0-320 H2AD mag issues?

david0tey

david0tey

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Aircraft is a 1977 172N with the H2AD engine that is past TBO.

During the runup about a week ago, I get a very rough running left mag with about a 250 RPM drop. Conveniently, I had already decided not to fly due to runway conditions but I wanted to run the engine anyway because it had been sitting for longer than usual. Because of this, I assume its a fouled plug and don't spend too much time trying to burn it off.

Fast forward a few days to when I am actually planning on flying. I get the same outcome after my initial runup. I run it at peak EGT and 2000 RPM for close to 3 minutes and try again, same thing. I continued running it hot for an additional minute and it clears up. So I did a couple laps around the pattern and went back to check the mags a third time. Now, its the right mag :dunno:.

We have had this particular airplane for about 6 months and I have put roughly 40 hours on it since that time. Never have I had any issues with fouled plugs. It just seems odd that it would seem to be persistent all of the sudden. I had a mechanic sit in with me today and couldn't recreate the problem.

Any ideas?
 
If it clears up, it's not the mag. Pull the plugs and take a look.
 
When was the last time anyone serviced the magneto? It seems like the dual magnetos need more attention than the individual magnetos.

If there is any question about how the engine is running I'd pull the magneto off and check the points gap.
 
mondtster said:
When was the last time anyone serviced the magneto? It seems like the dual magnetos need more attention than the individual magnetos.

If there is any question about how the engine is running I'd pull the magneto off and check the points gap.
Click to expand...

We don't check points gaps. We check E-gap, the angle of the rotor's rotation beyond the neutral point when the points open. Points gap checking is really inaccurate due to erosion and pitting/mounding of the points.

Condensers fail. Distributor parts fail. Coils fail. Spark plug leads fail. Lots of places the ignition system can make trouble. Even a P-lead chafing on something until it shorts can cause gremlins.

Those dual mags are in trouble parts-wise. When TCM bought Bendix they discontinued the dual mag, since it only fits Lycomings. I think someone--maybe Tempest--is going to make parts for them.

Another weak spot with that mag is its impulse spring. If that spring corrodes it can break, and if it breaks, the timing of both magnetos get so retarded that the power output of the engine is effectively zero.
 
Dan Thomas said:
We don't check points gaps. We check E-gap, the angle of the rotor's rotation beyond the neutral point when the points open. Points gap checking is really inaccurate due to erosion and pitting/mounding of the points.
Click to expand...

You're right. Incorrect wording on my part. What I mean is that I've seen multiple Bendix dual magnetos with the same problem, which manifests itself similar to what the OP is describing. The engine just doesn't run as smooth and when you open the magneto up to check it out you'll find that the points barely open at all and they don't fall within the gap speced out in the service manual when checked.

Once the internal timing is set everything goes back to normal and the engine will run smooth again.
 
Dan Thomas said:
We don't check points gaps. We check E-gap, the angle of the rotor's rotation beyond the neutral point when the points open. Points gap checking is really inaccurate due to erosion and pitting/mounding of the points.

Condensers fail. Distributor parts fail. Coils fail. Spark plug leads fail. Lots of places the ignition system can make trouble. Even a P-lead chafing on something until it shorts can cause gremlins.

Those dual mags are in trouble parts-wise. When TCM bought Bendix they discontinued the dual mag, since it only fits Lycomings. I think someone--maybe Tempest--is going to make parts for them.

Another weak spot with that mag is its impulse spring. If that spring corrodes it can break, and if it breaks, the timing of both magnetos get so retarded that the power output of the engine is effectively zero.
Click to expand...
Kelly (or whoever they are this week) makes blocks, coils, gears, bearings, felts, condensers, impulse springs and contacts. Only things missing are the coil retainer springs and nuts for the condensers.

The most current problem with the duals is the new blocks. The brass inserts that hold the contacts will start to pull out of the block if you are even close to the recommended torque, which causes the contacts to eventually slip. If the screws are under torqued, the contacts will still slip. Can't win. The mag starts to run rough on one side then that side fails. I've never heard of it clearing up, as the heat from running usually makes it worse, and it doesn't happen from sitting. It happens more often with a shower of sparks system due to the double contact on the left having more leverage and getting whacked twice as often, which manifests as a hard starting problem.

