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91 FX quits

Old 07-15-2002, 02:54 AM
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Default 91 FX quits

I have just became aware from reading the engine forum of the problems with OS 91 motors... I guess these problems are real because a friend of mine is at wits end trying to get his motor to run reliably. It dies after flying for a couple minutes as if the motor leans out and overheats. The engine forum in RCU has many examples of the identical problem he is having..Is there an Oring problem? He has sent it in before but the motor was not fixed. As a favor I am writing this to get some idea on what he can do to get his motor fixed. I am hoping for a response from Ann Marie Cross. I believe I have talked to her in the past... Thanks for your time..

Also why are there bubbles from the needle valve to the carb. That has got to mess thngs up as far as a constant metered fuel flow. Thanks again
Old 07-15-2002, 11:30 AM
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Default 91 FX quits

Hiflier,
The #1 issue encountered with the 91FX in the US is prop choice. The european, and esp Japanese fliers must prop very large due to noise considerations, but what we use here in the US is quite different.

Prop Size RPM
13x6 12,500
13x8 10,400 using a 13x8 prop,
13x10 9,500 Hobby Services tached thi
14x6 10,900 engine at 11,300 rpm
14x8 9,500 6-24-9
14x10 8,400
15x6 10,200
Old 07-15-2002, 02:03 PM
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Default 91 FX quits

What can I recommend to him to do to get the motor running reliably? The motor came with black Orings. Its hard to believe the prop size would cause the motor to quit. It turns about 10000 on Cool Power 15%. Not sure what prop he uses. Thanks again.

Also what can be done to get rid of the bubbles in the fuel line?
Old 07-15-2002, 06:16 PM
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Default 91 FX quits

We have changed out some orings, although teh stock orings hae been used with that carb for not less than 10 years on other engines. Please have him email us his mailing info and we will be happy to send him a separate set of o-rings.

Prop loading has tons to do with engine performance, particularly including overheating and going deadstick.
Old 07-16-2002, 06:21 AM
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Default 91 FX quits

AnnMarie thanks for the quick response. I don't think the problem is with the carb at all. Your right the OS carb has a great reputation. I think the problem is the relatively new rear high speed needle assembly. For some reason it has problems on some 91's. I have a OS160 that has the same needle assembly and it runs flawlessly. I'll let my friend know about your offer to send the Orings. Thanks again
Old 07-18-2002, 03:11 PM
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Default 91 FX quits

AnnMarie is it possible to get a new needle assemble by calling your service department? This may eliminate the bubbles which I am fairly sure is part of the problem...Thanks
Old 06-18-2003, 10:57 PM
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Default 91 FX quits

Hi i run several os 91fx. i have found that they like to turn at near 11,000. i use fai. fuel that i mix my self. i use castor oil. i also use a firewall mounted needle valve. i do have one that the needle on the eng. dose not work to well so i went remote on all instulations. a remote valve might help. hope this is of some help.

larry
tucson
Old 06-19-2003, 04:36 AM
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Default 91 fx

Having heating problems also, on both my 91's quiting in the air.....was wondering if a large finned os 91 Df head will work on my fx. I'm running 7d's and it helped a lot but still a little hot, actually real hot with a 12x8
Old 06-19-2003, 01:31 PM
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Default 91 FX quits

The cylinder head for the O.S. Max .91 VR-DF will not fit onto the .91 FX engine.
Old 07-04-2003, 03:30 PM
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Default 91 FX quits

Can I get some of those O rings also?
The address is:
PO Box 322
Good Thunder MN
56037-0322
I am also having the same issue with bubbles in the fuel line. I changed out all the tubing and the tank, but they are still present. The engine also exibits the same problems. Too hot, quits after a couple of minuets running.
Thanks,
Jaye Davidoski

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