912 Trigger Coil

[cainbird]

I started my engine this weekend and all went well. If I perform a mag check I get an awful vibration. I suspect I have a bad trigger coil. (used engine) I was wondering if anyone knows how to test the trigger coils and also what rpm do people use for a mag check and what is an acceptable drop.

Thanks in Advance~Cain B

[Dave S]

Cain,

Do you have a reason to suspect the trigger coil?

I'd start with the common/simple/easier stuff to rule out first.

With a misfire/rough run on a mag check - check the plugs first - check the spark plug wires - are they affixed properly @ both ends? - are they in one piece? - on the correct plug? - p-lead and mag switch? half loose screw on the mag switch? any chafing of any ignition wires anywhere? Any loose connections?

Hook up an auto timing light to each plug wire - is there spark indicated at all plug wires?

Sincerely,

Dave S

[jtpitkin06]

I agree with Dave, you go for the simpler things first.

Failing that, there is a lot of information on the Rotax Owners forums for specific information on the 912.
Here’s a note that is of particular interest for your problem.

“-I have a bad mag drop, what do I do?
First determine if you have a carburetor problem or an ignition problem; (WHAT?? Yes, the Rotax 9 series dual carb system can confuse the owner as a carb problem may show up as a high mag drop on one ignition system)
Procedure:
Run the engine at 4000rpm (approx 1700 prop rpm) and perform a mag check. Record the numbers.
Slowly pull the choke out half way, the RPM should rise about 200 RPM.
Try the mag drop and record numbers.
If the problem is worse or better the carbs are suspect.
If there is no change the carbs are not suspect, you likely have an ignition problem.

If you have an ignition problem use the Ignition troubleshooting guide to find the source.
If you have a carb problem see Carb troubleshooting”

[cainbird]

Thanks Dave and JT,
Excellent advise. Dave I was thinking the same way you were with using a timing light. Great idea! After buying this engine I discovered someone had removed a trigger coil. Somehow I had two of the trigger coils wired in reversed after I replaced the missing one. I just removed and swapped them with each other (without changing the wiring) and all is well. It runs great and mag drops are reasonable. What rpm do you guys use to check mags and what type of drop do you see?

Thanks~Cain B

[Dave S]

Hi Cain,

I can see now why you had reason to think about the trigger coil(s)

The Mag check specifications from the Rotax Manual for the 912ULS which came with my engine are:

1) Check at 4000 RPM
2) Maximum Drop 300 RPM
3) Maximum difference between R & L mag = 120 RPM

The Manual also says that prop setting will affect these numbers and the individual installation should be considered

The Mag Check values I use are the same as above except I am using 3800 RPM instead of 4000 RPM ( with the prop set 5200 RPM on climb-out )

If a person does the math & proportions to 3800:
1) Maximum Drop 285 RPM
2) Maximum difference between R & L mag = 114 RPM
But this isn't significantly different from the manual values for 4000 RPM

Experience with this engine over 150 hrs of operation:
Normal Drop = 150-175 RPM
Observed difference between R & L - Negligible - but has varied between 25-50 RPM on occasion

Glad to hear you have you engine running smoothly now.

Sincerely,

Dave S
KF 7 Trigear
912 ULS
Warp drive tapered tip

[cap01]

like dave , i use a little less rpm when checking mags . 3700-3800 , a lot easier to hold the airplane with the brakes and easier on the rudder system . whenever i do any running for maintenance , other than idle , i always tie the airplane down and dont even try and hold her with the brakes .