Run up and mag check

[Silicon Rallye]

During the first climb, I noted it was still full rich passing thru 3000msl, still rich passing thru 4000, and by 5000, I had to ask, "You don't lean during climb?" ... Anytime ATC changed our altitude, up or down, he went full rich for the climb/decent.

Good grief, he really must dislike his plugs. He should also speak with his mechanic who will probably tell him that, in most planes, "Full Rich" is really "Slightly over-Rich".

The guideline I use is to "tweak" the mixture (as necessary) every 1000 feet of change.

 

[PaSkyhawk]

I just had a CFI tell me about this. Not only a right left check but I quick off and back on to make certain there is no shorts in the ignition. It was such a common sense thing to do I was surprised I only now learned about it. Just goes to show, you can never fly with enough CFI's, I learn something from each new one I fly with.

Yeah, put it to off too long and blow up the muffler. If you get drops on both mags the ground wire is good.

Rich

 

[Bill]

I don't believe your friend is making max power by climbing full rich in a O-360 to 9/10,000 msl. If temps are a problem above 5000 feet I think something else might be wrong.

I don't think so either, the climb got real doggy after 8000. I know the Archer isn't a real high flyer, but it was worse than I thought it should be. Oil temperatures were just fine, although I wasn't PIC and flying, I watched over everything like my life depended on it.

 

[Bill]

Your method will do you fine, just don't forget to richen up during decent, if you try to go around at SL with a mixture leaned for 9000', you might get in a bind.

I usually remember to go full rich passing back thru 3000msl, and if I don't, I should catch it on the pre-landing checklist. But yeah, a go around that lean would probably get your attention QUICK!

 

[sere]

I also do a full run up each and every pre-flight. Unless I am in an international airport I use this time to also help bring the oil temp's up. On one cross country flight after refueling when I did a run up I found that I had a broken plug. So you never know when you will find a problem.

Larry

 

[Maverick]

I also do a full run up each and every pre-flight. Unless I am in an international airport I use this time to also help bring the oil temp's up. On one cross country flight after refueling when I did a run up I found that I had a broken plug. So you never know when you will find a problem.

Larry

Once when I was returning from a Civil Air Patrol event I stopped for fuel at a small airport near the Military field where the aircraft was based. When I was getting ready to leave after fueling up I found that I had a bad mag during my run up.

I know pilots who say they would not bother with another run up after they had just landed, fueled the airplane and took of again. But my experience as well as Larry's just shows that a failure can happen any time, not just over night.

Jeannie

 

[poadeleted20]

Just this week I flew a 5 hr IFR round trip (with an IFR rated buddy who was left seat) with much of it in IMC. During the first climb, I noted it was still full rich passing thru 3000msl, still rich passing thru 4000, and by 5000, I had to ask, "You don't lean during climb?" Response, "No, keeps the engine cool", and thus we stayed full rich to 9000 eastbound, and 10000 westbound. He then leaned the plane. Anytime ATC changed our altitude, up or down, he went full rich for the climb/decent.

I was taught to start leaning (100 ROP) at about 3000msl during the climb, and keep it there with minor tweaking, then lean to 50 ROP once in cruise.

Which is correct (if either) and why?

None of the above.

First, your buddy is the least correct unless he's flying a turbocharged engine (different story, different answer, save it for another time since I think most folks interested in the answer are flying carbureted non-turbo engines). The factory recommendations are all pretty much the same -- above 5000 density altitude, lean to peak RPM for takeoff and climb. If you don't, you a) give away critical power that was figured into the performance charts, and b) cool the combustion temps to the point where lead fouling is likely.

In addition, you NEVER enrich for descent unless/until you push the throttle forward at the lower altitude. A full-rich descent at reduced power is even worse for your engine than a full-rich climb above 5000. Carbureted engines actually run a bit richer when you pull back the throttle without changing the mixture.

