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OS Max 91fx Shutting off above 3/4 throttle, HELP!

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OS Max 91fx Shutting off above 3/4 throttle, HELP!

Old 07-19-2012, 10:44 AM
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MDFlyer83
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Default OS Max 91fx Shutting off above 3/4 throttle, HELP!

Hello all, im in the process of trying to figure out why my problematic OS seems to shut off above 3/4 throttle. It will start up fine and idle like a champ, but,if I gradually give it throttle once it reaches a notch past 3/4 the motor shuts off. I tore apart the motor only to find rust on the main bearing. So, I ended up getting a great deal on a almost NEW91 (free!). I bolted her in, fired her up, reved it slowly to a notch past 3/4 and BAM she shut off again? Here is a small list of what Ive done to remedy this problem:

1. New fuel lines
2. New glow plug (once replaced it ran better but 10 seconds later shut off)
3. Checked batteries both rx and tx but all were fully charged
4. Checked for bubbles in the lines but couldnt see any
5. Old motor had the cast high speed needle but the new one has the remote bracket
6. Running cool power 15% (should this be changed, perhaps killing glow plugs?)


The common factor that hasnt changed is the fuel tank, could it be that the tank is loosing pressure causing it to stall?Perhaps a slight pinhole letting the motor idle fine when there is a low demand for fuel but once it reaches 3/4 throttle pressure cannot be maintained?

Ive searched but havent been able to find a remedy for this. Please if anyone can shed some light on this id greatly appreciate it!



Thanks,
Adam
Old 07-19-2012, 11:24 AM
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jester_s1
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

You didn't mention tweaking the needle valves, so have you done that? A sudden shutoff is usually the result of a lean condition. If you have worked on your needle valves and there's no way to get it rich enough to run at full throttle then the only other things left for it to be is a clogged muffler pressure tap (eliminated in the new engine also came with a new muffler), a leaky tank (easy enough to test), bad fuel, an improper tank setup (not likely), and a bad front bearing (can allow air to leak in). When you tweak the needles, do you get a fairly consistent response from them or is the mix erratic as you make changes? Is the engine getting extra hot just before shutting down? What glow plugs are you using?
Old 07-19-2012, 11:28 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!



Is it mounted inverted

Bob
Old 07-19-2012, 12:13 PM
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WaffleMan
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

sounds like its lean.
Old 07-19-2012, 12:21 PM
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MDFlyer83
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

OS plugs, new fuel, low needle and high have been tweaked, its not inverted. Its wierd that this happens to BOTH motors.
Old 07-19-2012, 12:39 PM
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JohnBuckner
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

I too like Jester read no mention at all about the all important intial main needle setting for starting and nothing about the midrange needle equally important setting either.

Do you know where your midrange needle valve is located? Its an important question since you mentioned a few days ago you know nothing about airplanes engines. What does tweaked mean? You would be surprised at how often some fellows will carefully tweak their idle stop screws beleving that is the midrange of course that does not accomplish anything.

"OS plugs" which OS Plug?


John
Old 07-19-2012, 02:23 PM
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MDFlyer83
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

John,
            I use OS 8 plugs, to be honest I thought the 91 only has a low and high speed needle. Low for idle to about 3/4 throttle and high for 3/4 to full. If I am mistaken please feel free to correct me. As far as "mid range needle" no I was unaware it existed. Im relying on my buddy Alex who works at a hobby shop, been flying for 4 years, has a little knowledge with nitro's (not a genius). Perhaps he is tuning it incorrectly and I need to start off with a base tune then go from there. I do however believe he knows what he is doing and suggested getting a new tank.

Again im sorry if im leaving out key information so feel free to ask whatever question is needed.

btw Tweaked means we tried adjusting it little by little to no avail.
Old 07-19-2012, 04:04 PM
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JohnBuckner
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

OK first I feel you are simply to lean on your mid range needle and possibly the main needle also.

Let me first say I do not own a 91FX but I do own a 95AX and quite a few 61FX's among others.

The midrange needle on that engine should be located inside the throttle arm barrel on the right side, this is where the throttle pushrod is attached. I do not know what the default setting for that engine is but you will certainly find it in the instruction manual if you have one.

Also not sure if the #8 is the recommended plug for that engine or not. I must admit I do use the 'F' plug in a lot of the two stroke OS's and had good luck with such. But do go with whatever the manual booklet says.

Now if it were my engine and I had no info on the needles I would start out opening the main needle four full turns after having adjusted the mid range screw to flush with the edge of the throttle barrel when the throttle is closed completely.

I beleve you are just to lean.

If you can look into the venturi with the throttle completly closed you can see the end of the midrange needle protrude into the jet and I beleve there should be a few milimeters gap between the jet and the needle. At any rate find the manual for this engine and follow the instructions for setting the mid range needle.

John
Old 07-19-2012, 04:30 PM
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dbacque
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

I've flown the .91 FX for 4 years. The #8 plug is perfect.

It really does sound like you're running lean on one end of the throttle or the other. Try richening up the needle a little, just enough to drop the RPM a bit but not drop into a 4 cycle. Also turn the screw in the center of the throttle arm at least 1/4 turn counter clockwise. If it still does the same thing, turn the screw another 1/4 turn. Keep going until it gets better or gets worse. Keep count of the turns. If it gets worse, try going back to the original point and go the other way.

This is a great engine. All it needs is a little tuning. If you still have problems ask around at your field for the local engine guru. 4 years isn't much experience when it comes to fine tuning engines.

Dave
Old 07-19-2012, 07:54 PM
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jester_s1
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

To clarify, what JB is calling the midrange needle is the same thing you are calling the low speed needle.

