912 excessive RPM drop on lane A

Just to clarify a point...  Mag A fires the Top Plugs on 1-2,  Mag B fire the TOP plugs on 3-4.  This is the way it is properly configured.

When it is running 400 rpm slow, a handheld IR Thermometer pointed at the exhaust pipes will tell you quickly if a plug is not firing.

The dual ignitions are a redundant safety feature.  This means that either one will/should keep the engine running at nominal power.

The Top/Bottom, Bottom/Top plug arrangement is so that when doing a mag check, you are not killing all four top or bottom plugs at the same time.

This way the RPM drop due to the plug position in the cylinders will be the same for each Mag.

You should expect to see an RPM change, but it should be the same amount of change for each Mag.

You might expect to see "A" drop 200 and "B" drop 300, but you should NOT see A drop 400 and B drop Zero.

Zero Drop means Nothing happened.

- 

Both plugs in each cylinder fire simultaneously, or nearly so.

Whichever fires first, sets the timing.

4° BTDC during start and 26° BTDC at speed.

If your A ignition is stuck at 4°, the B ignition will mask the problem until the A mag check, when a 22° timing shift happens.

The A plug may still be firing, just not at the proper time.

A timing light pointed at the Stator, Not the Prop, would show the shift.

- - -

Please let's STOP calling the Ignitions, "LANES".  

The "Lane" term is reserved to identify the "A" and "B" ECM Computer Modules in the Fuel Injected engines. 912is, 915is

Rotax officially calls them Ignition A/B.  Mag A/B is OK by me.

- - -

Blindly changing parts (Shotgun Troubleshooting) is an expensive way to fix things.

I have always been a "WHYs Guy" myself! 

Robert did say he swapped the ignition modules and the problem stayed on the same side.

Great feedback Bill. I’m going to look into the timing. I hesitate performing any maintenance around trigger and charging coils because I feel I’m not qualified but I can perform a test with a timing light. I shut off mag A and lose 75 rpm and then B I lose 400.   75 was the norm for both carbs in the past. I’ve tried all rpm ranges and it’s a steady loss of 400 rpm doesn’t matter at5000 rpm or 2200 rpm.                            

I have only flown two planes in my life the one I trained in and the one I own. On both planes mag toggles were labeled  lanes.I am the victim of somebody’s miss labeling

Does switching the ignition electrics at the 6pin connector and the 400 drop doesn’t change reaffirm that mag A is not stuck?

I did not know if you had a keyed ACS-type ignition switch, which without the SB applied, or diode installed across the master and starter relays, is prone to internal contacts fouling;  or the dual toggles setup you have.  Asking why is good. That’s why. Aside from that, in my experience if an engine has carburetors folks tend to gravitate to them as the source of their problem when actually if there’s a problem it’s usually electrical, including grounds. You’re fortunate you have access to new carbs as a parts swap troubleshooting methodology, that should help immensely. If that fails to correct your issue, check that ignition switch, as toggle ground switches aside, your ignition switch may do more than start the engine and provide power to accessories if wired according to the Rotax installation manual. It routes power to the A and B ignition modules.