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Is it possible for a magneto to fire earlier than it is set for?

edbundy

Member
I've been chasing a high CHT (on all cylinders) issue for a while now. I've replaced the baffling, replaced intake tube gaskets, verified engine to mag timing, verified sufficient fuel flow, and everything else I can think of.

During climb all 4 CHT's will rise at roughly 1 degree/second and unless I aggressively get the airspeed up they will easily pass 400F. They never used to go above 370 in a climb.

The other day I thought to try isolating the mags during a climb. At about 1000' agl with both mags operating the CHT's were climbing rapidly. Switching to just the right mag didn't change anything. Switching to just the left mag not only immediately stopped the CHT's from climbing, but they then started going down fairly quickly. I switched back and forth several times and each time the right mag would make the temps climb and the left mag would bring them down.

It sure seems to me that the right mag has figured out a way to advance itself, but is it possible for a mag to fire earlier than the 25 degrees it is set for? These mags are absolutely set at 25 degrees, run up and mag drops are normal, starting is normal and the engine runs great just like always.
 
What type of fuel are you burning?
What CR pistons?
What kind of mags?
What are EGTs doing during this?
Etc.

The move info, the better.

Despite your last paragraph, it certainly sounds as if one mag is timed to the impulse/start/retard point coupling. Let us know.
 
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100LL exclusively.
0320 with standard compression.
Slick mags, left has impulse coupling.
EGT's normal when operating both mags, shows expected rise on individual mags.
Other than CHT's engine is operating normally.

I've triple checked the timing, they are both 25 degrees. If one were operating at a massively retarded setting wouldn't that show up when switching to that mag? And wouldn't that cause CHT's to go down?
 
certainly sounds like right mag is too far advanced. Suggest re-timing it. If it continues to be advanced, put an auto timing light on the rt #1 plug and confirm.' About the only way that a mag can readjust it's timing is for the hold down nuts to be too loose or improper timing procedures, such as not letting the impulse snap first or various other errors.

Larry
 
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@Larry. I’m not a mag guy. I can’t think of another scenario. I would assume the OP is at strong risk of a kick-back and starter damage if I’m picturing this correctly
 
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[QUOTE...I would assume the OP is at strong risk of a kick-back and starter damage ...[/QUOTE]

+1 Be very careful with this. The in flight mag check is a good idea.

I don't think I saw here when the last mag inspections were done. If I was anywhere close to the time, I'd pull these off and do the 500 hr inspection.

Dan
 
@Larry. I’m not a mag guy. I can’t think of another scenario. I would assume the OP is at strong risk of a kick-back and starter damage if I’m picturing this correctly

As the cam and points wear, the timing becomes more advanced. However, this takes a good amount of time, like 100's of hours in most cases, though slick has had QC problems with cams wearing out VERY early. OP implied this was new issue and still occurred after re-timing the mag. Re-timing the mag will adjust the ignition timing, regardless of point wear, but doesn't adjust for e gap timing, which only affects spark energy and not ignition timing. If I misunderstood that and the mag hasn't been touched in 100's of hours, wear could be the cause and in that case the e gap will need attention, as misfires will be in the OP's near future.

Kick back wouldn't apply if the OP starts on only the left mag, like many/most do.

Larry
 
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another step

One other step I would make is to verify the orientation of the P-leads.
Does the mag switch(es) actually turn off the Left mag, when the switch shows running on Right mag only? Or are they reversed? P-leads get reversed too often. It could be significant, if the Left mag is actually the problem, as a bad impulse coupling could cause timing drift.
And, has been mentioned already, if the mags have more than 500 hours, or there are a couple of outstanding SBs, then it is time to get them into the shop. One SB deals with bad rotors, which could be a factor....
 
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Lots of good ideas here.

I'm sure it is the right (non-impulse) mag. It would make more sense if it *were* the left. I only start on the left mag. Both mags drop 60rpm on runup mag check.