The other problem is availability of impulse couplings. Let one rust from sitting and it could be months to find a replacement.
 
tyndall said:
Kelly (or whoever they are this week) makes blocks, coils, gears, bearings, felts, condensers, impulse springs and contacts. Only things missing are the coil retainer springs and nuts for the condensers.

The most current problem with the duals is the new blocks. The brass inserts that hold the contacts will start to pull out of the block if you are even close to the recommended torque, which causes the contacts to eventually slip. If the screws are under torqued, the contacts will still slip. Can't win. The mag starts to run rough on one side then that side fails. I've never heard of it clearing up, as the heat from running usually makes it worse, and it doesn't happen from sitting. It happens more often with a shower of sparks system due to the double contact on the left having more leverage and getting whacked twice as often, which manifests as a hard starting problem.

The other problem is availability of impulse couplings. Let one rust from sitting and it could be months to find a replacement.
Click to expand...

Sounds like Kelly is making the stuff from nylon or some other thermoplastic instead of the old Bakelite (a thermosetting plastic). Typical. Should be an AD out on that any day if it's true.

There are numerous ADs on those mags, too. The one on the impulse couplings is often overlooked. Some flyweight pivot pins can come out and pieces fly everywhere in the accessory case and lunch the whole engine.

http://www.airweb.faa.gov/Regulator...fbdc286e86257020005a1cb9/$FILE/2005-12-06.pdf
 
The school plane I was flying had that same setup. We had a constant pop at 1000rpm. Sent it off to Aero Accessories in Van Nuys for a rebuild. About $800 from what I remember.

Of course, the CFI/A&P/owner/IA didn't tighten the lower nut when he reinstalled it and the timing jumped 28 degrees on the first flight, which was my fourth solo and first out of the pattern, but hey, I needed forced landing practice anyhow.


Sent from my iPhone using Tapatalk
 
Same issue again today. Almost sounds as if its skipping somewhere. I'm thinking its a mag timing issue but its still odd that it is happening with both mags. Could someone who's 'in the know' explain to me what makes the H2AD setup different in that regard? I hear that there is a single point failure on the mags with this engine model.
 
david0tey said:
Same issue again today. Almost sounds as if its skipping somewhere. I'm thinking its a mag timing issue but its still odd that it is happening with both mags. Could someone who's 'in the know' explain to me what makes the H2AD setup different in that regard? I hear that there is a single point failure on the mags with this engine model.
Click to expand...

That engine has two magnetos in a single housing, driven by one impulse coupling. The single failure point is that coupling.

387488832_Q4xu2-S.jpg


The coupling is on this end.

10-682555-14R%20(2).jpg


This view shows the two mags in one housing.
 
Yup, the beech duchess used the same setup. That plane is a rugged bastard but getting hard to find parts for them.

Sent from my phone thingy using the Tapatalk2 doohickey
 
david0tey said:
Aircraft is a 1977 172N with the H2AD engine that is past TBO.

During the runup about a week ago, I get a very rough running left mag with about a 250 RPM drop. Conveniently, I had already decided not to fly due to runway conditions but I wanted to run the engine anyway because it had been sitting for longer than usual. Because of this, I assume its a fouled plug and don't spend too much time trying to burn it off.

Fast forward a few days to when I am actually planning on flying. I get the same outcome after my initial runup. I run it at peak EGT and 2000 RPM for close to 3 minutes and try again, same thing. I continued running it hot for an additional minute and it clears up. So I did a couple laps around the pattern and went back to check the mags a third time. Now, its the right mag :dunno:.

We have had this particular airplane for about 6 months and I have put roughly 40 hours on it since that time. Never have I had any issues with fouled plugs. It just seems odd that it would seem to be persistent all of the sudden. I had a mechanic sit in with me today and couldn't recreate the problem.

Any ideas?
Click to expand...
Engines talk to you, Does this one have to send you an e-mail?

Maybe not always reserved, but always in your face. :)
 
Dan Thomas said:
We don't check points gaps. We check E-gap, the angle of the rotor's rotation beyond the neutral point when the points open. Points gap checking is really inaccurate due to erosion and pitting/mounding of the points.

Condensers fail. Distributor parts fail. Coils fail. Spark plug leads fail. Lots of places the ignition system can make trouble. Even a P-lead chafing on something until it shorts can cause gremlins.

Those dual mags are in trouble parts-wise. When TCM bought Bendix they discontinued the dual mag, since it only fits Lycomings. I think someone--maybe Tempest--is going to make parts for them.

