Which mag fires top/bottom plugs?

airguy

airguy

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airguy
I've got a fouled plug on my right mag on my #1 cylinder, Cessna 172 with Lyc IO360. The fouled plug does not scare me much (simple cleaning/replacement) but I have this question - the cylinders have top/bottom plugs, and the mags are listed as left/right. Which mag generally goes to which plugs?
 
The answer is "both." In most cases, one mag fires the plugs on the top on one side and the bottom on the other, and the other fires the other plugs. So, each mag fires half the top plugs and half the bottom plugs.
 
Ron is correct. The top and bottom plugs have slightly different characteristics typically, so by switching that evens it out.

Figure out which mag it's attached to (left/right) and then trace the harness lead. You'll find your plug. Also keep in mind you can sometimes clean them out on the ground depending on how badly fouled they are.

Typically, a fouled plug isn't something to worry about. But if it happens frequently, then there is probably an underlying issue to look into. I recently had to change the plugs on the right engine of the 310 - I was carbon fouling one about every 10 hours of flight time, and I run that plane quite lean. The plugs were just worn out and needed to be replaced. New set, and the engine is happy as can be.
 
Lycoming: Right mag fires top left plugs (cyls. 2 & 4) and bottom RH plugs (1 & 3). Left mag fires top RH and bottom LH.

Small Continentals: RH mag fires all the top plugs, LH all the bottom. The impulse mag is RH and the top plugs don't suffer oil fouling like the bottom plugs, so it makes sense to connect things that way.

Dan
 
Ron Levy said:
The answer is "both." In most cases, one mag fires the plugs on the top on one side and the bottom on the other, and the other fires the other plugs. So, each mag fires half the top plugs and half the bottom plugs.
Click to expand...

One notable exception to that general rule is the Cont O-300. One mag fires all the tops, the other mag all the bottoms.

I put a 172 into a bridge because some fool engineer didn't follow the standard convention.

Jim
 
When I have a fouled plug, I do a lean of peak mag check to see if the plug is fouled on the left or right mag and on which side of that mag. Then, I go to the mag and follow the wire to the plug.

Best,

Dave
 
airguy said:
I've got a fouled plug on my right mag on my #1 cylinder, Cessna 172 with Lyc IO360. The fouled plug does not scare me much (simple cleaning/replacement) but I have this question - the cylinders have top/bottom plugs, and the mags are listed as left/right. Which mag generally goes to which plugs?
Click to expand...

The overhaul manual gives no specific diagram for the IO-360 it gives a typical diagram for 4 cylinder engines but leaves the option up to the harness manufacturer.

follow your cables to be sure.
 
The plug still fires, but fires weak, I'm wondering if I migh have a wire problem. Ground runs when rich show just a slight roughness with fine RPM drop, lean ground run is a little more aggravated. In flight WOTLOP at 7500 it is quite evident from the engine monitor that the #1 R plug is not happy, and has not cleaned up after high power lean running. Looks like I'm gonna pull the plugs for cleaning and then repeat to check for further issues.

I suspect lead ball fouling from a steady diet of 100LL and a partner that won't lean for taxi, we'll find out!
 
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airguy said:
The plug still fires, but fires weak, I'm wondering if I migh have a wire problem. Ground runs when rich show just a slight roughness with fine RPM drop, lean ground run is a little more aggravated. In flight WOTLOP at 7500 it is quite evident from the engine monitor that the #1 R plug is not happy, and has not cleaned up after high power lean running. Looks like I'm gonna pull the plugs for cleaning and then repeat to check for further issues.

I suspect lead ball fouling from a steady diet of 100LL and a partner that won't lean for taxi, we'll find out!
Click to expand...


Sounds to me, like the resistor in the plug is breaking down. have you dropped it ?
 
Tom-D said:
Sounds to me, like the resistor in the plug is breaking down. have you dropped it ?
Click to expand...

Was thinking that, too. New plug will fix the problem.
 
Tom-D said:
Sounds to me, like the resistor in the plug is breaking down. have you dropped it ?
Click to expand...

Nope, they haven't been touched since replacement at the annual in August of last year, though I can't say for certain what type of treatment they saw at the shop that did the work. A replacement plug is cheap peace of mind and easy to do.
 
airguy said:
Nope, they haven't been touched since replacement at the annual in August of last year, though I can't say for certain what type of treatment they saw at the shop that did the work. A replacement plug is cheap peace of mind and easy to do.
Click to expand...

It would be nice to know if that fixes the problem.
 
I pulled the cowling Wednesday afternoon, looked over the mag and plug wires very carefully, they looked fine (no evidence of damage or arcing). I pulled the top and bottom plugs on the #1 cylinder, the top plug was in very good shape and went right back into the cylinder. The bottom was in pretty bad shape, with a few "lead balls" and a lot of oxide deposits packed around the insulation, it went into the "round file" and a new Champion REM38E went in the bottom hole. Buttoned it up and it's purring like a kitten, life is good.
 
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