Low RPM Vibration on Left Mag

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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3Dreaming
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Re: Low RPM Vibration on Left Mag

Post by 3Dreaming »

drseti wrote:Most likely an ignition/plug issue. There is no logical reason for a carb issue to manifest during a mag check. After all, the two carbs feed the right and left pairs of cylinders respectively, but the "right" and "left" ignition modules each drive all four cylinders.
Paul, I'm not saying it isn't an ignition issue, but I have seen a carb issue cause the problems you are describing. When I was younger I used to fly pipeline patrol. The person who taught me also taught me to use a mag check as a leaning tool. Lean the engine a little bit and check the mags, if it was smooth you were OK and if it was rough you were to lean.
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Re: Low RPM Vibration on Left Mag

Post by SportPilot »

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Last edited by SportPilot on Sun Nov 16, 2014 8:21 pm, edited 1 time in total.
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drseti
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Re: Low RPM Vibration on Left Mag

Post by drseti »

Not with a Rotax 912. There is no mixture control.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
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3Dreaming
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Re: Low RPM Vibration on Left Mag

Post by 3Dreaming »

Let me expand on the mixture and rough running thought. Most aircraft engines have 2 ignition systems with 2 sparkplugs per cylinder. For normal operation both plugs are firing at or near the same time. This provides 2 ignition points for the fuel air mixture in the cylinder allowing for a nice even fuel burn. When you turn off one ignition you change the burn pattern in the cylinder because you only have one point of ignition. Now most Lycoming engines are set up with each mag firing the top plugs on one side and the bottom on the other. Continental one mag fires all tops and the other all bottoms. Some Continentals have different timing by a couple degrees between the top and bottom plugs. They do this to control the fuel burn pattern in the cylinder. What I gather from the Rotax wiring is that they fire one top and bottom on each side for each ignition system. This should keep the ignition drops the same, but there is one more factor. It has to do with how the fuel air mixture flows into the cylinder in relation to the plug that is firing. For example a slightly lean mixture may burn OK with both plugs firing, but it might run rough with just one firing. It can also be dependent on whether it is the top or bottom plug because of the way the fuel air mixture flows into the cylinder and the point of ignition. With the engine running faster the flow and distribution of the fuel air mixture might be different leading to a smoother ignition check. Also different RPM's will rely on different circuits in the carburetor to provide fuel. At lower RPM's you will be running on the idle circuit, and at higher settings you will be on the main circuit. That is why I earlier said you could do a check while running on the starting circuit to see if there is a problem there.
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FastEddieB
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Re: Low RPM Vibration on Left Mag

Post by FastEddieB »

That was very clear.

Thanks!
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designrs
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Re: Low RPM Vibration on Left Mag

Post by designrs »

And don't forget, based on the examination of the old spark plugs, my motor was running richer on the front two cylinders and leaner on the back two calendars. (yes I really mean front and back, not left & right). Why that was the case we don't know. However when we pull the new plugs hopefully we will see if that is still the case or not, which might in some way be related to the lean mixture explanation above.
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