Another point is the right place to be when leaning in cruise. Peak CHT occurs at about 50-75 degrees rich of peak EGT, so leaning to that point creates a good bit more heat that needs to be disposed of. OTOH, leaning to peak EGT lowers CHT's significantly without losing much power at all. That gives you cooler, more economical operation at nearly the same speed -- hot damn! you CAN have your cake and eat it, too.

For the best guidance on leaning, consult your engine manufacturer's engine operating manual. Lycoming users can get one for about $18 by calling Lycoming at 800-258-3279 or 570-323-6181 with a credit card handy. You should also review Lycoming's Service Letters 185B, 192B, and 197A, and Service Instruction 1094D. You can find unofficial copies on the web at http://www.prime-mover.org/Engines/Lycoming/ or Lycoming will fax official copies to you free if you call them. Finally, I most strongly recommend a read-through of the Lycoming Key Reprints found at http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/index.html.

 

[John J]

Dave and Ron

Thanks for the post and comments. I always do a runup to check the "current state of the engine" and get the oil up to temp. Also I do a mag shut down just before shutting down the engine to check the P leads and grounding. I also do not ever touch the prop when pullng the plane out of the hanger for who knows if the P leads are still grounded. A visual inspection will not tell you enough for the wire can have internal chafing. I did loose a mag on a climb IFR and had to get back home for the engine sounded and felt like a "Bag of Hammers". Interesting the hold down for the mag had worked loose just enough to change the timing which caused the engine to bark and backfire.

As a side note there is a really nice article in the latest ABS Magazine about the Baron 56TC and John Miller comments about running LOP. He started flying in 1922. He owned a Baron 56TC for 30 years and ran it LOP and never had a problem running those engines LOP.

John J

 

[Henning]

Yeah, put it to off too long and blow up the muffler. If you get drops on both mags the ground wire is good.

Rich

All I do is flick the mag switches to off for a moment at idle before I shut it down, never in 20 years have I had a backfire. At idle, you don't always get a mag drop on one mag.

 

[RobertGerace]

As a mechanic, I have always done that from day one, although my reason is different, I shut down both mags momentarily to make sure I don't have a hot mag as I'm dragging/pushing the plane by the prop. Enlightened self interest I call it.

Hmmmmm...reminds me of yet ANOTHER mistake I made the other night. I posted about the pain of getting fuel (2 hours...takeoff delayed until midnight).

In my haste and anger, after taxiing to the 2nd FBO to try to get fuel, I shut down.

I finally got fuel, and then got in to restart. I noticed the mags had been left on.

Nobody touched either prop, and mixture was at cutoff, but still.

Damnit! How many of these mistakes...clearly taught and pointed out ahead of time to I need to make?

It seems the longer I fly, the more I have to add to my bag of, "Oh yes; I've done that." Sigh...

 

[Henning]

Hmmmmm...reminds me of yet ANOTHER mistake I made the other night. I posted about the pain of getting fuel (2 hours...takeoff delayed until midnight).

In my haste and anger, after taxiing to the 2nd FBO to try to get fuel, I shut down.

I finally got fuel, and then got in to restart. I noticed the mags had been left on.

Nobody touched either prop, and mixture was at cutoff, but still.

Damnit! How many of these mistakes...clearly taught and pointed out ahead of time to I need to make?

It seems the longer I fly, the more I have to add to my bag of, "Oh yes; I've done that." Sigh...

We're human, and we will make mistakes, especially when things aren't going routinely. The vast majority of the time, one mistake will not lead to an accident, that normally requires a chain of improper events (there are exceptions that I can note which include making nitroglycerin which only required one mistake, luckily the batch was small and I knew to run as soon as the boiling started, the shed did not survive) to create a casualty of any degree. The key is to catch and correct mistakes ASAP to interupt the chain, and try not to repeat them too often. The mag switch is so ingrained in me that I shut it off with the master as I was hanging from my belt upside down in a field.

 

[Ed Guthrie]

I think CFI's these days have their hands full trying to teach the proper skills to their students.

I am not quite so charitable. I think it reflects poor CFI skills. Proper engine management is a proper skill. Leaning the mixture properly is a fundamental operating skill and it should be taught to everyone.

