has caused a few wrecks already, none of witch were very bad due to REALLY high grass but, I can’t see risking my investment anymore. I NEED a reliable engine. I have a 40la, 46la, and a 46FX witch are ALL GREAT engines. Not a single problem with any of them. But I just ordered a thunder tiger 46 pro for my world models p-51 because I was scared of the O.S. engine. Please help.

Mark Mercier
808-422-4208
[email]mercierm001@hawaii.rr.com[/email]
p.s. this is the email I sent to O.S. (hobby services)
       Post #: 1

HEATED owner of an OVERHEATED 91FX - 3/15/2003 7:43:54 AM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		redbluffwingnut,
Never really CRASHED the plane. Had a couple scarry dead sticks that I saved, pretty good to if I do say so myself LOL. 2 of witch were during landing, one died on the approach and one died on a mis-approach, both I set down in some really tall heavy grass and saved the plane. Will check for air leak, there are ZERO bubbles in the line so I will start at the carb. The muffler I have has a big spacer in it. I would think that would be for the 91fx. Don't know but will check into it. the LHS had all sorts of O.S.s, maybe it got switched and someone is really having trouble with their 61fx somewhere...lol. thanks

(in reply to agentbishop)
       Post #: 2

HEATED owner of an OVERHEATED 91FX - 3/15/2003 9:18:09 AM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		Haven't tached it. I thought with a 2 stroke it wasn't really neccessary to tach it because you can hear the pitch difference. I did adjust the low speed needle as best I could. I started where the manual said to and then went from there. The slugishness goes away when I close the high speed needle.

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       Post #: 3

HEATED owner of an OVERHEATED 91FX - 3/15/2003 10:23:00 PM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		I will post any information O.S. gives me on this problem.

(in reply to agentbishop)
       Post #: 4

HEATED owner of an OVERHEATED 91FX - 3/17/2003 1:30:36 PM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		^ bump^ BAX, if I could get your thoughts too that would be great!

(in reply to agentbishop)
       Post #: 5

HEATED owner of an OVERHEATED 91FX - 3/18/2003 12:39:04 AM   
Bax


 

Posts: 12663
Joined: 4/26/2002
From: Champaign, IL, USA
Status: offline 		Here's the reply we sent in an email to reply. He did mention in his email to us that there were NO air bubbles in the line.

There are several things to check.

You don't mention which propeller you are using. Typically, a too-large propeller will tend to make the engine run too hot. We'd suggest a 14x6 or 14x8 prop.

If the engine tends to run too lean all of the time, you may have to use larger fuel line. Too many modelers use "standard" fuel line, but that's starting to get a bit small for the .91 FX. You'll also need to use larger-diameter tubing in your fuel tank and drill out the clunk.

Make sure the fuel tank is well-insulated against the engine's vibrations. Air can easily be agitated into the fuel when the model's in the air, and this leans the mixture.

Make sure the muffler you're using is not too restrictive. This is also a sitnificant source of overheating.

If the engine's cowled, you must make sure that air entering the cowl actually passes through the fins of the engine. If you can see the air outlet when you look into the air inlet, chances are that the air is following that path and not passing through the engine's cooling fins.

Finally, if the engine will run all day on the test stand and not in the model, then the problem is a result of the airframe/engine/mount/fuel system combination, and all avenues should be checked.

_____________________________

Bill Baxter, Manager Hobby Services/Futaba Service/North America
3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
Service Phone: 217 398-0007
Email: hobbyservices@hobbico.com

(in reply to agentbishop)
       Post #: 6

HEATED owner of an OVERHEATED 91FX - 3/18/2003 3:16:34 AM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		Bax,
Thanks for your reply. In the email I did forget to mention which prop I was using.

I had a 14X8 then switched to a 13X8 like the O.S. website suggests.

I did run it on a bench with the exact same results as in the plane.

I do not run it with the cowl on at all.

I didn't think about the fuel line itself. I was using the same size line as the line from the needle valve to the carb that the engine came with. Also the needle valve seems to be REALLY sensitive.
Did you say you guys replied to me? I did not receive that email.
I'm Not bashing O.S. here. I think O.S. is a GREAT engine. I bout this engine for a reason. This is the one I WANTED. I just want some help to fix it. Although I said I just bought a TT.46 pro, O.S. is still my engine of choice and I hope I don't regret going with TT.

(in reply to agentbishop)
       Post #: 7

HEATED owner of an OVERHEATED 91FX - 3/18/2003 9:46:35 PM   
Bax


 

Posts: 12663
Joined: 4/26/2002
From: Champaign, IL, USA
Status: offline 		When the needle's really sensitive, it can mean that the fuel delivery is a bit marginal. With a weak fuel flow, it doesn't take much change in the needle to cause a large effect.

When using larger fuel lines, don't forget the line to the muffler and the vent/filler line if you use one of those.

The only disadvantage to the larger fuel lines is that you'll have to tie the lines to the tank and engine nipples. They won't hold on just from pushing them into place.

_____________________________

Bill Baxter, Manager Hobby Services/Futaba Service/North America
3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
Service Phone: 217 398-0007
Email: hobbyservices@hobbico.com

(in reply to agentbishop)
       Post #: 8

HEATED owner of an OVERHEATED 91FX - 3/18/2003 11:38:43 PM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		Will try!! Thanks ALOT!

