Not making full power after a mag rebuild

BigBadLou

BigBadLou

Final Approach
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Lou
Hi guys,

I have a Cherokee 140 with an O-320 with less than 300 SMOH (2007 IIRC) and two Slick mags (installed as brand new at time of overhaul).

I lost a mag in flight recently (sudden loss of rpm) and safely landed at a nearby airport after flying maybe another 5 minutes on the good mag.
We did some basic troubleshooting on the ground with standard run-up and concluded that the right mag is poop. We disconnected the P-lead and it still would not provide ignition (suspected it might have been a grounded P-lead as well).

I had the local mechanic pull the mag, ship it off to Dallas for a rebuild and they found the cam flat. After a rebuild, the mechanic put the mag back on, timed it at 25 BTDC and verified that the left mag was also at 25.

Now I have a problem (which I did not have before the mag loss), she is not making full power.
We leaned her out drastically for a minute or two a few times with no change.
The run-up yields 100 rpm drop on each mag (used to be <100 at 1700). Carb heat drop is around 100, as usual. And the rpm recovers after the heat is removed. Static full-power rpm is just shy of 2200. During takeoff on the rwy, it takes longer (and understandably more distance) to reach Vr and the max rpm is barely 2400 (normally I see close to redline - 2700). Which forces me to abort the takeoffs.

I pulled the intake filter, checked for obstruction. Checked intake hose for obstruction. Checked carb heat door operation. Checked carb throat for obstruction. Looked up into the muffler through the exhaust pipe to check for obstruction.
She starts easy as usual, purrs like a kitten at idle but she just does not make full power. That also prevents me from flying her elsewhere (or even home) to get another mechanic to look at her.
Waiting on a friend test pilot to return to town to test her out and see if he is crazy enough to try to fly her and see how much she can climb.

I have run out of ideas and so has my mechanic. Any advice would be appreciated.

Thank you in advance.
Lou
 
Possible the throttle linkage got knocked out of adjustment? Visually verified the throttle opens all the way?
 
Chip Sylverne said:
Possible the throttle linkage got knocked out of adjustment? Visually verified the throttle opens all the way?
Click to expand...

:yeahthat:


Does the throttle and mixture arms hit both low and high end mechanical stops?


Does the carburetor heat hit the on & off stops?
 
Are all the engine control cables secured? If the sheath can slide it will mess up the travels.
 
bnt83 said:
Does the throttle and mixture arms hit both low and high end mechanical stops?


Does the carburetor heat hit the on & off stops?
Click to expand...

Thank you ALL, gentlemen, I did not think of checking the mixture and throttle linkages (did not have a reason to suspect them since they are on the "other side" (bottom) of the engine and have not been touched - but I am not saying that they could not have been). I will check them both tonight.

And yes, I checked the carb heat diverter flap, that was one of the first things I suspected.
 
Exhaust really old? Did something break internally and is now restricting the exhaust flow?
 
bnt83 said:
Exhaust really old? Did something break internally and is now restricting the exhaust flow?
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Muffler seems old, sure. Not to the point that I expect it to be falling apart. Though my usual mechanic did suggest (over the phone) to look up the exhaust pipe to check for any blockage. None found, though. There is a "strainer" of some sort on its top to prevent blockage which does not mean that there isn't a broken baffle inside blocking more. (if that is possible inside a Cherokee muffler - I am no expert)

FWIW, the exhaust pipes from the block to the muffler seem much newer.

We are thinking about unbolting the muffler connections and peeking inside. Is there a chance we can see anything useful that way?

Thank you
Lou
 
BigBadLou said:
Muffler seems old, sure. Not to the point that I expect it to be falling apart. Though my usual mechanic did suggest (over the phone) to look up the exhaust pipe to check for any blockage. None found, though. There is a "strainer" of some sort on its top to prevent blockage which does not mean that there isn't a broken baffle inside blocking more. (if that is possible inside a Cherokee muffler - I am no expert)

FWIW, the exhaust pipes from the block to the muffler seem much newer.

We are thinking about unbolting the muffler connections and peeking inside. Is there a chance we can see anything useful that way?

Thank you
Lou
Click to expand...

Always check easy stuff first.