It's easier for the student to not worry about the mixture until shutdown. This way they only have to worry about one knob. Also most of the operations done at flight schools are under 3000msl they can get away without worrying about it.

And sooner or later one of those students will promptly leave on a post pvt certificate x-country to Denver (or Lake Tahoe, or Cheyenne, WY, or...) where s/he crashes and burns because the Cessna 172 couldn't out climb terrain with a full rich mixture.

 

[gismo]

Now, before you switch to one mag. think about what will occur if one plug isn't firing properly. When you switch to the mag. on which that plug is located, you will accumulate fuel in that cylinder and the engine may run roughly. If you immediately switch back to both mags, you will ignite the unburned fuel which may have accumulated to a larger than normal amount which could cause a back fire. It is suggrested you do this at a lower powere setting until comfortable with the procedure. At a cruise setting, lthough it's counter intuitive, take you hand off the mag right after the switch over and put one hand on you MP lever. When you switch, if everything is running fine, EHTs will rise 10 or 15 degrees and CHTs will drop a few degrees on each cylinder.
If the engine runs rough, lower the power setting before switching back to both or you may cause a backfire and harm the engine.

I could be wrong about this but I doubt that there's any chance of engine or exhaust damage when checking mags at cruise power. First, I'm pretty darn sure that the 1400+F temp of the exhaust from an adjacent cylinder will manage to ignite any fuel that gets pumped out of a dead cylinder. Second no fuel will accumulate in the cylinder as the normal pumping action continues with or without ignition. Finally, when you do a "normal" mag check with full rich mixture at 1700 RPM you are flowing nearly the same amount of fuel as LOP cruise at altitude (at least for a n/a engine) so if this was a recipe for exhaust system damage, there'd be as many exhaust repairs as fouled plugs.

Now if you manage to kill both mags, or if one mag is completely dead, there is a good chance for an "explosion" in the exhaust if you switch back to a working mag without closing the throttle or pulling the mixture first. And that's also true if it happens during a runnup mag check.

 

[gismo]

Dave


Thanks for your post on Runups. I do them for it gives me time to check the temps and pressures. I never like to rush the departure. I have flown LOP in all the planes I have owned or shared for the last 40 years. Plug fouling has been non existent. My dad who loved those radials as an engineer during the 1930s ran everything LOP and when I took him for his first plane ride asked me how the cylinders and plugs were. I was a newly minted nieve pp. I did a runup. He told me so well that engines love oil, clean fuel and clean plugs to work.

There's a bunch of stories about the fire breathing B56TC in the latest ABS magazine including one by John Miller (He's a still living Ernie Gahn type). And guess what? He was operating his engines LOP 35 years ago and routinely exceeded TBO on engines that are considered fragile today.

 

[gismo]

Just this week I flew a 5 hr IFR round trip (with an IFR rated buddy who was left seat) with much of it in IMC. During the first climb, I noted it was still full rich passing thru 3000msl, still rich passing thru 4000, and by 5000, I had to ask, "You don't lean during climb?" Response, "No, keeps the engine cool", and thus we stayed full rich to 9000 eastbound, and 10000 westbound. He then leaned the plane. Anytime ATC changed our altitude, up or down, he went full rich for the climb/decent.

I was taught to start leaning (100 ROP) at about 3000msl during the climb, and keep it there with minor tweaking, then lean to 50 ROP once in cruise.

Which is correct (if either) and why?

You were taught correctly, at least more correctly than your buddy. Primary students are often taught to leave the red knob alone, or at least return to full rich anytime except cruise. I assume this comes from the fears of their CFI's that they might misuse the mixture and damage the engine coupled with the rather limited knowledge many of these CFI's have WRT mixture management. Some of those students (I consider myself in this category) manage to learn a bit more about the correct and safe operation of their engines later in their flying careers, and some blindly follow that original simple but inadequate advice.

 

[gismo]

I pretty much use your friends modus operandi...

