romair Posted November 27, 2012 Report Posted November 27, 2012 Have a questions regarding excessive magneto drop. I just had my magneto IRANed. After all that needed to be replaced was replaced, the magneto was bench tested and seemed to be working fine. Put it on the plane and when checking for the individual magnetos the left one dropped 150 and the right one 250. Previously it was about 130 drop on one and 150 on the other. Checked the spark wires under tension - they seem to be good. Checked the spark pugs under tension and cleaned them as well, they check good too. Then, since I changed one of the capacitos that is in the harness we decided to change the connections going from the capacitos to the magnetos. Tried it again, and this time the left magneto dropped 250 and the right one dropped 150 (totally reversed). Case solved, capacitor is the problem...not really!! Went to the shop that did my mags (thankfully it was local, and he is the same guy Don Maxwell uses), and we bench tested the capacitors. They seem to be good. Regardless, got a pair of different capacitors (used, but only 400 hours on them), put those on, and again...250 drop on the left, 150 on the right. We then ran the engine for 15 minutes on the "bad" magneto and checked temperature with infrared gun for the spark plugs, intake and exhaust stacks. Within reason they are about the same (colder on the sides that were not firing because it was running on one mag only). then did a compression check of the cylinders...all above 75/80. Checked the tach with a window tach to make sure that I was getting correct readings, and it correlates. At this point I pulled the entire dual magneto (back side included), as well as all the wires and going to the magneto shop to have it bench tested. With the exception of the excessive mag drop, the engine runs smooth, and no back-firing at all. Do you guys have any thoughts? The internal timing is good (points close together at the same time) and the external timing has to be pretty good too, as the engine runs quite smooth... Thanks Stefan 1989 M20J Quote
jetdriven Posted November 27, 2012 Report Posted November 27, 2012 Stefan, where was the mixture? If it is full rich and the density altitude is over 2K that might be your excessive mag drop. Setting 2000 RPM, leaning to peak RPM, then doing the mag check lessens the mag drop quite a bit Quote
N201MKTurbo Posted November 27, 2012 Report Posted November 27, 2012 The magneto can be put on upside down from the last time it was installed and it works just fine, it just swaps one side for the other. I think it is your internal timing. The points should be set to open at the timing mark, not close. Quote
romair Posted November 27, 2012 Author Report Posted November 27, 2012 mixture was not full rich...and I live in Dallas, today was extra cold, so I don't think its density altitude... Quote
romair Posted November 27, 2012 Author Report Posted November 27, 2012 It could be internal timing, hopefully he will be able to figure it out now that I am taking the whole thing back...what I was trying to say (and I am pretty ignorant, this is a steep learning curve for me), when the mechanic was trying to do the external timing using his meter, when the "beep" would come on when moving the prop, there was a single beep. This made him think that the 2 magnetos are firing in the same time, as opposed to if there had been 2 discrete beeps which would show that the two magnetos are not timed corectly to each other. I hope this makes sense... Quote
jetdriven Posted November 27, 2012 Report Posted November 27, 2012 a dual mag has very tight internal timing and both mag lights will light up at pretty much the exact same time. Quote
N201MKTurbo Posted November 27, 2012 Report Posted November 27, 2012 If the internal timing is set correctly. It usually takes me about an hour to set the points on a D3000, but its my plane and I'm picky about that. Quote
smccray Posted November 28, 2012 Report Posted November 28, 2012 We then ran the engine for 15 minutes on the "bad" magneto and checked temperature with infrared gun for the spark plugs, intake and exhaust stacks. Within reason they are about the same (colder on the sides that were not firing because it was running on one mag only). then did a compression check of the cylinders...all above 75/80. Checked the tach with a window tach to make sure that I was getting correct readings, and it correlates. At this point I pulled the entire dual magneto (back side included), as well as all the wires and going to the magneto shop to have it bench tested. Not sure what you mean by colder on the sides that were not firing. When I had an issue with my ignition system we traced it to the lead, but the lead test was inconclusive. I went ahead and replaced the leads and it solved my problem. Where are you in Dallas? I'm at ADS- if you could use an extra hand let me know. Quote
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