Another weak spot with that mag is its impulse spring. If that spring corrodes it can break, and if it breaks, the timing of both magnetos get so retarded that the power output of the engine is effectively zero.
Click to expand...

Implulse coupling spring is not in play during a mag check, thus can not cause a weak start, or a wide spread in timing.

none of the other problems you mention will cause a problem to be intermittent.

Engine at or over TBO.. I'd bet the mag is also.
 
Tom-D said:
Implulse coupling spring is not in play during a mag check, thus can not cause a weak start, or a wide spread in timing.

none of the other problems you mention will cause a problem to be intermittent.

Engine at or over TBO.. I'd bet the mag is also.
Click to expand...

If the spring is broke the timing can't wonder from near 0 BTDC to 28 BTDC?
 
A broken impulse spring would not cause a weak start, but it would keep the timing at the badly retarded position used for start. You wouldn't get off the ground, or if it broke in flight, you'd be on the ground real soon.
 
bnt83 said:
If the spring is broke the timing can't wonder from near 0 BTDC to 28 BTDC?
Click to expand...

Read the first post, then asks yourself when was the last time you did a mag check during start.
 
Dan Thomas said:
A broken impulse spring would not cause a weak start, but it would keep the timing at the badly retarded position used for start. You wouldn't get off the ground, or if it broke in flight, you'd be on the ground real soon.
Click to expand...

The weights in the impulse coupling will go to full advance and stay there the entire time the engine is running. It will have no effect on timing while the engine is at or above 350 RPM.
 
Tom-D said:
The weights in the impulse coupling will go to full advance and stay there the entire time the engine is running. It will have no effect on timing while the engine is at or above 350 RPM.
Click to expand...

Those aren't advance weights like in an old car's distributor. They are pawls that catch the stop pins and stop the magneto rotor while the drive keeps turning and winds up the spring while the crankshaft moves close to TDC. The drive housing trips the pawls when it's time to fire so that the spring flings the magneto's rotor forward rapidly to make a hot spark.

The pawls actually retract when the engine is running. They do nothing at all after start.
 
Tom-D said:
The weights in the impulse coupling will go to full advance and stay there the entire time the engine is running. It will have no effect on timing while the engine is at or above 350 RPM.
Click to expand...
Comedy gold, right there. :rofl: :rolleyes:

The spring keeps it at full advance. Spring breaks and it goes full retard. The only time it will advance with a broken spring is when you cut the throttle and the engine slows down quicker than the mag. Doesn't matter if it's Slick, Bendix or Eisemann, they're all the same.

Then there's the A&P's that have no clue how an impulse works, and will just re-time a mag that mysteriously jumped 20º, then screw with the carb when it starts hard and backfires when adjusting the throttle. :dunno:
 
tyndall said:
Comedy gold, right there. :rofl: :rolleyes:

The spring keeps it at full advance. Spring breaks and it goes full retard. The only time it will advance with a broken spring is when you cut the throttle and the engine slows down quicker than the mag. Doesn't matter if it's Slick, Bendix or Eisemann, they're all the same.

Then there's the A&P's that have no clue how an impulse works, and will just re-time a mag that mysteriously jumped 20º, then screw with the carb when it starts hard and backfires when adjusting the throttle. :dunno:
Click to expand...

Best study how they are driven, it is impossible for a mag to rotate different than the crank/gear drive.

The impulse coupling has no function above 350 RPM. Simply because the fly weights are holding the prawl with drawn and it an not engage the stop pin.

The function of an impulse coupling is to allow the mag to retard for starting and to enable a better spark by spinning faster as the points open to provide hotter spark.
 
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Dan Thomas said:
Those aren't advance weights like in an old car's distributor. They are pawls that catch the stop pins and stop the magneto rotor while the drive keeps turning and winds up the spring while the crankshaft moves close to TDC. The drive housing trips the pawls when it's time to fire so that the spring flings the magneto's rotor forward rapidly to make a hot spark.

The pawls actually retract when the engine is running. They do nothing at all after start.
Click to expand...

What retracts them? A. the fly weights.

It is true the impulse does nothing after start.

the basics

http://www204.pair.com/bbg46/FM Mag Manual/Mag Impulse Couplings(43-48).pdf
 
The impulse spring is working at all times to position the halves opposite of the position the pawls do at low RPM. When that spring breaks you get this:





On a dual magneto neither one would work properly with the lone only impulse spring broke.