However, I am always leaned in cruise, on the ground and in initial climb. Having an accurate, four probe engine analyzer is key for me in the Tiger.

Sounds to me like your MO is more like Bill's than his buddy's (who runs full rich during climb and descent regardless of altitude).

 

[RogerT]

.

Which is correct (if either) and why?

I was taught full rich on climb and descent. And down lower I
do that. I tend to lean it a little up higher on climb but watch the EGT
and stay a little richer than I would at cruise.

 

[Dave Siciliano]

Now if you manage to kill both mags, or if one mag is completely dead, there is a good chance for an "explosion" in the exhaust if you switch back to a working mag without closing the throttle or pulling the mixture first. And that's also true if it happens during a runnup mag check.[/QUOTE]

Afraid I'll have to take issue with you on this; it's happened to several folks performing the LOP mag check--that's why the procedure has been modified in the manner I stated. I did it once, there was a bad plug and I just swithched back. Bang!! Just like a back fire on a car.

You're Mag check on the ground is quite a bit different. Your MP is limited and you're burn timing is quite a bit different. Believe me, you don't want to be at a high power setting, close to peak EGT, turning plugs off, then back on. And, if you do, you sure as heck want to reduce power before turning things back on.

If you can get a plane I can demonstrate in, I'll be happy to----it won't be mine. Been there, done that, got the T-shirt!!

Dave

 

[RogerT]

On the Bonanza Net, there has been a lot of discussion about whether one should do a run up and mag check before each flight.

I've done a mag check at runup on every flight I've ever done. And I've
found mags that were flakey that needed some attention. Guess I'll
keep doing them.

 

[gismo]

I don't believe your friend is making max power by climbing full rich in a O-360 to 9/10,000 msl. If temps are a problem above 5000 feet I think something else might be wrong. Climbing too steep, baffling. You're also burning alot of extra 100LL.

And you won't make the POH fuel consumtion figures since they generally assume leaning during climb above 5000 MSL. Running full rich means that you will burn fuel faster for a longer period of time.

 

[gismo]

We're human, and we will make mistakes, especially when things aren't going routinely. The vast majority of the time, one mistake will not lead to an accident, that normally requires a chain of improper events (there are exceptions that I can note which include making nitroglycerin which only required one mistake, luckily the batch was small and I knew to run as soon as the boiling started, the shed did not survive) to create a casualty of any degree. The key is to catch and correct mistakes ASAP to interupt the chain, and try not to repeat them too often. The mag switch is so ingrained in me that I shut it off with the master as I was hanging from my belt upside down in a field.

I taught myself to "always" chant "mixture, mags, master" when shutting down yet I still manage to leave the mags on once in a great while. Now I'm trying to build the habit of checking the switches before touching the props.

 

[gismo]

Afraid I'll have to take issue with you on this; it's happened to several folks performing the LOP mag check--that's why the procedure has been modified in the manner I stated. I did it once, there was a bad plug and I just swithched back. Bang!! Just like a back fire on a car.

You're Mag check on the ground is quite a bit different. Your MP is limited and you're burn timing is quite a bit different. Believe me, you don't want to be at a high power setting, close to peak EGT, turning plugs off, then back on. And, if you do, you sure as heck want to reduce power before turning things back on.

If you can get a plane I can demonstrate in, I'll be happy to----it won't be mine. Been there, done that, got the T-shirt!!

I think I'll take your word for this for now, although I'd still like to find an explanation that makes sense to me. Maybe Braly has the answer.

 

[Henning]

There's a bunch of stories about the fire breathing B56TC in the latest ABS magazine including one by John Miller (He's a still living Ernie Gahn type). And guess what? He was operating his engines LOP 35 years ago and routinely exceeded TBO on engines that are considered fragile today.

I always thought that that plane/engine got a bad rap. I flew one quite a bit including taking it to HI. I almost bought one that was a runner and abandoned in Indiana, but it was gonna cost me 80K to get it running.

