914 UL Mag Drop
in 912 / 914 Technical Questions
5 years ago
Gentlemen,
I have a new (20hrs) 914 UL installed in a MTOsport gyroplane. I had been only testing the mags @ 3000rpm due to the gyro skidding on the turf runway. I recently did some ops on hard surface runway and did the run-up checks at the recommended 4000rpm. I had bee seeing 50-75rpm drops at the 3000rpm, but at the higher rpm I'm seeing 100-150 drop with "A" mag off, and 250-300 drop with "B" mag off.
I've verified that my #1 Mag switch is connected to the "A" ignition & the #2 Mag switch is the "B" ignition.
I next swapped the output connectors from the A & B modules to the coil packs. On run-up, I noted that the problem appeared to swap, "A" mag off now had 350-400rpm drop, "B" mag off only had 50-100rpm drop. The problem switched sides and got worse while the opposite mag drop improved!
I then swapped the ignition module output connectors back to original then pulled all 8 plugs for inspection. The PTO side plugs were somewhat sooty, but the center insulator looked normal. The mag side plugs were all identical, no soot at all and center insulator almost white. I then re-terminated all the "A" plug boots (1-4top, 2-3bottom). I then ran the engine again and noted the identical rpm drops as noted above, somewhat marginal with the "B" mag off (same as first paragraph).
What would you suggest I try next? Should I be concerned about the sooty plugs on cyls 1 & 2? Note that on this engine I had to remove and reinstall the heads due to the ASB for the exhaust valves. I cannot say if this was a problem previous to pulling the heads as I had not checked the mag drop at 4000rpm.
Could this be related to the 914 UL having different ignition timing between the A & B circuits (MMH ref, 74-00-00, page 6)? Edit, added screen shot of this MMH refernence.
Thanks & regards,
Pete Melby
I have a new (20hrs) 914 UL installed in a MTOsport gyroplane. I had been only testing the mags @ 3000rpm due to the gyro skidding on the turf runway. I recently did some ops on hard surface runway and did the run-up checks at the recommended 4000rpm. I had bee seeing 50-75rpm drops at the 3000rpm, but at the higher rpm I'm seeing 100-150 drop with "A" mag off, and 250-300 drop with "B" mag off.
I've verified that my #1 Mag switch is connected to the "A" ignition & the #2 Mag switch is the "B" ignition.
I next swapped the output connectors from the A & B modules to the coil packs. On run-up, I noted that the problem appeared to swap, "A" mag off now had 350-400rpm drop, "B" mag off only had 50-100rpm drop. The problem switched sides and got worse while the opposite mag drop improved!
I then swapped the ignition module output connectors back to original then pulled all 8 plugs for inspection. The PTO side plugs were somewhat sooty, but the center insulator looked normal. The mag side plugs were all identical, no soot at all and center insulator almost white. I then re-terminated all the "A" plug boots (1-4top, 2-3bottom). I then ran the engine again and noted the identical rpm drops as noted above, somewhat marginal with the "B" mag off (same as first paragraph).
What would you suggest I try next? Should I be concerned about the sooty plugs on cyls 1 & 2? Note that on this engine I had to remove and reinstall the heads due to the ASB for the exhaust valves. I cannot say if this was a problem previous to pulling the heads as I had not checked the mag drop at 4000rpm.
Could this be related to the 914 UL having different ignition timing between the A & B circuits (MMH ref, 74-00-00, page 6)? Edit, added screen shot of this MMH refernence.
Thanks & regards,
Pete Melby
Screenshot2019-04-0117.03.07.png (You do not have access to download this file.)