Both mags are definitely overdue for service at 700 hours. I'm going to replace the left mag with a Surefly. I plan on keeping a Slick on the right side, but that brings up another, probably more salient point - the mags are the originals from when the engine was new, purchased when I built the plane 25 years ago. (I know, I don't fly enough) I'm thinking rather than 500 hour the right mag perhaps I should just buy a new one?
 
I have seen and dealt with cylinders with excessive lead deposit formation that would cause CHT's to skyrocket as you describe. As the cylinder pressures come up the deposits caused preignition. Fix was to remove cylinders, hone, new rings, and clean pistons up. Problem went away thereafter.
 
Following with similar issue. Cylinders 2,3,4. C1 runs cool.

Did a mag check at 5000 ft a few days ago.. similar outcome - when running on R only (Electronic Ignition in my case), temperature went down considerably. Actually first couple of times I switched to R only - heard a 'flutter' that wasn't sure what the cause was. RPMs did seem to increase when turning L back on.

Magneto has 650 hours since new (2006). Planning to have it rebuilt soon. Slick impulse 4371

Also - Leaning seems to help bring 3,4 temps down but raises #2. At cruise, leaning can get all cylinders to be closely matched (but high - near 390)

The points in the mags do wear and when this happens, the timing advances. Owners should be checking timing at each CI if they have mags. If the timing has changed appreciably from the last CI (indicating a strong likelihood that the points have worn), they should be re-adjusting the E gap inside the mag.

Larry
 
Are you sure the mags are timed correctly? Did you turn the engine and allow the left mag to snap the impulse coupling, then back it up and set the timing? Otherwise, if you were to set the timing without snapping the impulse coupling and setting both to 25, the leftover e would actually be 25 degrees retarded from the right side, or the right being 25 degrees advanced from the left, depending on your perspective.
 
Mags with 650/750 hrs and never been serviced, geez....
Ya gotta wonder what other maintenance has been neglected.

My 2c, take care of your stuff, have someone else take care of it, or get rid of it before something bad happens and the rest of us have our insurance go up.
 
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I was having starting issues on my O-320 with 320 hours since overhaul. Suspected the problem was magnetos so took them in for overhaul. It turned out that the points in both magnetos were out of adjustment and the impulse coupling was firing the left magneto at 13 degrees after TDC. All internal components were good so had a cheap 500 hour overhaul on both and it fixed the starting issue. I was puzzled by the starting issue as the engine appeared to start when I moved the start switch from start to both. The hugely advanced impulse was not starting the engine and going to both retarded the start to something more suitable to start using the right magneto. So lesson learnt was not to rely on the mags lasting to 500 hours without problems. One last thing was that both magnetos were timed correctly at 25 degrees BTDC so all looked good on the surface.
 
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SLICK Service Bulletin SB1-15A

The Sb above relates to rotor arms moving around inside magneto which changes the spark advance without being affected by mag points or installed mag to engine timing. I bet you have a rotor with a loose arm!
FYI - 500 hr strongly recommended service interval.
PS - Your phone number in the White pages is dead
Steve
 
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The Sb above relates to rotor arms moving around inside magneto which changes the spark advance without being affected by mag points or installed mag to engine timing. I bet you have a rotor with a loose arm!
FYI - 500 hr strongly recommended service interval.
PS - Your phone number in the White pages is dead
Steve

The loose rotor arms can cause various issues, but advancing the timing is not one of them.
 
I politely however vigorously disagree with your comment and urge EdBundy along with any others curious readers to check with Champion Slick 1st hand. The loose arms can & DO move internally relative to the towers of the distributor cap producing both advance & retard conditions based on where the flailing arm is momentarily stuck at.
 
I politely however vigorously disagree with your comment and urge EdBundy along with any others curious readers to check with Champion Slick 1st hand. The loose arms can & DO move internally relative to the towers of the distributor cap producing both advance & retard conditions based on where the flailing arm is momentarily stuck at.