One question that hasn't been asked is whether or not the new engine had been run by the previous owner before you got it? If so, has anything at all been changed aside from what it's mounted on (ie, did you change the needle settings at all before starting it the first time)? It's possible that your old engine had a problem, but this new one does too. It's not unheard of for RC'ers to sell off their problem equipment instead of fixing it themselves.
Old 07-19-2012, 08:47 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

It is known to have happened...check that the clunk is not too close to the tank rear wall. As the fuel flow rate increases, the clunk can (and has!) 'sucked' itself to the inner tank skin, and shut off the fuel flow. As this only happens at higher revs, and sometimes only with airborne (unloaded) revs, it can be a very frustrating problem to troubleshoot...
Evan, WB #12.
Old 07-20-2012, 02:30 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

Either the high speed needle is too lean or the fuel flow is being restricted by the tank. Have you checked for kinks in the brass tubing and are you running at least medium size fuel tubing? I run my 91 with an OS 8 and 15% Cool Power and its been a flawless engine for many years. Another possibility is if you are running an after-market muffler. The pressure tap may not be feeding enough pressure to the tank for full-throttle runs, particularly if your tank is not up against the firewall.
Old 07-20-2012, 06:36 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

I had a similar problem with my Enya 90 4C. New fuel system in new plane. Turned out the fuel tank was jammed to tight to the firewall pinching the fuel line slightly. All you had to do was think start and it started and would idle all day long. You could hold the plane vertical (nose up) and it would run just fine. Nose down and it would richen just like it was supposed to. Pressure tested the fuel system and it held a couple PSI for 15 minutes. I removed part of a former o allow the tank to slide back a 1/4 inch and all is well.

You are to lean. I was SO SURE of my new fuel system/tank installation, that was the last thing I checked. heck I even had the engine and carb in a total tear down a couple of times looking for the issue that did not exist in that part of the plane.

Ken
Old 07-20-2012, 09:34 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

The old motor was Alex's, he sold it to someone who never used it and it sat for over a year. The tank is not wedged or pinching the line at all. The newer motor was Alex's as well and was barely run, it too does the same thing. None of the needles were messed with prior to the initial run of the second motor. If we richen up the high end and get it to where it will run for a few (5) seconds as soon as I back down the throttle (quickly) to idle it dies. Its a loose loose. I dont think the clunk is touching the tank. They both have the factory pipe on them.
Old 07-20-2012, 10:32 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

Did Alex have either of these running well for himself? If so, you might pick his brain on how to get them going for you. I will tell you that an engine that will run on top but shuts down immediately when you close the throttle is lean on the bottom. So before taking stuff apart you might try opening the low end about half a turn and see what that gets you. I'd also go ahead and pull the HSN and flush the passage with fuel just in case there's some debris blocking the fuel flow.
Old 07-20-2012, 10:56 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!



Yeah he had both engine running great and hes the one trying to get this thing figured out. Hes suggesting the tank butI feel as though this is a shot in the dark buy and dont want to waste the money (6$) for nothing. I know its not that much but no point in purchasing it if I DONT need it. I cleaned the first carb out and it did the same thing, I might try cleaning the new one out though perhaps it has something in it as well. This is worse than troubleshooting a vehicle!

Old 07-20-2012, 11:42 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

I'm thinking it's tank pressure, or tank plug loose. It's the only thing common to both engine's.

JED
Old 07-20-2012, 11:56 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

Good point, On my 91fx a cheap chinease ARF tank may have caused some problems that i thought were becausethe enginewas inverted. I flipped it upright and installed a new tank and that fixed it, so mabye his fuel tank bung breaks the seal at higher pressures???
Old 07-20-2012, 01:04 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!


ORIGINAL: kenh3497

I was SO SURE of my new fuel system/tank installation, that was the last thing I checked.
Just sayin'..................
Old 07-20-2012, 02:36 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

As much fuel as you're burning trying to get these things right, not to mention the value of your time, $6 is a great investment to eliminate the tank as the cause. Even if it turns out there's nothing wrong with it, knowing that will get you that much closer to finding the cause. But since both engines have run well before, you can be 100% certain it's either the lines, the tank, the fuel, or the tuning.
Old 07-20-2012, 03:08 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

I cannot find a tank online. Most are out of stock!
Old 07-20-2012, 03:28 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

Couple of thoughts, 1. did any engine ever run right in this airframe? 2. Borrow a tank of fuel from a fellow rcer to see if your fuel may have condensation(water) in it, even if you can't visually see it, and 3. the tank is common to both engine problems, I have used a small tank that is plumbed correctly and just rubberband it to the fuse externally to see if it cures the problem. Good luck.
Old 07-20-2012, 05:07 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

As some one else stated, make sure the clunk is not to close to the back of the tank resulting in the engine starving for gas and also make sure the clunk you are using has a good size hole.. I once had a O.S. 160 that was doing the same thing and it ended up being the clunk was to small and the gas was not passing through fast enough to keep the engine running at higher speeds..
Old 07-20-2012, 05:58 PM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

I would put both engines on a test stand and get them to run correctly. It sure sounds like a tank or fuel line problem since both engines share the same symptoms. When you get your new tank, put it together and use it on your test stand. When both engines run correctly with the new tank and fuel lines, put the tank in the plane and run it. It should be fine at that time, and you can fly. I agree with most posters, it sounds lean or has a severe fuel delivery problem.
Old 07-21-2012, 08:01 AM
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Default RE: OS Max 91fx Shutting off above 3/4 throttle, HELP!

Im starting to believe its the tank as well, I need to source one asap! Anyone??

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