Rough Mag Check

[isaacpr7]

  During runup I noticed the mag drop to be at 200RPM on the left side. After leaning the engine at higher RPM and allowing it to run for about a minute, the drop remains between 100-125RPM. Most people I spoke to suggested that I might have had fouled plugs so I replaced them all with Tempest UREM38E (massives). After running up with new plugs, the RPM drop was normal but the engine ran rough on the same side (left). I also noticed that when full throttle is applied the RPM does not go any higher than 2500-2600. Has anyone experienced this before that might be able to shed some light as to what might be going on?

[BigTex]

What kind of Mags do you have and how many hours do you have on them?

[John Pleisse]

rd..

[rbridges]

do you have an engine monitor to see if the temps drop in a specific cylinder when you switch to the "bad" side?  I could see a substantial drop when I switched to the mag that fired the bad plug.  If it's a plug/wire, I would think the temp drop would be localized to a cylinder.  If it's a mag, it should not be a single cylinder.

[Bob - S50]

Not a maintenance guy here and only going on what I've learned on the internet (scary).  From what I've read, a leaned engine at runnup will have larger than normal RPM drop so that alone would not bother me too much personally.  However, a difference between the left and right and a rough engine would.

 

Have you had them check the Magneto timing?

 

If the left Mag is advanced too much, it would cause peak pressure closer to TDC.  That would cause higher peak pressure, higher CHT (neither of which I think you would notice during a runnup) and a reduction in power.  The engine would be pretty unhappy too.

 

Just a thought.

 

Bob

[isaacpr7]

What kind of Mags do you have and how many hours do you have on them?

They are Bendix Slick Mags with around 600hrs on them.

[isaacpr7]

do you have an engine monitor to see if the temps drop in a specific cylinder when you switch to the "bad" side?  I could see a substantial drop when I switched to the mag that fired the bad plug.  If it's a plug/wire, I would think the temp drop would be localized to a cylinder.  If it's a mag, it should not be a single cylinder.

I still have the original engine cluster so I have no way of monitoring each individual cylinder.

[BigTex]

They are Bendix Slick Mags with around 600hrs on them.

Isaac - I'd consider having your Mag's IRAN'ed by a top notch mag shop.  At 600 hrs, you're about 100 hours beyond what is recommended.  Plan on spending around $650 per mag.

[Hank]

Isaac,

My 70 model permits up to 150 RPM drop but no more than 50 difference between left & right. Do you lean on the ground? I pull the mixture about halfway back, as long as she will taxi and run smooth.

Having the mags IRAN'ed should also help.

Good luck! Let us know what you find out.

[isaacpr7]

Isaac,

My 70 model permits up to 150 RPM drop but no more than 50 difference between left & right. Do you lean on the ground? I pull the mixture about halfway back, as long as she will taxi and run smooth.

Having the mags IRAN'ed should also help.

Good luck! Let us know what you find out.

Hank,

 

The POH on my 68C has the numbers at 125. I know I am at the limit and it is okay but my concern is that the egine runs rough on it and I cannot get full power at take off (RPM does not go higher than 2500 to 2600). As soon as my MX gets back from from vacation I will try to figure this out with him and let you guys know. As usual, I am thankful for everyone's help.

[N601RX]

They are Bendix Slick Mags with around 600hrs on them.

They are either Bendix or Slicks, not both. Since you don't have a engine monitor, you can crank it and immediately switch to the bad mag. Let it run for a short while at medium power. The cool cylinder is the one causing the problem.

[isaacpr7]

They are either Bendix or Slicks, not both. Since you don't have a engine monitor, you can crank it and immediately switch to the bad mag. Let it run for a short while at medium power. The cool cylinder is the one causing the problem.

Sorry about that. Kinda new to all this    I was confused because I went through my log books and someone wrote Bendix with model number on engine equipment list sheet and I saw another reference at another sheet refering to them as slicks. I will head to the hangar tomorrow and read it straight from the data plate.