 

[Jim Chumley]

I taught myself to "always" chant "mixture, mags, master" when shutting down yet I still manage to leave the mags on once in a great while. Now I'm trying to build the habit of checking the switches before touching the props.

My mantra is similar: "Electrics, mixture, mags, master". "Electrics" being the radios and such.

Jim

 

[PaSkyhawk]

All I do is flick the mag switches to off for a moment at idle before I shut it down, never in 20 years have I had a backfire. At idle, you don't always get a mag drop on one mag.

Yes you have to be at run up rpms, but a regular mag check will tell you if you have a broken mag ground wire.

 

[Dave Siciliano]

I think I'll take your word for this for now, although I'd still like to find an explanation that makes sense to me. Maybe Braly has the answer.

One explaination may be you have to turn off the left while going to the right. The other is that at WOT and 17gph in cruise, the timing of the burn is pretty close to stochiometric combustion; where as, in run up, you'd be very ROP which would delay the burn. Let me see if I can get you something more engineeresk!!

Dave

 

[Dave Krall CFII]

I am not quite so charitable. I think it reflects poor CFI skills. Proper engine management is a proper skill. Leaning the mixture properly is a fundamental operating skill and it should be taught to everyone.



And sooner or later one of those students will promptly leave on a post pvt certificate x-country to Denver (or Lake Tahoe, or Cheyenne, WY, or...) where s/he crashes and burns because the Cessna 172 couldn't out climb terrain with a full rich mixture.

...and the pilot forgot how to do 180. ...and forgot how to read their POH ...and forgot their FAA written exam material and....

 

[Ed Guthrie]

Originally Posted by Ed Guthrie
I am not quite so charitable. I think it reflects poor CFI skills. Proper engine management is a proper skill. Leaning the mixture properly is a fundamental operating skill and it should be taught to everyone.

And sooner or later one of those students will promptly leave on a post pvt certificate x-country to Denver (or Lake Tahoe, or Cheyenne, WY, or...) where s/he crashes and burns because the Cessna 172 couldn't out climb terrain with a full rich mixture.

...and the pilot forgot how to do 180. ...and forgot how to read their POH ...and forgot their FAA written exam material and....

What's your point?

Regarding a 180: If the aircraft won't out climb rising terrain on take-off (or some other critical moments) then a 180 may be equally out of the question.

Regarding the POH and written, if those sources were sufficient learning experience then why is proper leaning ignored in training? Seems to me if those were sufficient nearly every student would be using the mixture appropriately during all training flights. Read a few posts in this thread and you'll realize immediately that it just isn't so. Your suggested sufficient sources obviously don't work, which is to be expected, actually. The POH and the FAA written are rather abstract and sometimes insufficiently specific. Those abstract sources become rapidly negated when the CFI doesn't use the mixture in real practice during flight training.

 

[John J]

I do remember so well doing a runup at JeffCo in the early 1960'. I was young and nieve,on a very warm summer day in an old 172. The CFI who checked me out watched as I did the mag check and then I pulled the mixture out and checked again. He was one of these wonderfull "Grey Beards" who said to me " Are you shure everything is ok" I told him " I am not sure". Well he fired walled the Continental and pulled the mixture out way beyond what I had done and we listened and listened to the engine. It ran a little rough and then smoothed out. He said" How does it sound and feel? "Is it strong and ready to go?" I said yes and we took off and I never forgot that lesson. it was one of the best instruction flight I have ever had. The plane had an Alcor single cylinder EGT guage. We flew LOP for the flight. It was the runup and aggressive leaning taught me that these engines like to run hard and lean. Ever since the planes that I have owned or shared the engines have always past TBO. The runup is so important so that the pilot can listen and feel to his/her engine to see if it is ready to go.

John J

 

[Dave Krall CFII]

Yep. Every pilot does a prop 3 times and says "good to go". I ask them:
(1) Why 3 times?
(2) What are you looking for?
Most say that they want to be sure the oil is circulating (which is a good answer) but I ask what gauges they're also checking. A few give me a funny look suggesting that's it.

