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  #1  
Old 07-07-2019, 06:58 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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Default Misfire "A" Ignition...

I asked this question over in the Rotax 912/914 Technical Forum and didn’t get a response... I have an early S/N RV-12 with 912ULS now with 450TT. Carbs are synched beautifully with electronic CarbMate. Synch is good at idle and just off-idle. L&R EGT’s are very closely matched from idle to WOT.

Problem…. Most of the time the engine runs smooth 1800 – 3000 RPM. Sometimes the engine has a miss at this low speed. When “B” ignition is turned off the miss continues. When “A” ignition is turned off the engine runs smooth. So, problem is definitely with “A” ignition. I have changed sparkplug caps to NGK Spark Plug Resistor Cover - 90 Elbow Type (LB05F) and installed new NGK DCPR8E Spark Plugs gapped to 0.025”. The new spark plug caps improved COM radio noise but didn’t fix misfire. I thought maybe problem is temperature or moisture related but that doesn’t seem to be the case. The misfire almost seems like induction/cross-talk between the high-tension wires. Engine always runs smooth at 4000 + RPM and extremely smooth at 5500 RPM cruise. Either ignition can be shut off at cruise RPM and engine continues to run smooth.

Possible next steps:
  • Replace high-tension ignition wires (7mm copper)
  • Maybe there is problem with magnetic timing sensor?
  • Maybe timing sensor gap is incorrect?
  • Maybe one of the new spark plugs is bad?
I have an oil change coming up in 20 hours so I’m trying to get ducks lined up for when I pull the cowling.

Any suggestions appreciated. Thanks in advance…
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  #2  
Old 07-07-2019, 01:52 PM
Azjulian Azjulian is offline
 
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Location: Scottsdale, Arizona
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Default

One thing that would help you diagnose if this is an ignition issue would be to use a probe on the ignition wires in turn and look at the waveform on a scope. You could either use a laptop as a scope with a cheap usb scope online or you may have access to one.

The probe is for sale here https://www.aeswave.com/15-inch-Wyze...COP-p8764.html

Here is a guy using it on a car https://youtu.be/iFLaRd_5wc8
What you describe could also be attributed to sinking floats, have you weighed those ?
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  #3  
Old 07-07-2019, 11:19 PM
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I know nothing about 912’s, but I replaced an intermittent pickup coil in a 582 recently. Same type of symptom. You can ohm it out.
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  #4  
Old 07-08-2019, 05:57 AM
Dave12 Dave12 is offline
 
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Try swapping the ignition modules and see if the misfire follows it. I had a similar situation with my first 12. It eventually stopped firing at all.
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  #5  
Old 07-08-2019, 12:12 PM
RFSchaller RFSchaller is offline
 
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I don’t know if this simple test would even apply, but I remember the instructor at the ROTAX course talking about a dark test. You remove the cowling in a dark area of the airport at night and run the engine while an observer looks for any sign of arcing from the ignition leads or connectors. Might not turn up anything, but it’s easy and you might get lucky.
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  #6  
Old 07-09-2019, 12:57 PM
NinerBikes NinerBikes is online now
 
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Quote:
Originally Posted by RFSchaller View Post
I don’t know if this simple test would even apply, but I remember the instructor at the ROTAX course talking about a dark test. You remove the cowling in a dark area of the airport at night and run the engine while an observer looks for any sign of arcing from the ignition leads or connectors. Might not turn up anything, but it’s easy and you might get lucky.
That works, sometimes. Putting your hand on the outside of spark plug wires on cars (which don't have a prop that can chop you up) and seeing which wire zapped you is how we did it in Auto shop in high school.

Really can't recommend that with a prop whirling around so close by, in the dark.


There is a higher tendency to blow the sparks out of spark plug and misfire from engine fueling rates and have misfires at higher rpms/ higher loads, if the problem is electrical.

Location of spark plugs on top or on bottom of cylinder head, in conjunction with the carbs running richer at lower rpms, which is where your misses are showing

up, would lead me to believe the problems is in the Carb adjustments or the floats again, and not an electrical problem.

Pull your floats again and weigh them, see what you get, since you put in a spare set of dried out sinker floats, last time.

Last edited by NinerBikes : 07-09-2019 at 01:04 PM.
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