Not the OPs problem.
 
david0tey said:
Plugs pulled. Plugs cleaned. Engine happy.
Click to expand...

We had this issue for awhile as well. We had to get the plugs cleaned every 5-10 hours due to an oil leak and unseated rings. If it keeps happening, it wouldn't hurt to have the cylinders rebuilt or have rings replaced.
 
bnt83 said:
On a dual magneto neither one would work properly with the lone only impulse spring broke.

Not the OPs problem.
Click to expand...
Even with two mags the engine will not pass a mag check, select one with a broke spring the engine will barely run if at all.
L=50 R=1000 Both 250 low.

Remember the engine will run normally (nearly so) on the mag that fires first. very few have the plugs firing simultaneously, very few mechs bother to set them that way, and several engines require timing 2 degrees apart like the 0-300/ R-28 +-1 L-26 +-1

you throw one out 20+ degrees, you will notice it very quickly.

the OP doesn't have this problem.

My best guess at the OP's problem, is water in the mag, as in condensation it gets warm, water goes away, the mag gets cold, water is back, maybe not on the same side. ( it is one case)
 
Tom-D said:
Even with two mags the engine will not pass a mag check, select one with a broke spring the engine will barely run if at all.
L=50 R=1000 Both 250 low.
Click to expand...

Not quite

Assume an O-320E2D (popular Cessna 172 and Cherokee powerplant)with Slicks and the coupling spring breaks after getting the engine started, because most likely the engine wouldn't start at all if the spring was already broke.

If the engine is running and a spring breaks that mag will fire after the good mag does. During mag check you won't have an RPM drop when you select the good mag (from BOTH) and you'll have a VERY high drop when the bad mag is selected.

The spring ONLY holds the coupling in position to fire the mag at 25 BTDC (O320E2D), when it breaks, it will work its way towards 5 BTDC (Slick p/n 4371 has a 20 degrees lag coupling, 25 minus 20 = 5 BTDC).
 
Tom-D said:
Implulse coupling spring is not in play during a mag check, thus can not cause a weak start, or a wide spread in timing.

none of the other problems you mention will cause a problem to be intermittent.

Engine at or over TBO.. I'd bet the mag is also.
Click to expand...

But, Tom,

In this thread, http://www.pilotsofamerica.com/forum/showthread.php?t=46963

You said this.

"It is what drives the mag, with out it, your brand new mag will not start the engine, and it could allow your brand new mag to run 25 degrees retarded."
 
Tom-D said:
the OP doesn't have this problem.
Click to expand...

Nobody said he did. We were discussing the shortcomings and failure points of the dual magneto, one of them being the single drive with its impulse spring that can break and essentially fail both mags at the same time.
 
Dan Thomas said:
Nobody said he did. We were discussing the shortcomings and failure points of the dual magneto, one of them being the single drive with its impulse spring that can break and essentially fail both mags at the same time.
Click to expand...

This whole thread was hijacked when some one took a guess the spring was the problem.

Things that break, won't be intermittent.

Pretty much standard procedure here.
 
Tom-D said:
This whole thread was hijacked when some one took a guess the spring was the problem.
Click to expand...
Just stop already. You're just embarrasing yourself further.

Your understanding of the flow of this thread is almost as poor as your understanding of impulse coupling failures. Nobody said the spring was the problem, only that it is one of the mags weak points. And yes, a broken spring can cause an intermittent problem in one specific case.
 
tyndall said:
Just stop already. You're just embarrasing yourself further.

Your understanding of the flow of this thread is almost as poor as your understanding of impulse coupling failures. Nobody said the spring was the problem, only that it is one of the mags weak points. And yes, a broken spring can cause an intermittent problem in one specific case.
Click to expand...

RE-read post 4.

''Another weak spot with that mag is its impulse spring. If that spring corrodes it can break, and if it breaks, the timing of both magnetos get so retarded that the power output of the engine is effectively zero.""


So give it up. this is a normal thing here. While I'm responding to one point you are responding to another post, and the whole problem of the OP is lost, because you missed the whole point of "this isn't the OPS problem". because you are off on a tangent to the question trying to rationalizing the spring
 
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tyndall said:
And yes, a broken spring can cause an intermittent problem in one specific case.
Click to expand...
there ain't noway in hell a broken spring can fix itself, and be intermittent.
 
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