Excessive RPM drop during runup when full rich

azure

azure

Final Approach
Joined
Apr 2, 2005
Messages
8,293
Location
Varmint Country
Display Name
Display name:
azure
All summer I have been noticing this, and my A&P has seemed unconcerned about it due to the high density altitudes we've been having, but lately DAs have been in the mid-2000s and I am still noticing it. These conditions are not too much worse than last summer when it did NOT happen.

So what I observe is this: following the POH procedure, with mixture full rich and at 1800 rpm, I see a ~200 rpm drop when running on each mag. The POH says that 150 rpm absolute, or 50 between, is the limit. The drop is slightly greater on R than on L, but within the 50 rpm between limit.

The POH also says that repeating the test with leaner mixtures or at higher rpms will "confirm whether a deficiency exists". So as instructed by my A&P, I have been repeating the test with the mixture leaned out close to peak, and there the rpm drop is only 100-125 on each side, with even less difference between.

So what does this mean? That a deficiency does not exist because the rpm drop is within limits at a leaner mixture? Then why didn't this happen last summer? My A&P has checked the magneto timings at least twice now, as well as the plugs themselves and he has regapped them. He says the next step is to replace the air filter. If that doesn't fix it, then he says maybe the fuel servo is feeding the wrong mixture at certain throttle settings.

Additional information: On the JPI readout, all the EGTs rise smoothly on one mag, some faster than others, but if you wait long enough they even out. The only thing that might be significant is that on L, the initial EGT DIFF reading is less than either on both, or on R.

I've yet to try doing the check at higher RPMs (I know, dumb) -- I'm going out now to do that. My A&P reports that at higher and lower power settings, the rpm drop is within limits. Only in the 1500-2000 rpm range, is it acting funny.

Thoughts?
 
I think your full rich mixture is too rich for the density altitude, and that your magnetoes are also affected by temperature (could be damn hot in that compartment) and density altitude.

The fact that neither mag seems worse than the other, and that when you lean the mixture out they both perform correctly, is reassuring to me. I don't think you have a problem that requires maintenance, though it does bear continued watching.
 
I am glad you posted this. I had another run up issue on this past weekend (solo) but after I re-checked and changed RPM (I was high) it got better. Also, they had done the 100-hr the day before so I am not sure if that matters.
 
IME the most common cause of excessive mag drop without roughness is retarded timing. If this engine has timing marks and the mechanic uses them to set the mags the marks could be off.
 
gismo said:
IME the most common cause of excessive mag drop without roughness is retarded timing. If this engine has timing marks and the mechanic uses them to set the mags the marks could be off.
Click to expand...
...and mag timing can drift with operating time.
 
Ron Levy said:
...and mag timing can drift with operating time.
Click to expand...

And in so doing, it will lessen the quality of the spark, by lessening the dwell time of the coil.

as the cam follower wears the points open late and close early. lessor the dwell the less spark.

But that should not effect the mag drop, or the difference between mags, as much as mixture.
 
TMetzinger said:
I think your full rich mixture is too rich for the density altitude, and that your magneto's are also affected by temperature (could be damn hot in that compartment) and density altitude.

The fact that neither mag seems worse than the other, and that when you lean the mixture out they both perform correctly, is reassuring to me. I don't think you have a problem that requires maintenance, though it does bear continued watching.
Click to expand...

In this scenario I would advise she simply advance power to 1800RPM, switch to 1 mag, lean to a slight vibration, richen until it runs smooth, then switch to both mags and wait 1 full minute and do the mag check.
 
gismo said:
IME the most common cause of excessive mag drop without roughness is retarded timing. If this engine has timing marks and the mechanic uses them to set the mags the marks could be off.
Click to expand...

If he uses the degree wheel as required he won't use the marks.
 
What model of engine are we talking about here? What carb or is it injected? What mags? There have been several SBs lately from several manufacturers, one or more of which might be at play here. Defective stuff, some of it dangerously so.

Dan
 
I experienced the same thing on my T182T. So, I tried leaning it prior to the check and it all seemed fine. That became my standard practice. Then, recently, I received a SB from Lycoming stating that leaning may become necessary to get the drops within specs.
 
Thanks to all for the advice. The engine is a fuel injected 200hp Lycoming, IO-360-A1B6 with about 180 hours since zero time factory rebuild. Since the previous engine was an A1B6D, we've assumed that the mags were also zero time. But the engine was installed by the previous owner and the logs aren't clear on that point (at least, not to me), so we aren't 100% sure of their age.

