503 DCDI engine failure.

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ToddK

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Rotax has had some issues with the carb floats, there is an AD to weigh them. If they fail, get the marvel epoxy floats.
 

rv7charlie

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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).
 

Ken Powell

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This 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.
 

Tuneturkey

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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).
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.
No indication of heat, the engine id not cowled. Have not checked the fan belt, but at 60 mph, it should have had enough cooloing even without the fan.
Yes, the prop turned over with no binding, and it restarted within a few minutes of the shutdown. First time immediately, second time 6 or 7 pulls.
where did you get a cheap inodcope?
 

Tuneturkey

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This 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.
on 1. two shutdowns, each on different plugs. 2nd time on new ones
2. - 4. done
5. not done. inspected and no corrosion
6 - 7 Stator charging winding checked out ok.
Thanks
 

rv7charlie

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The 'heat' reference is about heat related electrical failure; not general engine temps. Electronics can shut down when hot, and work perfectly when cooled off. I once fixed a numerically controlled printed circuit board drilling machine using the improvised tool of rubbing alcohol in an eyedropper, putting a drop on each integrated circuit on the circuit board until the problem cleared up. Replaced the IC that 'responded' to the alcohol cooling; done.

Note that a coil of wire, or a solder joint, or even a screw terminal, can have similar responses to heat/cold. The idea of using a temperature probe is to see if there's a correlation between the temperature of the component in question and the engine stoppage.

But do the obvious stuff first. Assuming that the newly installed fuel pump is making the correct (not excessive) pressure, you obviously have an issue with at least one of the carb float bowls; possibly both, depending on how the vents are plumbed.
 

TFF

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I assume you have two kill switches. I have chased 912 ignition problems and to my surprise it’s always been wires in the system not the system. I have changed a stator, that did help with hotter ignition on one engine. I have been able to swap out components that would be fine on another engine every time. I do suspect the ignition boxes degrade over the years, but it feels like a gut reaction because they always work just good enough.
 

quick582

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I had almost this exact same issue and two engine failures, one with a glide to land, the other on the ground ( I would not fly it agian until I figured out the problem) I went through seemingly everything, had the engine totally apart and back together again. Went through all the electrical. Carbs apart and back together. But I did NOT remove all the jets yet, just looked to see the sizes. I was in the process of checking the float bowls and levels again, ( they were fine) and I was installing them when I noticed brass thing on the ground. Astonishingly it was my pilot jet, which had fallen out when I dumped the fuel out. Apparently the jet was barely screwed in, and when running would suck up into the threads and run until the throttle was reduced. Then it would slip down and the flood of fuel would cause the engine to run super rough and then quit. No amount of cranking would make it "run" except very roughly and then die. This is exactly what happened on approach to landing on the engine out. This engine was new to me and I was told that it ran inconsistantly, but I never expected this as the answer. Luckily caution prevailed and with the jet properly tightened it ran perfectly and has continued to do so. So I thought I'd pass on my experience. It is very easy to overlook a loose pilot jet
 

Rattler1 1

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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.
Thanks.
 

Ken Powell

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Dec 8, 2020
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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.
Thanks.
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.
 
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