Iagree on the CDI's. The engine stopped abruptly like the kill switches were activated. Didn't runon one cdi/cylinder, drop rpm or anything.Again, I can't be specific to the 503's ignition system, but...
It seems extremely unlikely that you'd have two independent CDIs consistently inconsistently (parse that...) fail at exactly the same instant. If either continued to work, even for a few seconds, you should be able to hear the engine go slightly rough or the rpm drop off slightly when running on one ignition, just as it would when doing a 'mag check' prior to takeoff.
You could check the electronic components for any heat correlation with your failures using a cyl head temp probe, if you can find a way to securely attach it to the component of interest.
Does it turn over normally immediately after an unplanned shutdown? Will it re-start immediately, or do you have to wait for minutes or hours to get it to restart?
You can inspect the cyl walls through a spark plug hole using a cheap endoscope and a cell phone or tablet; I've used one on my Lyc engines at annual to check cyl walls and even the valves (trickier, with the cheap ones).
on 1. two shutdowns, each on different plugs. 2nd time on new onesThis is a fairly common occurrence in the dirt bike world of 2-strokes which usually run Kokusan ignition and charging systems. Stator failures are the most common issue followed by CDI failures.
1. Replace the spark plugs even if they are new
2. disconnect the kill switch
3. Ohm out the wiring harness to check for wire faults
4. Verify no corrosion on any grounds
5. Remove 1/4" from both ends of the spark plug wires to remove corrosion.
6. Check the ignition coils for correct resistance values
7. Check the stator for correct values (my bet on the issue)
8. CDI's are next to impossible to check. Try to find someone who will loan you a known good unit for testing
Good luck - sometimes these are hard to find.
Yes, After installing the elec fuel pump, had fuel squirt thru vent. Possible flooding. Carb inspection and cleaning pending.lRotax has had some issues with the carb floats, there is an AD to weigh them. If they fail, get the marvel epoxy floats.
Tank venting is another common problem we see in the dirt bike world. That both cylinders are quitting should make diagnosis easier but there is nothing easy about this thread.You've got a tuff one here. It does sound like its electrical but check your fuel tank vent. A plugged vent could somewhat act this way. The hard part here is there are two separate ignition systems that don't have a common part unless you only have one kill switch. You also have two separate fuel systems after the fuel pump. So I'm trying to think of a part that would affect both cylinders. The carbs are really simple. If one has a problem the other still works. If one coil has a problem the other still works. If it was mine, I would gravity feed the carbs from a separate tank to eliminate the whole fuel system before the pump. Pull the plugs to make sure there is not metal in them. (scored pistons) Compression check. Unplug the wire harness to eliminate the kill switch system. If all else fails, I would pull off the flywheel and check the stater. Keep us updated on what you find.