Hmm, the spark is generated when the points open, which has nothing to do with where the rotor is pointing, so I don't see how an actual timing change would be possible. The rotor may be leading or lagging or even on the wrong tower, but the spark is still generated at point opening.
 
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Quote "The rotor may be leading or lagging or even on the wrong tower, but the spark is still generated at point opening."

Thanks Walt - You just described exactly HOW. As the arm moves around lots of arcing occurs, builds carbon tracking and exacerbates the issue. I will not try and be an expert or support a debate - I'm only pointing to a probable cause & trying to help the original poster - this is my last as this is the wrong place for an argument.
 
Walt is correct. Let's not confuse when the spark is generated with where it goes. For a non-impulse mag with tight base nuts, spark timing has a fixed relationship to crank position...the points cam is on the magneto's main shaft, geared to the crank.

Walk through the piston positions. Firing order is 1-3-2-4. Assume intended spark is 25 BTDC for #1, which is then on the compression stroke, valves closed.

When the points open...

#3 is 25 BBDC on the intake stroke. The intake valve will still be open for another (ballpark) 40 degrees or so.

#2 is 25 BTDC on the exhaust stroke, with the intake valve opening shortly.

#4 is 25 BBDC on the power stroke, with the exhaust valve opening.

A loose finger can easily result in misfire, i.e. failure to light #1 because the spark went elsewhere, but it would be a waste spark when delivered to any other cylinder. None of the other cylinders are ready for inflammation.
 
Timing

I've been chasing a high CHT (on all cylinders) issue for a while now. I've replaced the baffling, replaced intake tube gaskets, verified engine to mag timing, verified sufficient fuel flow, and everything else I can think of.

During climb all 4 CHT's will rise at roughly 1 degree/second and unless I aggressively get the airspeed up they will easily pass 400F. They never used to go above 370 in a climb.

The other day I thought to try isolating the mags during a climb. At about 1000' agl with both mags operating the CHT's were climbing rapidly. Switching to just the right mag didn't change anything. Switching to just the left mag not only immediately stopped the CHT's from climbing, but they then started going down fairly quickly. I switched back and forth several times and each time the right mag would make the temps climb and the left mag would bring them down.

It sure seems to me that the right mag has figured out a way to advance itself, but is it possible for a mag to fire earlier than the 25 degrees it is set for? These mags are absolutely set at 25 degrees, run up and mag drops are normal, starting is normal and the engine runs great just like always.

Normally with two properly timed mags, shutting off either mag reduces the effective timing of the combustion events and you will see a drop in CHT and an increase in EGT. In your case you are seeing a drop when switched to the left mag only and no change when switched to the right mag only. This means that either the right mag is firing well in advance of the left mag or that the P-Lead on the left mag is broken. Have you performed a "shutdown" mag check to ensure that momentarily switching both bags off together causes the engine to momentarily stop firing (don't do this in flight at high power, unless you want to hear a really loud after-fire bang!).

To aid with your trouble-shooting, you'll want to verify that your mag switch(s) is wired to the correct mags i.e. when the right mag is selected, the left mag P-lead should be grounded, and when the left mag is selected the right P-lead should be grounded. Swapped P-Leads is not uncommon.

Assuming each mag is shutting off as expected and as indicted by switch position, then I would certainly verify that the right mag is correctly timed. Do you have an impulse coupled mag(s) or retard breaker mag(s) and if so, have you verified which one(s)? As previously mentioned, if an impulse coupled mag were timed while in the retarded position (before the impulse coupling has fired), then you would see significantly advanced timing from that mag while the engine is running and this would certainly cause the high and rapidly rising CHT's that you are seeing at high power.

If your trouble-shooting has been led astray by a broken or incorrectly wired P-lead then your high EGT could be caused by something other than improper timing.

Skylor
 
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