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Inverted OS 91 4 Stroke Surging

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Old 05-23-2012, 02:31 AM
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dasintex
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Default Inverted OS 91 4 Stroke Surging

I have an OS 91 4 Stroke Inverted with a Slimline Compact Pitts Muffler in a Hanger 9 60 Size Spitfire; the engine starts without a problem, low end runs clean with a smooth transition, but once in the air it starts to surge a little at mid to full throttle, what I mean by surging, the throttle goes up and down on its' own; throttle linkage and servo are solid, no free play.

Since then I have made some changes, I thought heat was not venting out properly or sufficiently, so I built an air dam in front of the exit vent to create some negative pressure to draw the heated cowl air out.

Also, I read somewhere, that the exhaust pressure nipple may not be in the best place to properly pressurize the fuel tank, not sure if this is the problem, the nipple was already in place when I received the muffler, I tried to upload a photo but it didn't work, basically, it is placed on the short header between the engine and the canister.

Would plugging one of the exhaust tubes help?

I will try uploading a picture later.

I have not ran the plane since I built the Air Dam, just thought someone may have the same problem, so while I have the cowl off, what better time to change the nipple placement or plug an exhaust.

Thanks.
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Old 05-23-2012, 05:54 AM
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Default RE: Inverted OS 91 4 Stroke Surging

A few things to consider:

1) An "air dam" at the outlet is really a stalled flap. It is supposed to reduce pressure at the outlet so that air can flow through the cowl. Actually, you really need to make sure that air going IN is actually passing through the engine's fins, especially the head fins. Baffles may be needed. If you can see the outlet when you look into the inlet, that's how the air is going, and not necessarily through the engine's fins.

2) It may be too rich. A too-rich setting can cause surging when the model's in the air.

3) Make sure all of the plumbing is not clogged, and has no cuts or holes in it.

4) Make sure the high-speed needle's not vibrating around, and that the carb barrel isn't moving in and out due to vibration.

All of these can contribute to surging.
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