I suggest
1. Oil pressure .. should show little or zero change ('nuff oil)
2. Manifold pressure .. should show an increase (pitch changed)
3. RPM .. should show decrease (verify pitch changed)

Did a BFR with a buddy/owner of a Commander SEL last week who just cycled twice, siting wear & tear issues opposed to cyling thrice.

Valid point I guessed, so I suggested at least looking at two guages simultaneously during one of the cycles.

 

[Iceman]

I thought you guys might want to see what the inside of a complex prop looks like. Very simplified: there is a chamber that contains oil under pressure. As more oil is forced into it it pushes the piston further forward against the spring tension and changes the pitch of the prop. The oil is not used to lubricate the inner mechanisms of the prop. Enjoy the picture...I wished I had taken a movie of the demonstration which went through each cycle of the prop and what takes place.

 

[John J]

I thought you guys might want to see what the inside of a complex prop looks like. Very simplified: there is a chamber that contains oil under pressure. As more oil is forced into it it pushes the piston further forward against the spring tension and changes the pitch of the prop. The oil is not used to lubricate the inner mechanisms of the prop. Enjoy the picture...I wished I had taken a movie of the demonstration which went through each cycle of the prop and what takes place.

Chris;

Thanks for the photo. It would be great to see a film of the prop working.

Thanks again and yes we will always remember this picture as we cycle the prop.

John J

 

[Dave Krall CFII]

Originally Posted by Ed Guthrie
I am not quite so charitable. I think it reflects poor CFI skills. Proper engine management is a proper skill. Leaning the mixture properly is a fundamental operating skill and it should be taught to everyone.

And sooner or later one of those students will promptly leave on a post pvt certificate x-country to Denver (or Lake Tahoe, or Cheyenne, WY, or...) where s/he crashes and burns because the Cessna 172 couldn't out climb terrain with a full rich mixture.



What's your point?

Regarding a 180: If the aircraft won't out climb rising terrain on take-off (or some other critical moments) then a 180 may be equally out of the question.

Regarding the POH and written, if those sources were sufficient learning experience then why is proper leaning ignored in training? Seems to me if those were sufficient nearly every student would be using the mixture appropriately during all training flights. Read a few posts in this thread and you'll realize immediately that it just isn't so. Your suggested sufficient sources obviously don't work, which is to be expected, actually. The POH and the FAA written are rather abstract and sometimes insufficiently specific. Those abstract sources become rapidly negated when the CFI doesn't use the mixture in real practice during flight training.

If they're so low as to be unable to do a 180, then they're so far behind the airplane they don't deserve to be flying PIC and their piloting problems are deeper than improper leaning.

IOW, my point is, flight ratings are not only licenses to learn but PIC means just that -if you're not ready to glean neccessary info for the flight you're going to make, then you shouldn't be PIC. Beyond that, in reality, few pilots study everything as much as they could.

Yeah, I've read all the above posts here on leaning and recall from my private all the way through over 20 CFIs at a dozen different facilities the the CFIs had proceedures, plus it was often written in by hand in the POH to "lean during taxi", etc.

Whether it's supposedly never using a radio (different thread) during all private training AND the FAA flight exam, or complete and total abscense of leaning procedures by a student's CFI and FAA examiner, I find it hard to believe.

But if it really happens, it must happen when one deficient CFI somehow takes a student all the way to PPL with no other CFI phase checks AND gets some Bubba FAA examiner/DE ...of which I've never seen any.

 

[poadeleted20]

I was taught full rich on climb and descent. And down lower I
do that. I tend to lean it a little up higher on climb but watch the EGT
and stay a little richer than I would at cruise.

I'm with you on climb, but not on descent. If you enrich as you reduce throttle, you're double-overriching the engine, and that's a recipe for lead fouling that you won't discover until you throw the throttle forward for a go-around, and don't get all the ponies under the hood digging in their hooves.

