Originally Posted by
Crash one
I'm sorry but I certainly don't understand that. There are two mags in order to provide a full burn, not as a redundancy / choice exercise. Both mags should fire at the same time (possibly one may be slightly advanced?) Misfiring when both are on, not misfiring when only one is on does not compute very well?
Could this vibration have been caused by the rain shower / carb ice? switching off one mag so reducing the amount of "bang" & thereby reducing power & vibration?
Most likely there was either some level of water contamination in the the suspect mag causing intermittent/off-time firing or a slight breach of the insulation for one of the plugs (causing it to not properly fire). When both mags/plugs fire you get a full power stroke and when one messes up you get slightly less power. This difference causes the vibration. When you switch to one mag (either the dry mag or the one not powering the duff plug/wiring) you get slightly less power on all cylinders so no vibration.
In a fully instrumented engine you can see this weakness in EGT spread well before you can feel it and then have the issue sorted on the ground (as you can relative easily isolate the plug/plugs in question).