Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

OS 46 with Macs pipe surging at high speed

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Old 10-21-2013, 03:44 PM
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rideon67
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Default OS 46 with Macs pipe surging at high speed

This engine/pipe combo has run OK for years on a quickie 500 airframe. Now it can't seem to get enough fuel and surges at anything over 2/3 throttle. screwing in the high speed needle will lean the engine out, but screwing out the needle will not richen up at high speed. I cleaned out the whole fuel supply system, put in new tubing, and a new o-ring on the remote needle valve and flushed out the needle valve body. I'm at a loss for what to try next.

Anybody ever run into this? Thanks.
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Old 10-21-2013, 05:46 PM
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Old fuel, plug, using pipe pressure? If it is in the same plane it sounds odd. Could be a loose fitting on the pipe if you are using pipe pressure, leaking backplate. Smaller prop maybe. That would make it worse but it might just be at the point of getting on and can't do it. I'm running out of guesses.
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Old 10-21-2013, 05:56 PM
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I had a high time 28 FSR that started doing that and it ended up being a bad bearing. Somehow air got through it. Replaced it and it was back to being its usual reliable self.
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Old 10-21-2013, 06:29 PM
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A high percentage of my engine troubles are in the fuel tank.
The hardest one to find is tank foaming because you can cancel out the foaming when you hold the model in your hands.
If the engine wont richen up and you have ruled out the fittings at the engine that is just a portion of the entire fuel delivery system.
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Old 10-21-2013, 06:40 PM
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Did this problem happen progressively or all of a sudden? It's evidently starving for fuel, or sucking air. Sounds like you have eliminated fuel blockages or air leaks in the fuel supply system.

Some questions that come to mind:

- by fuel supply system does that include the pressure line? And checking the pressure tap on the pipe that it is clear?
- if it happened between one season and the next, corrosion taking out a bearing? (Do you even have flying seasons? - dumb Canuck) As thailazer suggested perhaps one has gone bad for some reason, could happen gradually or all of a sudden.
- are you eating plugs?
- if you're not, how old is the plug?
- leaky header/cylinder seal?
- leaky pipe coupler?

Hmm.. what else..
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Old 10-22-2013, 08:04 AM
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carb base o-ring may be leaking
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Old 10-22-2013, 10:22 AM
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A strange [and rare] thing that will most likely cause the high speed running to suffer but allow the low rpm running to be OK is to have the liner rotate slightly in the case. I do not know if the liner in this engine is pinned or not.
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Old 10-22-2013, 11:37 AM
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a loose backplate or leaking gasket can cause this as well
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Old 10-22-2013, 04:04 PM
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Thanks all for the great gremlin ideas. I've got a to do list going -- starting with change the bearings since this starts with complete engine disassy and inspection tonite. I didn't mention it, but saw some rusty afterrun oil under the engine where it sits nose down when stored. The problem started after the plane was sitting for a few months. If that doesn't help, I'll change out this engine for another OS46.
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Old 10-22-2013, 06:31 PM
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Make sure the needle is set just above four stroking. Smoking alot! For speed I ran 9x8 - 9x10 on piped .45's Your engine is unloading and you are going lean causing surging. Set it fat! Ditto on the plug. Keep the plug medium- cold and keep it fresh. KB 1L is a good one if it is still around. Been a Lecky for 5 years, but ran fast glow for 25 years. You want to go fast? Try electric! What prop are you running? What kind of airframe?

Brooks
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Old 10-23-2013, 05:10 AM
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The thing is, his setup was running well before, now it isn't. So unless he's forgotten how to set the needle (which admittedly is possible given my own experience) I tend to think something has changed. To me a strong piece of evidence is that it started misbehaving after sitting around for a while.. corrosion.. bearings.. but it could be something else of course. Disassembly, cleaning and complete inspection should reveal the issue(s). Sure sounds like an air leak somewhere. Same symptoms from fuel foaming, so maybe a prop change put it out of balance. No matter, if there was grungy looking oil coming out, strip 'er down and check 'er out regardless would be my advice.

So rideon, what's the verdict after tear down?
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Old 10-23-2013, 05:27 AM
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I had a couple planes dribbling from the carb as I hang them tail up. It is usually because of fuel still in the tank and it dries out. The bung gets loose, one softened up the balsa in the fuse so much the battery fell out. Tightened the bung and everything was fine, ran much better. I hope you figure it out, probably something simple.
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Old 10-23-2013, 11:20 AM
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A pressure test is a good thing to have. Automotive hobbyists use this idea as well as guys who repair 2 stroke powered equipment.
It is pretty easy to rig up on any engine. Take a known healthy engine to see what the leakdown looks like typically. You just need to block off the exhaust port and adapt a sealed hose to the intake. Dunk the engine in water, blow low pressure air into the hose and watch for air bubbles escaping from the engine. You will need to slowly turn the crank to observe as much as you can. A hose attached to the block off plate over the exhaust port and plugging off the intake does about the same thing. Besides these tests, simply priming the engine and bringing the piston up to TDC, the cylinder ought to remain pumped up with air for several seconds at least while it is cold. Again, take a known healthy engine and perform these tests to acquire "The Feel" for what seems right or wrong.
Slowly turning an engine while applying radial force to the crank will reveal flaws in the bearings.
The "water dunk" pressure test can reveal porosity flaws in the crankcase that escape the naked eye.
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Old 10-30-2013, 06:12 PM
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Sorry for the lack of feedback. Great flying weather and the World Series have cut down on work time. It's still on top of the bench....
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