912 excessive RPM drop on lane A

Paul,

As long as you are looking for things to check...

And assuming you have the latest ignition electronics with Two 6-pin connectors.

On the 6-Pin connector that feeds to the Ignition Coils, there is a Violet Wire.

This is the soft-start signal that delays the spark timing while cranking.

Both Violet wires are supposed to be connected to the Non-Grounded (+12v) side of the Starter Solenoid.

During starting the Violet wire will have 12 volts on it.

Once the starter is released this wire will be grounded through the Solenoid and show Zero volts.

If one of these wires is Broken, Disconnected, or otherwise Floating, all bets are Off!

The Ignition Module could switch back and forth from start to Run mode at random.

If you can do it, make sure the violet wires are grounded (0.0v or <1.0v at most ) when the engine is running and you are seeing the rpm loss.

Ref.

0v = 26° BTDC Normal Timing.

12v = 4° BTDC Delayed Timing for easy Starting.

 I think you've got it!!!

Swapping the 6-pin connector swaps the ignition coils and the spark plugs.

Swapping the 4 pin connector swaps the trigger coil.

Swapping the 1-pin connectors swaps the Mag switches.

- - -

So now we know it is NOT the Ignition (Spark) coils or the spark plugs.

Swapping the 4-pin connectors will eliminate the Trigger coils if the problem stays on "A".

Finally swapping the 1-Pin connectors should make it swap sides unless there is something magical about the switches. (Extremely unlikely but do it just to cover all remote possibilities.)

If that is the only time the problem moves that makes the Electronics module suspect number one!

- - -

I have been thinking ... I have been told this is dangerous!  (My thinking that is!)

Getting access to the stator coil to Point a timing light at them is not an easy thing to do and pointing the light at the propeller will not be informative because the prop and the crankshaft only resync once every 21 revolutions of the prop.  But there is hope.

If you were to attach Two Timing lights to the engine, one to an upper plug and one to a lower plug on a single cylinder, we would expect the strobes to be firing simultaneously.

The strobed prop is going to be jumping around but you should only be seeing One prop at any location.

One degree of crankshaft rotation is approximately 1/4 of an inch of tip rotation on a 65-70" prop.

So, as long as the plugs are firing within one degree of each other you should see the strobed blades within 1/4 inch of each other.

If one spark is 22° delayed, the two blades would show an offset of 5-6 inches; about the full width of a blade.

Now that's what I call thinking out of the box!

I hope that at least sounded like English!!!  

Bill,get some sleep.That was brilliant. I know a guy that can break that down for me I’ll have him read your post while I recover from pilot overload. I received two new carbs in the mail yesterday. I’ll put them on so I can eliminate if it’s a carb problem. If it is not the problem and you suspect it’s the modules then I’m going to buy a brand new module and try that.

I thought switching the 6 pin connectors also switched the A B modules 

There goes that English thing again!  

Let me rephrase it.

Swapping the 6-pin connector swaps the Ignition Modules with the Ignition Coils and Spark Plugs. (Coils and Plugs as a group)

Swapping the 4 pin connector swaps the Ignition Modules with the trigger coils.

Swapping the 1-pin connectors swaps the Ignition Modules with the Mag switches.