[N601RX]

When you find the cool cylinder, you can follow the mag wire back to the mag that is rough an make sure its not rubbed or burned through. You might want to recheck the plugs on that cylinder also.

[larryb]

I still have the original engine cluster so I have no way of monitoring each individual cylinder.

An engine monitor was my very first upgrade after purchase.

Sent from my iPad using Tapatalk

[Mooneymite]

Are you sure your carb heat is completely shutting off?

[isaacpr7]

An engine monitor was my very first upgrade after purchase.

Sent from my iPad using Tapatalk

 

That is exactly what I am thinking about doing myself. Just waiiting patiently to see if anyone wants to get rid of one for cheap when they upgrade.

[isaacpr7]

Are you sure your carb heat is completely shutting off?

 

I will take a look at that too. Thanks

[isaacpr7]

When you find the cool cylinder, you can follow the mag wire back to the mag that is rough an make sure its not rubbed or burned through. You might want to recheck the plugs on that cylinder also.

 

I don't think it's the plugs because I just replaced them all but I will check for the cold cylinder as you suggested. I will check with spruce to see how much would a new harness set me back. I saw something there that was priced at 67 cents a foot but I am not sure I was looking at the right thing.

[isaacpr7]

Another question for you guys.  What would be the consequenses of running my engine if in fact the mag was bad? Is this going to hurt anything other than have to put up with a rough engine? I just want to understand how the systems work in a deeper sense.

[Bob - S50]

Another question for you guys.  What would be the consequenses of running my engine if in fact the mag was bad? Is this going to hurt anything other than have to put up with a rough engine? I just want to understand how the systems work in a deeper sense.

If the Magneto truly is bad it could hurt the engine.  Spark plugs firing too early either due to mis-set timing or internal failures could cause the peak cylinder pressure to occur too close to TDC which can pound the bearings in the engine and could even lead to detonation or pre-ignition if bad enough.

 

Here is a link to a webinar about magnetos by Mike Busch:

 

http://www.eaavideo.org/video.aspx?v=1367499350001

 

Not that I would fly it that way, but if the engine runs smoothly on one of the two mags but not the other, I'd rather run on the one mag that gives me smooth operation than on both (one of which was bad).

 

Bob

[isaacpr7]

If the Magneto truly is bad it could hurt the engine.  Spark plugs firing too early either due to mis-set timing or internal failures could cause the peak cylinder pressure to occur too close to TDC which can pound the bearings in the engine and could even lead to detonation or pre-ignition if bad enough.

 

Here is a link to a webinar about magnetos by Mike Busch:

 

http://www.eaavideo.org/video.aspx?v=1367499350001

 

Not that I would fly it that way, but if the engine runs smoothly on one of the two mags but not the other, I'd rather run on the one mag that gives me smooth operation than on both (one of which was bad).

 

Bob

Thank you for the info Bob.  I will take a look at the webinar. I am becoming more and more fascinated by the amount of knowledge found on this site through you mooniacs

[N601RX]

You should not fly it with a rough mag. If the 2nd mag has the same hrs on it, it cold be just a few hrs away from developing a similar problem leaving you with 2 bad mags and looking for somewhere to land.

[isaacpr7]

My MX is not back yet from vacation but here is a picture I took of the left mag for those who were asking what brand I had.

 

[flyboy]

Sounds to me like its time for a set of new points in the mags. Not really that hard to do if you are handy with a wrench, time consuming however.

[isaacpr7]

So I had my MX finally come over and check out the mags. I thought the problem was in the left mag because that was the side that ran rough during the mag check. My MX tested and pointed out that it was my right mag with the issue instead. When I select the left mag on my ignition switch I am actually shutting off the left mag and running the engine with the right mag. We took the mag appart and everything in there, to include the points and plastig gear, look completely new to my surprise. He is thinking that it is the harness (visually it looks beat up) but he will check the mag timing after installation on Monday just to make sure.