Pulling the exhaust or the muffler or just about anything else exhaust related kinda sucks. If you can't see anything but that wire "bail" thingy up the pipe then its probably fine.
 
bnt83 said:
Always check easy stuff first.
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Yup, will be checking more tonight. I will have another pilot at my disposal so we can easily check the travel on all cables etc.
Thanks, everybody, for the advice.

bnt83 said:
Pulling the exhaust or the muffler or just about anything else exhaust related kinda sucks.
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You are preaching to the choir. Just last weekend, I finished putting the rear axle, exhaust, driveshaft and everything else back into my Z28 after being forced to replace the fuel pump (just as its warranty expired :rolleyes: ). I am getting old for these "fun projects". :)

bnt83 said:
If you can't see anything but that wire "bail" thingy up the pipe then its probably fine.
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Thank you, sir.
 
bnt83 said:
Always check easy stuff first.
Click to expand...

Always check the obvious stuff first. The mag was just worked on and retimed to the engine. That whole thing needs review. I have seen mags improperly timed that still let the engine run. If the engine is supposed to have the mags set to the same angle BTDC, then they need to be as close as possible. A few degrees difference can cause power loss. One mag will fire earlier and the cylinder pressures start rising to the point that the late mag can't make a spark, so you're essentially running on one mag. It will not show up on a mag check, since the mags will work separately just fine.

Dan
 
You'll get it figured out bud! I like the linkage test on the throttle & mixture and maybe a sleeve dislodging and stopping the full travel.

Hopefully it's something very simple, and you're able to get it home before Saturday morning. That way you'll call me so we can fly to the airshow!
 
I agree with Dan, the first thing I'd have checked is mag timing.
 
Mechanic swears that both mags are at 25 BTDC, on the dot. I have no way of verifying besides paying another mechanic to drive out and check the timing.

We checked all cables' travel last night and they are all good, stop to stop (on the carb). We also pulled the plugs and they are clean and white, as expected after burning them lean. All wires are securely attached.

The mechanic has a friend with a bore scope which he will try to borrow today and he will snake the muffler to check for any internal blockage.

I will keep you guys posted. Keep the good suggestions coming.
 
Checkout_my_Six said:
Bingo.....:yesnod:

Start with the internal mag timing.....then the mag to engine timing. Either could present the problem you are experiencing.
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What do you mean by " Internal mag timing"?

anything in the mag that is wrong the mag check will be way wide. if it runs at all.
 
The magneto was shipped off and came back as an assembled unit so I assume that the rebuilder timed it internally. I know, it is just an assumption. So let me go find the paperwork to confirm that they actually did their job. :)
 
Tom-D said:
Do you really believe any of that can be wrong and still get off the test bench?
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How is bench testing supposted to tell if the mag is timed properly?
Do you believe that mag could be timed to the wrong cylinder and still pass a mag check?
Click to expand...
Nobody said "to the wrong cylinder" and yes a mistimed mag can escape many people's mag checks because they say "hey look hardly any mag drop, these rebuilt mags are really great" when in fact that is a significant indication as to what is going on.
 
flyingron said:
How is bench testing supposted to tell if the mag is timed properly?
Click to expand...
It won't. But running it from 100 to 3000 rpm with gaps over 1/8", while blowing hot air on it will weed out the ones that aren't timed properly.
 
flyingron said:
How is bench testing supposted to tell if the mag is timed properly?
Click to expand...

Sixey, was talking internal timing, not mag to engine, or at least that is what his little vid was about.

The test bench certainly will tell you that the mag has been put together wrong.

Mag to engine timing is often messed up.
 
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Tom-D said:
Mag to engine timing is often messed up.
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Yup, and some Lycomings have been known in the past to have their timing marks on the flywheel to be off a few degrees. Maybe the mechanic previous to this one knew that and used a spinner protractor and got it right, while the recent guy used marks that were off some.

It's not really likely but still worth checking.

Dan
 
Timed using a piston stop and a protractor or marks on the crackshaft flywheel that may not be correct?
 
You said the other mag was verified at 25 which leads me to believe no adjustment was made. If that's correct it adds a bit more mystery
 
Are you using auto fuel? how long did the aircraft set between flights while dealing with the mag issue?
 
compression test? sticky valves not making full compression
 
Maybe your sudden loss of rpm wasn't the mag?
Fuel pressure? Flow? TRied both tanks? Tank selector full in the detent?
 
k...next try cleaning and inspecting the lower plugs. They could be fouled.
 
bnt83 said:
You said the other mag was verified at 25 which leads me to believe no adjustment was made. If that's correct it adds a bit more mystery
Click to expand...
Yes, mechanic said he did not touch the left mag because it was right at 25 so he timed the right mag to match the left one at 25.

