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OS 91 FX problems - need help

Old 08-31-2015, 06:14 PM
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prodgodq
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Default OS 91 FX problems - need help

Having a perplexing problem with this engine. Two weeks ago it was running fine. Used after run oil. Brought it out to the field yesterday. Started up fine and there were no problems with the usual run up procedures, including running at full throttle with no adjustments to the high speed needle. Also ran at full throttle with the plane pointing straight up.
No problems.
However, the engine died at about 15 feet in the air after take off. Brought the plane back. Changed the glow plug even though the existing plug was fine. Did the same run up procedure with no problem. This time, the engine died just as I was getting to altitude.
What could the problem be? The engine was tuned in fine.
Air leak? bad fuel?
Any ideas would be greatly appreciated.
The engine is mounted in a Sig Four Star 120.
Old 09-01-2015, 07:07 AM
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Bax
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If the engine dies quickly after takeoff, check to make sure that the pressure line to the tank is good. Also make sure that there are no obstructions to any of the lines in the tank. The vent is one place to start looking. Make sure the fuel system is totally intact, with no cuts or leaks in the fuel lines. You'll likely find something there.
Old 09-01-2015, 12:18 PM
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prodgodq
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Hi Bill, thanks for getting back to me.

It's a simple two line system, fuel line and vent line. Another symptom is that the engine has a tendency to die when I give it throttle right after starting it. That goes away after the engine has warmed up a little.
So if the vent line is blocked in the tank this could be the problem?
I'm planning on taking out the tank and blowing out both lines.
Also, I have a fuel filter on the fuel line and was thinking of removing that. The fuel already goes through two filters before it gets to the tank anyway so it's a little redundant.
Any other ideas?
Old 09-02-2015, 05:53 AM
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Bax
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You may have a problem with the piston/liner/ring fit. Throttle handling is the first thing to go when the compression starts to deteriorate. You may also just have a bad carb setting. The basic way to adjust it is to have a rich idle and rich top-end, get the engine at full throttle and adjust the high-speed mixture to just rich of peak RPM, and then retard the throttle and find the best idle mixture and RPM.

When an engine dies on throttle-up, it's usually a mixture problem.

If the engine needs to warm up a bit, but runs well after than, then you just need to warm it up before flight. You do that with full size aircraft engines.

A blocked vent line will cause the engine to quit after a few minutes because a partial vacuum will build in the tank until the engine can no longer draw fuel against the vacuum. Then it quits.
Old 09-02-2015, 06:03 AM
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prodgodq
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OK Bill, thanks. I checked the fuel tank, it looks OK but I may replace it anyway. The fuel tubing is old so I'll swap it out. I just replaced the cylinder head over the winter due to a stripped thread so I don't think it's compression. The engine hasn't been run that hard and I oil it after every flight. I've had problems keeping the engine in tune in the past and have gotten a lot of advice from people in my club, and as you can guess that can get confusing!

What is your step-by-step process for tuning? Maybe I should start from scratch.
Old 09-03-2015, 04:43 AM
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When we adjust the carb of a glow engine, we just start out with a very rich idle mixture setting, a rich high-speed needle setting, and start the engine at part throttle. We keep the battery on the glow plug as we accelerate the engine to full throttle. We then get the high-speed needle adjusted to peak RPM and then richen slightly. When we're sure of the correct high-speed setting, we then slowly retard the throttle and adjust the low-speed mixture to keep the engine running well. The low-speed mixture adjustment usually starts having an effect once you've gone below 1/2-throttle or so. We then find the lowest RPM setting and low-speed mixture setting that's consistent with good throttle changes...up and down.

It's all try and see.

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