Mags and plugs

[BDPetersen]

Not a Mooney, but maybe interesting for general knowledge. 

While troubleshooting hot start issue with my son’s Extra 300 we discovered the mags had been timed to 25 instead of 20 per the data plate. Waiting to see if that helped, meanwhile was interested to learn that both mags had impulse couplings. I guess that’s a thing with fuel injection as opposed to the left mag only on the carbureted engines I relate to.? But even more interesting was to discover each mag fires both top and bottom plugs on its side of the engine, left serving even # cylinders, right doing the odd ones. I’ve never seen that and I’m not sure how to confirm it. I suppose the Lycoming manual may address that. Any thoughts?

[Hank]

The firing order is weird. When I lost a mag in flight, I'm really glad that all four cylinders were still running, not just two of them. Only lost a little speed, reached my destination and called the local A&P.

[EricJ]

58 minutes ago, BDPetersen said:

Not a Mooney, but maybe interesting for general knowledge. 

While troubleshooting hot start issue with my son’s Extra 300 we discovered the mags had been timed to 25 instead of 20 per the data plate. Waiting to see if that helped, meanwhile was interested to learn that both mags had impulse couplings. I guess that’s a thing with fuel injection as opposed to the left mag only on the carbureted engines I relate to.?

Some do that on experimentals just to get better starts.   It's not a bad idea other than adding a little complexity.  I don't think it has much to do with whether it's fuel injected or carbureted.

58 minutes ago, BDPetersen said:

But even more interesting was to discover each mag fires both top and bottom plugs on its side of the engine, left serving even # cylinders, right doing the odd ones. I’ve never seen that and I’m not sure how to confirm it. I suppose the Lycoming manual may address that. Any thoughts?

That sounds very unusual.   It's fairly common to have one mag fire the top on one side and the bottom on the other, but, as Hank mentioned, you'd lose redundancy if a mag fired only one side (which sounds like it'd take a special mag, anyway).

[N201MKTurbo]

1 hour ago, BDPetersen said:

Not a Mooney, but maybe interesting for general knowledge. 

While troubleshooting hot start issue with my son’s Extra 300 we discovered the mags had been timed to 25 instead of 20 per the data plate. Waiting to see if that helped, meanwhile was interested to learn that both mags had impulse couplings. I guess that’s a thing with fuel injection as opposed to the left mag only on the carbureted engines I relate to.? But even more interesting was to discover each mag fires both top and bottom plugs on its side of the engine, left serving even # cylinders, right doing the odd ones. I’ve never seen that and I’m not sure how to confirm it. I suppose the Lycoming manual may address that. Any thoughts?

Are you sure the harness was installed correctly? 

[BDPetersen]

Ok, this was a ridiculous post and I apologize. It’s obvious I misread the plug wires or it would not have run on one mag (3 cylinders). Even with my newly cataract free eyes, I can make mistakes. 
Good news is we had 2 successful hot starts.

I’m reminded of a captain I flew with once who made a lengthy p/a to the passengers but mistakenly picked up the wrong mic and broadcast it on comm one. Before he finished he realized what he was doing and continued by saying to his audience of airline pilots “ I’m just going to hang up now and take off my headset, so make all the comments you want, I’m not listening “

Ya’all have a great day!

[Ragsf15e]

3 hours ago, BDPetersen said:

Ok, this was a ridiculous post and I apologize. It’s obvious I misread the plug wires or it would not have run on one mag (3 cylinders). Even with my newly cataract free eyes, I can make mistakes. 
Good news is we had 2 successful hot starts.

I’m reminded of a captain I flew with once who made a lengthy p/a to the passengers but mistakenly picked up the wrong mic and broadcast it on comm one. Before he finished he realized what he was doing and continued by saying to his audience of airline pilots “ I’m just going to hang up now and take off my headset, so make all the comments you want, I’m not listening “

Ya’all have a great day!

I was surprised to learn that my m20k has two impulse mags and is wired to start on both mags.  My F model with an io-360 was injected as well but only used one mag for starting.  Your whole post wasn’t crazy, just a tiny bit! 

[N201MKTurbo]

The ‘56 172 I used to own had two impulse mags. It also had the mags wired kind of odd. One mag did all six top plugs and the other did all the bottoms. The timing was different for the two mags. One was at 26 and the other was at 28.

[1980Mooney]

3 hours ago, N201MKTurbo said:

The ‘56 172 I used to own had two impulse mags. It also had the mags wired kind of odd. One mag did all six top plugs and the other did all the bottoms. The timing was different for the two mags. One was at 26 and the other was at 28.

"One was at 26 and the other was at 28."  Maybe for longer spark/burn duration?

[N201MKTurbo]

25 minutes ago, 1980Mooney said:

"One was at 26 and the other was at 28."  Maybe for longer spark/burn duration?

I think the intake and exhaust ports were at the bottom of the cylinders. I think they fired the bottoms first and then fired the tops when the flame front got to the top.

That 0-300 was high tech. It as designed in 1938.

[CCAS]

5 hours ago, N201MKTurbo said:

The ‘56 172 I used to own had two impulse mags. It also had the mags wired kind of odd. One mag did all six top plugs and the other did all the bottoms. The timing was different for the two mags. One was at 26 and the other was at 28.

Seems to be a good example of staggered timing. The mag with the 28 deg BTDC setting should have correlated to the spark plugs that were closest to the exhaust valves. Goal would be for that combustion event to occur slightly before the opposite set of spark plugs (firing at 26 deg BTDC) so that the two flame fronts meet at the center of the combustion chamber and produce the most effective power stroke.  

[Pinecone]

On 9/12/2025 at 7:03 PM, N201MKTurbo said:

The ‘56 172 I used to own had two impulse mags. It also had the mags wired kind of odd. One mag did all six top plugs and the other did all the bottoms. The timing was different for the two mags. One was at 26 and the other was at 28.

That setup is not uncommon with one electronic ignition. 

Just installed a Surefly on my 252.  We put it on the right side.  The left is the original Slick with impulse coupling.  It sure starts smartly.

I plan on replacing the harness with a Maggies and will have the SF fire bottom plugs and the mags the top ones.