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OS 91FX Anybody like them?

851 Views 9 Replies 7 Participants Last post by  wagas
I was given 2 91 FX's by a friend. I've already had one of them on a fun fly (Harrier) but had problems everytime I run it. It starts to sag after running for about a minute or two the when you throttle back it wants to quit. Anybody got any advice on these engines? What props work best on them. Also it's inverted and I wonder if that's the problem.

10% fuel 15 x 4 APC and 14 x 4 APC
No fuel tank problems
Engine runs great for the first minute or so.
This also happens when the engine is either leaned out or running so rich it looks like a smoke system..
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not enough prop and overheating is my guess. try a little more prop. 14-4 is more 60 size. I'd try a 16-4 or 5, those engines have a lot of torque.
These have a common problem with the rear mounted needle valve leaking air caused by heat and vibration. Find a remote needle valve assembly and skip the one on the backplate. Mount it on the fireway or somewhere convenient. That has cured most peoples problems. After this fix they are an excellent running engine. 15-4 ProZinger has been my prop of choice for a couple of years. 14-6 APC may wirk well also but haven't tried one. 15-4W APC is a dog on spool up but pulls well when it gets wound up.
I know a fellow ProBro who owned two of these.

One bought used, and the other new.

Both ran like CRAP!!!

I'd give basmntdweller's tip a try. If that doesn't fix them, then my advice would be to get rid of them.
Yeah there is not one 91 FX in our club that doesn't have problems running. Some of them just quit with no reason what so ever. I was calling fot a buddy of mine at a pattern monthly and the test and first flight the engine was running like a charm. But two dead sticks without warning on second and last round. Since that day the thing keeps doing it.

Try air sealing the whole engine. We've gotten a couple of them to run ok since we've been doing this. The whole needle valve ass thing and everything you can think off needs to be siliconed or something.
I used 15-6 apc's and zinger pro's on mine. You can buy the GMS 120 needle valve assbly from tower pretty cheap to replace the crappy one that comes with the motor and any air bubble problems will go away. beyond the needle valve it's a solid well built motor
DANG , Thanks Guys, WOW great replies. I'll have to try the tips given! I hate to mess with engines if they can't be fixed in less than 20-30 minutes worth of work.

Sounds Like I should just sell them both and buy a Pro 46 and a new PROFILE!!!!

I'll try the moving the needle valve first from the back plate as one of the engines already has thise broken off......
As I recall, there was a problem with a run of the "O" rings in the needle of the 91FX.

Try a set of new ones and see if that's the problem.

Also, they don't really like high nitro, in my expreience. 10% is plenty.

A 15x6 to 16x4 is OK, just don't get in a hurry for rpm on the 16.
Well I'm gonna give mine a second chance. The airplane that I had this on also gave me a fit with both a OS 1.08 and webra 1.20.

Took me 3 engines to figure out that the fuel tank in the Harrier was defective....STUPID!!DUHHH!
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