 

[Iceman]

I'm with you on climb, but not on descent. If you enrich as you reduce throttle, you're double-overriching the engine, and that's a recipe for lead fouling that you won't discover until you throw the throttle forward for a go-around, and don't get all the ponies under the hood digging in their hooves.

100% correct. Another problem is the shock cooling of the cylinders because of the increased air flow/increased fuel cooling.

 

[Ed Guthrie]

If they're so low as to be unable to do a 180, then they're so far behind the airplane they don't deserve to be flying PIC and their piloting problems are deeper than improper leaning.

We appear to have different scenarios in mind. You appear to assume mixture induced altitude/climb problems only occur in cruise flight. What about take-off scenarios? As an example, look at TLV (Lake Tahoe, CA). Where's the 180 turn to be made on a north departure before impacting trees?

 

[Dave Krall CFII]

I'm with you on climb, but not on descent. If you enrich as you reduce throttle, you're double-overriching the engine, and that's a recipe for lead fouling that you won't discover until you throw the throttle forward for a go-around, and don't get all the ponies under the hood digging in their hooves.

After leaning for cruise, an early CFI told me that the FBO's A&P mechanic said to enrichen their SkyHawks less than one turn of the mixture knob per 1000 feet of descent... Right or wrong ?

 

[Dave Siciliano]

Lance:

Here's a better explaination:

Dave
-------------------------------------------------------------------------
If one plug is DEAD and you go to that mag, the fuel in that cylinder will be pumped straight through the cylinder and into the exhaust. In Bonanzas, there is not much run before that fuel would be ignited and you would get a POP in the exhaust. On a one-cylinder basis, this would be pretty small and I have not worried about it at all. I've had it happen a few times and have noted no problems. I do the mag check at between 85 and 90% power, so I'm not at some pre-determined lower power setting.

If the entire mag were dead, and all six cylinders were to pump raw fuel into the exhaust, the thought is that there could be enough fuel

in the exhaust to result in a PARTS YARD SALE. That's why we recommend not going back to BOTH without pulling the mixture to Idle Cutoff and allowing the system to clear of raw fuel. That said, we, in the name of doing research FOR YOU GUYS, have done this without going to Idle Cutoff when LOP and have not noted that it's a problem. I would still pull to idle cutoff to be better safe than sorry.

The afterfire (it's not a backfire) is not from turning the mag back on. It's from the raw fuel being ignited in the exhaust by the fire from the other cylinders.
Walter

 

[Dave Siciliano]

Add to the above that 2500 is a lot faster rpm than 1700 for the mag. check on the ground, so fuel is pumping faster. However, even on the ground, with on dead mag, it can cause quite a backfire or after fire as Walter refers to it. Difference is what happens after the after fire. On the ground, one may be shaken--not stirred

Dave

 

[gismo]

Lance:

Here's a better explaination:

Dave
-------------------------------------------------------------------------
If one plug is DEAD and you go to that mag, the fuel in that cylinder will be pumped straight through the cylinder and into the exhaust. In Bonanzas, there is not much run before that fuel would be ignited and you would get a POP in the exhaust. On a one-cylinder basis, this would be pretty small and I have not worried about it at all. I've had it happen a few times and have noted no problems. I do the mag check at between 85 and 90% power, so I'm not at some pre-determined lower power setting.

If the entire mag were dead, and all six cylinders were to pump raw fuel into the exhaust, the thought is that there could be enough fuel

in the exhaust to result in a PARTS YARD SALE. That's why we recommend not going back to BOTH without pulling the mixture to Idle Cutoff and allowing the system to clear of raw fuel. That said, we, in the name of doing research FOR YOU GUYS, have done this without going to Idle Cutoff when LOP and have not noted that it's a problem. I would still pull to idle cutoff to be better safe than sorry.

Dave, that's pretty much in line with what I posted earlier, IE a single non-firing cylinder isn't a likely cause of exhaust damage, but several or all cylinders pumping unburned fuel can be very problematic if you relight while fuel is still in the exhaust. As to being safe vs sorry, I can't disagree with that at all.