(in reply to agentbishop)
       Post #: 9

HEATED owner of an OVERHEATED 91FX - 3/20/2003 11:05:50 AM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		No snow here in Hawaii. I'll keep this thread updated with anything that happens. If you read this post you should have all the recommendations you need. I will let you know how the fuel line thing works out. I'm hopeful it will solve my problems.

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       Post #: 10

HEATED owner of an OVERHEATED 91FX - 3/20/2003 11:58:54 AM   
dc3capt


 

Posts: 28
Joined: 2/17/2002
From: Stocton, CA
Status: offline 		If your going to have trouble with an OS the 91FX will be the one. I've got 2 that I'm running and one new in the box. The only way I can get good performance and reliablity was to shim the head, lowering compression, and run a perry pump. Both of mine had exactly the same symptons you have. It may sound like a lot of trouble to go to but it does perform well for what it is ( a 61 case). I paid about 24.00 bucks for the perry pumps and under 5.00 for the OS head shims. I used medium size fuel line. Byron 5% fuel. Profile Giles and Hangar 9 stick. Bob

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       Post #: 11

HEATED owner of an OVERHEATED 91FX - 3/20/2003 12:26:16 PM   
agentbishop


 

Posts: 44
Joined: 3/11/2003
From: Honolulu, HI, USA
Status: offline 		I'm going to try the large fuel lines first then go from there. I feel I shouldn't have to do anything additional to an engine to make it run correctly. I hope O.S. will do something about it before I have to resort to that.

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       Post #: 12

HEATED owner of an OVERHEATED 91FX - 3/24/2003 8:32:40 PM   
walshy


 

Posts: 24
Joined: 3/24/2003
From: Bundaberg, AUSTRALIA
Status: offline 		I have two 91 fx and believe they are just fantastic One in a pattern plane I use in competitions, never a dead stick in 97 flights. Standard muffler less the baffle, OS A5 plug is definatley the best plug, APC 14x10 or 15x8 (this motor loves a big prop). 20%nitro 22% synthetic oil (I use coolpower oil) add one head shim, Tune for peak rpm less 100, I use no pump,don't need to.Tune the bottom end nice and lean for smooth transition. I have noticed that this motor has a unique ting-ting type rattle at half to three quarter throttle, no big deal. When you open the throttle it really goes to work. I have (9) OS motors there is no such thing as a bad one in my experience. my 160 and 46's all behave much the same as the 91fx. I have noticed that many people under prop the engines and subsequently rev the guts out of them. read the manual , OS recommend the props I have suggested. I hope this info may help out.

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       Post #: 13

os 91 fx - 3/25/2003 7:54:20 PM   
Mark DiMonte


 

Posts: 150
Joined: 4/11/2002
From: Richmond Hill Ontario, ON, CANADA
Status: offline 		I too have had the 91fx problems and after many tests and fixes, I finally found the one that works and is not that expensive. The problem with the gray O-ring suggests there is a problem with the remote high speed needle. After changing fuel, multiple props, and the O-rings I tried the remote needle from an OS 40 LA. It is a plastic unit that mounts on to the backplate with a metal bracket. I mounted the metal bracket to the firewall and turned it over. BINGO I now have a 91 that behaves like a true OS Engine. Gobs of power and perfect transition. I am running an APC 13X8 and that seems to be the right prop. You get better transition using the smaller props and I have not had the overheating problems that I had before. PS if you have the money, buy an OS 7L carb and forget about all of your troubbles. It is infinately adjustable and will make you engine sing. I tried one from a friends engine, but could not afford it, Plus it is a crime to spend $150.00 more for a carb that should work from the start. MAD out.

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       Post #: 14

HEATED owner of an OVERHEATED 91FX - 3/26/2003 5:36:16 PM   
walshy


 

Posts: 24
Joined: 3/24/2003
From: Bundaberg, AUSTRALIA
Status: offline 		Hey thanks for the reply and good info! can,t help but notice that fuel pumps are very popular in the states, in aussie land no one uses them that I know of. I have 140rx and a surpass 120 that have pumps standard but I have never used a generic pump . Is it a big advantage and what is the best to use. I tend to think they are just extra weight and more stuff to go wrong. Also notice that many of you use little props even smaller than the manufacturer recommends?? mostly the people I fly with ( I travel around a bit flying pattern. ) use the largest props we can get away with.....usually the largest that the manufacturer recommends, It keeps the noise down and generally makes the models easier to fly, particularly at lower air speeds there is more air flow over the control surfaces with a larger prop.The only dissadvantage I can think of is that there may be more Gyroscopic forces and loads placed on the bearings in sudden yaw and pitch changes. food for thought.

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       Post #: 15

OS 91 fx - 3/26/2003 7:35:17 PM   
Mark DiMonte


 

Posts: 150
Joined: 4/11/2002
From: Richmond Hill Ontario, ON, CANADA
Status: offline 		I have used the perry fuel punp and carb on a nasty ASP 75 and it turned it into a fire breathing dragon. It worked great! The only reason I am using smaller size props is I find the two strokes like to run fast. Large props seem to suit 4 stokes better. I prefer thrust over speed and you get that with the bigger props ,but the eingines seem to like the smaller ones. MAD out.

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