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OS 91FX High Speed Needle Not Responsive

Old 09-13-2006, 07:47 PM
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chris8105
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Default OS 91FX High Speed Needle Not Responsive

I have a brand new (about 2 weeks old) OS 91 FX engine, when I turn the high speed needle I get no change in engine rpm. I am running 15% Wildcat fuel with 18% synthetic oil content, bisson pitts exhaust, #8 plug, zinger 14x6 prop. If I turn the needle in far enough the engine will die and the same thing if I turn it out far enough, but with no change in engine rpm, and I have alot of oil coming out of the exhaust. I have had the needle valve off and blew through it to check for restrictions, the same with the carb. I plugged one of the outlets on the exhaust thinking that it was a pressure problem but that didnt help at all. I changed out all the fuel lines, checked the clunks in the tank, submerged the tank under water and blew into the line to check for bubbles but I did not see any. I reset the low end mixture per what the manual says, 2 1/2 turns out, and I started with high speed about 3 turns out, once I turned it about 1 turn in the engine started to die, with no difference in engine rpm. I also checked the head bolts to make sure they were tight. I dont know what else to check as I have checked everything, I am wondering if there is something wrong with the high speed needle assembly or maybe the carb. I have no air bubbles going from the high speed needle to the carb.
Old 09-13-2006, 08:22 PM
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Cyclic Hardover
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Default RE: OS 91FX High Speed Needle Not Responsive


Bill , I spent the darn near 2 hours on line with this guy trying to solve the problem, so I thought I would tell you what we did to save some emails. He cannot get this thing to go lean on the high end.

1. He reset the low end and high. and had him check rpms at and idle after removeing fuel tube. There was a slight increase which tells me the low end is slightly rich but okay.

2. He changed the plug

3. Checked the head bolts

4. The fuel line looked a bit bent to me so he lengthened it a bit just for testing and nothing.

5. He has flushed it a couple times.

6. I had him put some fuel tubing over the high speed needle just to check for a leak-nothing.

I had him try ever trick I knew and nothing. My best guess still tells me it is in the remote.

I am also curious if that nipple on the remote is not lined up. Now he did say he was seeing fuel in the line to the carb but I have experience first hand thatjust because you get fuel in that part of the line does not mean its right. Example is my G90 I had on a P-51 once. The short story is the fuel line coming out of the firewall to the carb was bent too much. Even though there appeared to be a steady fuel flow to the carb and needle, I could not dial it in. After I lengthend the line and got rid of than bend, Ran fine for 3 years.

Somehow I think the problem is before the fuel gets to that needle on his remote

Without being their its hard, but beats me.
Old 09-14-2006, 09:51 AM
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Bax
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Default RE: OS 91FX High Speed Needle Not Responsive

I'd have to agree. If the high-speed needle valve does little or nothing when you try to adjust it, it give the impression that the engine's just not getting enough fuel pressure to the needle. It's possible that there's something in the needle assembly that's causing the difficulty, or it's external to the engine. We'd really need to see the engine to determine exactly what's wrong.

After a good physical examination of the engine, we'd mount the engine on our test stand with the stock muffler and make sure the fuel system was well-plumbed. We'd then see how it runs.

If you have the engine mounted in a model, remove it and put it onto a test fixture. That way, you can control all of the elements involved. If it runs properly on the test stand, then you know for sure it's something in the airframe/engine combination. That's a LOT more common than people want to believe. The test stand is an excellent way to isolate the engine so you can really see what's what.

You can most certainly send the engine to the service center for us to take a look at it.
Old 09-14-2006, 11:23 AM
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chris8105
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Default RE: OS 91FX High Speed Needle Not Responsive

I tried a few things this morning...

1. I switched props out to a APC 13x6, I could hear more of a change in rpm's, it is making alot more difference now. And I didnt have a puddle of oil on the garage floor as well.

2. I switched out the engine on the plane to a magnum 91 four stroke and everything worked great, which leads me to believe that everything before engine is good.

The crankcase pressure seems to be good as it will run on the ground fine with the pressure line disconnected from the exhaust.

I lied it did leave for a pretty big puddle of oil on my garage floor for just running about 30 seconds.

I can get the engine to lean out now though by pinching the fuel line to the needle valve, which it wouldnt do before. Maybe it just needs to be ran some more, it has about 4 tanks through it.

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