Tom-D said:
Are you using auto fuel? how long did the aircraft set between flights while dealing with the mag issue?
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Negative.
The plane sat "only" 2 weeks while waiting for the mag to be rebuilt. Not any longer than other times.

yetti said:
compression test? sticky valves not making full compression
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We pulled the prop through and felt for compression and did not feel anything wrong.
But pressure check is being done today.

Chip Sylverne said:
Maybe your sudden loss of rpm wasn't the mag?
Fuel pressure? Flow? TRied both tanks? Tank selector full in the detent?
Click to expand...
Negative. It was the mag, verified.
Good fuel pressure, no matter which tank.

Checkout_my_Six said:
k...next try cleaning and inspecting the lower plugs. They could be fouled.
Click to expand...
Plugs are clean with a light color on the electrodes (from running lean). Gaps are good. Wires are secure.


I will know more this afternoon (hoping). Will post back.
 
Probably not Germaine to your issue but I just replaced two Slick mags on a Maule with 300 hours on the engine. We did an initial run up and mag check for an annual and we had noticed when the owner flew the airplane that it seemed difficult to start but he was used to it. (Ignorance is bliss and if you don't know what right is then how will you recognize wrong) the timing was supposed to be 23 degrees BTDC and one was 23 and one was 28. We called Lycoming and told them (factory overhaul) and they said to ship them the mags and they sent two new ones. Lycoming and the FAA said that they were having problems with "drifting" with Slick mags lately. Put on the new mags and the owners first comment was that it started in two blades, ran smoother and flew faster.
 
MD11Pilot said:
Probably not Germaine to your issue but I just replaced two Slick mags on a Maule with 300 hours on the engine. We did an initial run up and mag check for an annual and we had noticed when the owner flew the airplane that it seemed difficult to start but he was used to it. (Ignorance is bliss and if you don't know what right is then how will you recognize wrong) the timing was supposed to be 23 degrees BTDC and one was 23 and one was 28. We called Lycoming and told them (factory overhaul) and they said to ship them the mags and they sent two new ones. Lycoming and the FAA said that they were having problems with "drifting" with Slick mags lately. Put on the new mags and the owners first comment was that it started in two blades, ran smoother and flew faster.
Click to expand...


I smell an AD right around the corner......:rolleyes:
 
N801BH said:
So....

Will someone please remind me why FAA Approved, Quality controlled Aviation parts cost so much. Since they are manufactured to a higher standard...:dunno:.......:mad2:.......:redface:
Click to expand...

Because it has nothing to do with the actual quality, just the generation of paper.
 
flyingron said:
Because it has nothing to do with the actual quality, just the generation of paper.
Click to expand...

Exactly. I get tired of printing reports that are part of an STC that say absolutely nothing. There was a 30 page electrical load analysis for installing an single LED Nav & Strobe Anti collision light that replaced an incandescent & strobe one in an STC I signed off yesterday.
 
Friends don't let friends fly with Slicks.

Our local mechanic won't "trouble shoot" a Slick, other than to pull it out and put a new one in. Says they are more misery than they are worth.

My mags are more than 6000 hours old (Bendix, of course) They have been overhauled a few times. The engines start immediately - assuming the brain dead pilot did what he was spoze to.
 
N801BH said:
So....

Will someone please remind me why FAA Approved, Quality controlled Aviation parts cost so much. Since they are manufactured to a higher standard...:dunno:.......:mad2:.......:redface:
Click to expand...

They seem to be manufactured to a standard that was in place about 75 years ago. SInce then, automobiles have incorporated so many quality improvements that aircraft stuff, which is still stuck in 1940, looks pretty poor. What other industry puts felt bearing seals in their wheels? Or still uses Buna-N O-rings when there are many superior materials available? Or still uses contact points in their magnetos? Even lawnmowers, for the last 15 or 20 years, have used a breakerless magneto ignition. Seems to me that Mercury outboards incorporated that in about 1972. Bendix and Slick could probably eliminate about half the maintenance, and double the reliability, if they used it.

Improvements imply, to a hungry lawyer, that the old stuff was defective and worthy of a lawsuit, if he can find someone who has come to grief with the old stuff. So things move forward very slowly. And sometimes very quietly, as with the Champion spark plug integral resistor. And most of the time any improvement is effectively resisted by government, which imposes an enormous burden on the manufacturer who modifies anything or comes up with a better way.

Dan
 
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