Sounds like it is dropping 2 cylinders when you perform the mag check. It clearly runs on circuit A & circuit B & the inability to start on B could be misleading your diagnosis. Focus on the mag drop first.
I would check the following.
Ignition trigger for damage/shorted.
Damaged wire from the triggers
Damaged or disconnected coil earth lead
Damaged or disconnected coil supply wire.
Sounds like it is dropping 2 cylinders when you perform the mag check. It clearly runs on circuit A & circuit B & the inability to start on B could be misleading your diagnosis. Focus on the mag drop first.
Is there is a inexpensive tool to check what spark plugs are not firing?
What I mean is: start the engine and see what spark plug are not firing (with the tool).... then check that CDI wiring, etc?
I dont want to hijacking Mark Gray post but I am interested in this because I have a similar problem like him.
On checking one Mag everything is ok and engine does not shake, when checking the other Mag, the RPM drops like 400 RPM and the engine runs rough and shakes. Then, when I put both Mags, the engine still runs rough and shakes.........Carbs are balanced and sycro too.
Roger,
I switched over the a & b modules, and the problem stayed with the lower module ie, changing them made no difference.
So with both modules connected, at 2500rpm, closing the A magneto switch gives 500 rpm drop, runs very rough, closing B mag switch drops abt 50 rpm.
At 1600 rpm, removing plug leads one at a time indicates there is nil or little spark in any of the top plugs. hmmmmm!
This motor is new and when it was installed in the ring mount, a bracket was removed and modified to allow the fitting. I'm wondering if this bracket also holds the trigger coils B 3/4 & A 1/2 , (Bracket is # 36 in Fig. 74-25, maint manual.) and we have altered the trigger coils / cam offset. Is that possible ? Thanks, mark.
Here is something to consider for mag drops. Mag drops can be equal or they may have a spread which is also normal. If you do a mag drop at 4000 as the book says verses 3500 you will just get a bigger spread or larger drop. There is nothing magical about 4000 rpm for a mag drop. But here is the kicker that I was reminded of the other day during a carb balance. If you sync the carbs at 2500 and then advance the throttle up to 4000 the sync will be off. 2500 carb sync is too low. I always sync at 3500 and up to 4000. If you sync the carbs at 3500 and do a mag check there then the carbs are synced there and run smoother so the carb sync is synced with the mag drop if you use 3500 for a mag check. I synced some carbs the other day at 3800 and they were out of sync at 2500 and then came back into sync closer to idle. Many engines have a little rough spot some where in their rpm range.
What I'm trying to say is;
If you sync carbs at 2500 and do a mag check at 4000 they are out of sync, will run rougher and have a bigger mag spread between the two mags because the carbs aren't in sync there. It is better to sync the carbs at 3500-4000 and then the mag check there too to get a better and more true mag check.
This is one of the reasons I don't believe in a 2500 rpm carb sync. It is too low and too close to idle and we don't fly near idle rpm.
Try a carb sync at 3500 and then a mag drop at 3500 and I bet the mag drop spread is much closer and smaller.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
To check which cylinders aren't running on any engine obtain a infra-red type heat sensor from your local auto parts store. Shine the red light on the exhaust pipes, one at a time. The cool pipe is not running, or running poorly, depending on how much cooler the temp is.
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