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Uneven Mag Drop 912 ULS
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BARRY CHECK 6



Joined: 15 Mar 2011
Posts: 738

PostPosted: Mon May 26, 2014 12:19 pm    Post subject: Uneven Mag Drop 912 ULS Reply with quote

John,

Easy ways just don't work!  Well, they do, but only for those people that won the lottery.   

In simple terms, even if you had a exhaust leak, it very we'll Would Not give you  a
 Mag drop.  Nor would an 
out of tune Carb.  Nor would the point of running With or Without the cowling on.  That logic would follow checking the air pressure in your tires for the Mag drop!
IF the Mag drop was good on one Mag, you are STILL using the SAME Carb for the Good Mag as for the Bad Mag , no reason why it would change for the second Mag
 ...  Do ya see the logic there?
Next is the Exhaust System.  If there was an Exhaust leak, it  would depend where the leak was...  An exhaust valve leak would leak on either Mag so No difference in Mag check 
with this scenario.  If it was a pipe or muffler leaking  it would again be the same leak no matter which Mag you were on, 
 so No difference in Mag check
with this scenario either.  

This is the way you go about checking out a Mag Drop Problem:

1 - Clean the plugs.

2 - TRIPLE check the Timing.

3 - Check the condition AND resistance of the spark plug wires.

4 - Did you ever DROP a spark plug?  If so - Replace it, don't think about it, do it.  OR, test it on a Pressure-Voltage Spark Plug checking station. Those are your ONLY options when it come to plugs.

5 - This is a bit more difficult - Check the Voltage coming out of your Mag.  You will need a High Voltage Probe Volt Meter, not many people have them.

6 - Question:  Do you get a Mag drop at BOTH low RPM and high RPM?  If so, then go back to #1 and run through the steps...  1 thru 5.  If it only happens at High RPM then there could be a voltage breakdown of a component and an overhaul is needed.  Problem being - You have a Rotax and both Low and High are both High when you consider the small delta between the two ranges.  SO...  What Next?  SWAP out the Mag with one of known GOOD working condition.


These are your options.  Go through the steps and I'm positive you will find the problem.


Barry





On Mon, May 26, 2014 at 2:07 PM, JohnF <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)> wrote:
[quote] --> RotaxEngines-List message posted by: "JohnF" <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)>

Progress on the mag drop !

Taking suggestions from you folks,  I  trimmed back the plug wires on the ignition side that I had not yet done.

As expected, mag balance was perfect.

As to exhaust leak...prior experience has shown me that a serious leak, one pipe 1/2 way out of the 'socket' on the muffler...definitely and repeatedly showed that mag drop was OK with the top cowling off, but bad with it one. An A&P and an experienced builder working with me saw this at least three times before I figured out what the problem was. Fixing the exhaust pipe into the muffler fixed everything.

As to today....the mag drop, average of 10 'runs' is now only 14 rpm difference between A and B sides. I still am not happy with the drop, its 227 rpm drop on A side, and 241 rpm drop on B side. I have buttoned up the cowling and will fly tomorrow, weather permitting, and run the plane hard for about 30 minutes and see if that changes anything.

My guess now is that the wire into the boot on cyl #4, bottom, was too far back in;  trimming it as Roger and others suggested fixed the large difference in mag drops.

My appreciation for all the help.

John




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