Dual mag Engine won’t start

[Martin S.]

Evening everybody,

any suggestions what could cause an A3b6d with dual mags not to start, or only after long trying. Engine didn’t run before so no hot start.never saw this behavior before.

Any ideas how to trouble shoot the ignition?

Greets and many thanks,

 

Martin

Edited August 12, 2023 by Martin S.

[EricJ]

Troubleshooting a dual mag is essentially the same as separate mags.    You can remove a spark plug and appropriately lay it on a cylinder to verify that it is sparking, or just check the timing to make sure that the points are opening correctly.   First step might be to make sure the p-leads aren't grounded.

Are you sure it's getting fuel to the cylinders?

[PT20J]

With the ignition off, you might slowly rotate the prop and listen for the click to make sure the impulse coupling is working. 

[A64Pilot]

If your ignition is a little weak, if you over prime or just flood it, it won’t start. The wet plugs won’t fire

[N201MKTurbo]

Your impulse coupling could be bad, do you hear it snapping, 

I think some ignition switches will only start on one mag. That mag could be bad. The best way to check is with a mag timer.

When it does start, do both mags work?

If one mag is bad, and it only starts on one mag, you can sometimes get it to start by releasing the key while the engine is spinning and letting it snap back to both.

[PT20J]

13 hours ago, N201MKTurbo said:

Your impulse coupling could be bad, do you hear it snapping, 

I think some ignition switches will only start on one mag. That mag could be bad. The best way to check is with a mag timer.

When it does start, do both mags work?

If one mag is bad, and it only starts on one mag, you can sometimes get it to start by releasing the key while the engine is spinning and letting it snap back to both.

In two mag installations, a jumper on the ignition switch grounds out the right mag during starting so that only the left mag fires because the left mag has the impulse coupling and is retarded for starting. The dual mag impulse coupling will operate both left and right mags, so Mooney SIM20-59A allows removal of the jumper so that both mags can fire during starting. But, on a dual mag, if the impulse coupling is bad, neither mag will fire.

[A64Pilot]

As a a side note I never have understood the logic of only one Mag having an impulse coupling, to save money is the only reason I can come up with.

Of course a dual mag there isn’t any other choice, I can hear my impulse coupling snapping on start. I don’t listen for it, but think if it wasn’t snapping I’d notice that.

 

[skykrawler]

10 minutes ago, A64Pilot said:

As a a side note I never have understood the logic of only one Mag having an impulse coupling, to save money is the only reason I can come up with.

Of course a dual mag there isn’t any other choice, I can hear my impulse coupling snapping on start. I don’t listen for it, but think if it wasn’t snapping I’d notice that.

 

Strictly speaking, a second impulse coupler is another point of failure.   Two plugs/magneto are for 1) redundancy 2) higher performance.  If one's engine starts poorly because the impulse coupled mag is not performing well, it should probably be inspected/iran'd.

For the this dual mag,  the OP implies that this is "change in behavior."  Its possible its the mag - also possible to be fouled plugs.   Clean the plugs, check the timing.  Check the fuel - load of water? 

 

[A64Pilot]

My 1946 C-85 engine’s two magnetos are identical, both have impulse couplings.

If that was any kind of safety issue one supposes that at sometime in the past 75 years one would have been removed, as the old C-140 doesn’t have a key, it has independent mag switches you can start on one mag, I’ve tried out of curiosity and it starts better on two, better meaning quicker.

Yeah I know the party line has been forever that dual mags are for redundancy, but that’s not really the case or not the whole truth anyway, if it was just redundancy then it would run just as good on one as it does on two, but it doesn’t, you make significantly less power on one mag than two.

Top fuel and funny cars run dual mags, not for redundancy, but because they make more power that way.

 

On this case I’m sort of hanging my hat on a flooded engine with wet plugs, the more fouled they are, the easier it is to happen, and massives are more prone than fine wires. I had it happen once, right on the shore of the Arctic Ocean in a town called Tuktoyoktuk, with no place to stay, but I had a really good battery and after it sat a couple of hours it started, that had a lot to do with me going to fine wires

 

[M20F]

18 hours ago, EricJ said:

Troubleshooting a dual mag is essentially the same as separate mags.    You can remove a spark plug and appropriately lay it on a cylinder to verify that it is sparking,

Get an in-line plug tester, cheap, predictable, and easy to see the spark.  Grounding the plug is iffy at best and trying to see the spark outside of total darkness is a bitch. 

[N201MKTurbo]

1 hour ago, M20F said:

Get an in-line plug tester, cheap, predictable, and easy to see the spark.  Grounding the plug is iffy at best and trying to see the spark outside of total darkness is a bitch. 

I usually prop the plug up on the fuel injection line to ground it even though it should be grounded through the lead. I’ve never found the spark hard to see. That’s with the top plugs removed and the bottom leads removed. If you don’t remove the bottom leads, there may be enough residue fuel in the cylinders to ignite. So, you are pulling the prop through looking at the end of the spark plug and waiting for that tiny little snap of a spark when BAM! It sounds like someone just fired a gun next to your head. I now not only remove the bottom leads, but I blow compressed air into all the cylinders to blow out any fuel vapor.

[skykrawler]

All leads removed should be placed to ground.  Not doing so can potentially damage the secondary coils which will find a path to ground through the coil insulation.