VAF Forums

VAF Forums (
-   RV-12/RV-12iS (
-   -   912uls Spark Plug Diagram (

trecool44 12-17-2020 07:20 AM

912uls Spark Plug Diagram
Hello, I am diagnosing a rough running / bad mag drop issue on my RV12. I t runs very rough when revving to 4000 for the mag test. It will smooth out after running higher rpm for a minute like the plugs are loading up. It is always ignition A that is rough. Does anyone have the diagram that shows exactly which spark plug locations are connected to ignition A so I can replace those plugs and take a look. I have already replaced the carb floats with no notable difference. Thanks in advance for the help.

John-G 12-17-2020 07:54 AM

Have a look at the Rotax Heavy Maintenance Manual section 74-00-00 ... page 19 shows the trigger coil assignments. (Not at the airport, but my reference is from revision 5, so the page reference may be off a page or two in a newer manual).

You will find the complete wiring diagram in section 74-00-00 page 25.

Per the manual:
Trigger coil assignment (on the 912 Series)
Trigger coil A1/2 serves the top spark plug of cylinder 1 and 2
Trigger coil A3/4 serves the bottom spark plug of cylinder 3 and 4.
Trigger coil B1/2 serves the bottom spark plug of cylinder 1 and 2
Trigger coil B3/4 serves the top spark plug of cylinder 3 and 4.

Hope that helps.

Piper J3 12-17-2020 08:14 AM

Sparkplugs are cheap - replace all @ $3.50 ea. Use high temp thermal paste on sparkplug threads per Rotax manual for correct heat transfer.

Could still be carb problem. Did you weigh floats?

trecool44 12-17-2020 10:12 AM

Thank you for the quick responses. Yes I weighed the floats, the current set was the old style but were still in tolerance for weight. I changed to a new set that was also in tolerance for weight.

Piper J3 12-17-2020 11:20 AM

I would not discount carbs. Your problem does not sound like ignition or plugs.

Next time you fly - do an in-flight ignition check by cycling each ignition at cruise RPM.

Another thought... if you have completed the torsion spring upgrade for the throttle arms, then add back the original extension spring. Many folks (me included) notice smoother operation in mid-range RPM. 2nd spring adds more tension to make cable pull(s) more uniform.

NinerBikes 12-17-2020 02:44 PM

Check the coil pack mount brackets first to see if any of them are now broken. Pull the air filters, probably clean them too, while you have them off and re oil, and inspect those funky "L" brackets that hold the coils in place.

If they are broken, the coils can rub on the engine mount and rub through and short out the coils windings. If they are broken, it's highly possible you have a vibration problem that caused them to fail. Balance the carbs, balance the prop, and verify the Clutch, Dogs, and Bellvue torque washers are in spec and not worn on the gearbox, causing vibration problems that break those brackets. On a Rotax 912ULS... not balancing causes a lot of failures of components, you have to stay on top of it and keep things in proper adjustment. The motor running a 11.0 :1 compression ratio is pretty violent if the engine RPMS are too low, making things speed up and slow down, lack of rpms creates lack of momentum, so speed things up a little bit.

Always have the owner idle the engine at somewhere very smooth running between 2000 and 2500 rpms on start up through warm up.

Always a good place to look, do a search on broken coil mounts here in the RV-12 section of VAF.

trecool44 12-18-2020 09:29 AM

More great information, I have performed an in-flight ignition check. About 20 rpm more drop on ignition A but still smooth. Once the engine smooths out, it is smooth for the remainder of the flight. I will be doing the mount check this weekend.

Piper J3 12-18-2020 09:56 AM

The in-flight check should show a little drop. Sounds like carb problem...

trecool44 12-21-2020 12:37 PM

I did find the long bracket to the case was broken. I didn’t see any wear on the coils. I think it’s time to send the carbs to Lockwood. Thanks for the information that led to finding the broken bracket.

Cammie Patch 12-27-2020 03:45 PM

Before you assume the ignition system is the problem on an unusual mag drop, make sure it's not a mixture issue.
To check this, do your normal mag check, they pull the choke out about halfway, and do another mag check like that. If the results are different, it's your carbs not your ignition.
4000 RPM is the leanest range, and is the best place to test the drop. You may even notice that your drop is less in the summer, since a leaner mixture is better then.

All times are GMT -6. The time now is 01:25 PM.