Misfire "A" Ignition...

[Piper J3]

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?

 

[Azjulian]

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-WyzeProbe-for-Secondary-Ignition-and-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 ?

 

[fl-mike]

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.

 

[Dave12]

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.

 

[RFSchaller]

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.

 

[NinerBikes]

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.

 

[Piper J3]

See my original post #1 above.

I decided to get serious and find the source of rough running engine at low / mid-range speeds. I contacted Lockwood Aviation to ask if they can test ignition modules for intermittent operation. They can only test for spark - no spark.

Dean Vogel at Lockwood suggested several troubleshooting steps to identify if problem is related to carb or ignition. He said to start with carb first. Here is the troubleshooting step he describes? Put the engine where it?s unhappy ? say, 3500 rpm on ignition circuit B ? and pull the choke full ON. You are likely to get an rpm rise. Ignore that. What you are looking for is, does the roughness get better or worse, or does it stay the same. If it gets better or worse, the problem is likely to be carburetion. If the roughness stays the same, the problem is likely to be ignition.

I did this carb test and found that my engine runs extremely rough with the choke pulled out on either/both ignitions. I did the same test on a friend?s RV-12 and it ran much smoother with the choke pulled out. So, I bought a Bing 64 rebuild kit ($252) and disassembled one carb at a time. Carbs were very clean with 480TT. I run Mogas 93E10 exclusively. I changed all O-Rings, gaskets, diaphragm, and float needle. I cleaned and blow-out all passages with Gumout Carb spray cleaner.

One caution to note: The Rotax Heavy Maintenance Manual has an error for assembling the enrichener choke. The manual says choke labeled ?L? is for carb cylinders 2/4 and choke labeled ?R? is for carb cylinders 1/3. This is incorrect and indeed the opposite is true.

Below is excerpt from Rotax Heavy MM?



Even though the original symptom seemed to indicate a fault with the ?A? ignition it turned out to be carburation. Overhauling the carbs with all new rubber parts including rubber-tipped float needle has solved the problem. A very careful synchronization provides smooth RPM transition from idle to full throttle and back to idle. This is particularly noticeable when reducing throttle in the air and the engine is being back-driven in a glide.

Here are good YouTube videos that explain idle circuit and enrichener for Bing Constant Velocity(CV) carbs?

https://www.youtube.com/watch?v=S-fUkEEiork

https://www.youtube.com/watch?v=hDsJH1Y9W_U

 

[Harvey rv12]

Jim
Did the rebuild kit contain new jet needle and needle jet?, and was there any wear on these or other moving parts, ie slide, shaft bushings etc?.
My -12 has about the same hours (480) and I am looking at doing the same to my carbs soon.

 

[Piper J3]

Jim
Did the rebuild kit contain new jet needle and needle jet?, and was there any wear on these or other moving parts, ie slide, shaft bushings etc?.
My -12 has about the same hours (480) and I am looking at doing the same to my carbs soon.

No jets or main needle included in the rebuild kit. Only wear I could see was lower edge of the slide - I forget if front or back side. Needle looked good with no apparent wear on the main jet.