From what he's told me I gather that my A&P has checked the timing between mags but not the internal timing. But from what I've read, a timing issue does fit the way it acts. When you switch from both to one mag, the initial rpm drop is normal at first, about 100 rpm, but then it continues to drop slowly and gradually levels off at ~200 below both.

I will try Tom-D's leaning suggestion next chance I get. Over the next couple of days A&P will be installing my new flap cable (finally).
 
The Precision Airmotive engineers once told me that Lycoming specifies a rather rich mixture (at max setting) on their carbs and injection systems. That makes them run roughly when the DA is high and that's why the POH often tells you to lean if the drops are too big, or to lean for takeoff if the DA is above 3000'ASL or so.

But if you didn't have to lean before when the DAs were what they are now, either something has slipped on the fuel controller (did someone fiddle with the throttle-to-fuel valve adjustment?) or the spark is weak. If those mags haven't been opened for awhile they should be done now. Probably Slicks, and Slick has had two SBs since '08. One dealt with a poor points cam material (an "improved" version) that wears out and screws up the internal timing, and the other dealing with bent coil tabs that chew up the distributor spark brush.

Check the plugs for internal arcing or failed resistors.

According to one of my old mechanics teachers, 90% of engine trouble is electrical. That was 43 years ago, and and I've found that he was right. Spark problems, most of the time. The timing might look ok but it doesn't tell you how strong that spark is.

Dan
 
gismo said:
Agreed but I've witnessed some that don't bother with the TDC finder and degree wheel.
Click to expand...

On a Lycoming 4 cylinder it is pretty simple its marked on the fly wheel, but on the big continentals they put it on a tiny wheel in the accessory section and they are often wrong.

The 0-300 has the marks on the back of the prop flange, brain dead simple
 
I had this discussion with someone a couple weeks ago. 172M with 0320 that I had flown fairly regularly for a few years now. Full rich, 1700 RPM, w/R mag selected, it ran really rough.

It's been hot before, this aircraft passes the mag check when everything is right. Yes, I tried leaning and burning deposits off several times.

I taxied back to FBO, he tried with the mixture pulled out about 1" and it mag checked good. I taxied out and checked using the same old checklist and it failed again. I rejected the airplane and cancelled.

The owner showed me a Lycoming SB on the topic. It talked about RPM drops.

I saw engine roughness and I heard backfire/afterfire (I'm not sure which).

A week later, I went back to fly and it worked just fine. I also noticed the carb heat knob range of travel was back the way it was a few years ago.

It mysteriously would not travel back all the way to the panel in full cold for the last year or so, now it does again! I assume somebody worked on it.

These are training rentals. With so many users, it's a different airplane every time. I'm OK with slightly uneven mag drop and variations in size of drop, carb heat has to drop RPM some, but I won't take it with a mis fire.
 
Last edited:
All, FWIW, I attached the Lycoming SB that discuses mixture and mag checks on hot days.
 

Attachments

  • Y10M06D18_Lycoming _I_1132B.pdf
    272.2 KB · Views: 55
@azure did you ever find out the root cause of this? I'm experiencing some very similar (actually identical) symptoms on my RG, the only difference being that I'm still on the dual mags.
 
azure said:
So what I observe is this: following the POH procedure, with mixture full rich and at 1800 rpm, I see a ~200 rpm drop when running on each mag. The POH says that 150 rpm absolute, or 50 between, is the limit. The drop is slightly greater on R than on L, but within the 50 rpm between limit.
Thoughts?
Click to expand...
I believe your aircraft is to be operated IAW the POH as directed by its TCDS. Does the POH say to lean then try again?

I think you have pending mag failure, your aircraft is talking to you, are you listening?
 
kontiki said:
All, FWIW, I attached the Lycoming SB that discuses mixture and mag checks on hot days.
Click to expand...
Aircraft are not certified by SB, the OEM's operating instructions take precedence.
 
CMongoose said:
@azure did you ever find out the root cause of this? I'm experiencing some very similar (actually identical) symptoms on my RG, the only difference being that I'm still on the dual mags.
Click to expand...
Nope. The mags have even been overhauled since then, and this behavior is still very much present. It does appear to be a mixture setting/DA issue since it does not happen during the winter, the only question is why it didn't happen when the engine was new, during the first summer I owned the plane which was 2010.

Are your symptoms new? How long have you been flying the plane?
 
Ravioli said:
@Tom-D din't wake the dead... @CMongoose did.
Click to expand...
Yep, I didn't say he did. That's how it usually happens, someone wakes a thread and someone replies to a long-ago post thinking it's new. Every post has a data stamp, it would help if people would check it before replying.

I wish I had useful additional info though. I think my symptoms are probably benign since they've continued for so long, but @CMongoose's may or not be.
 
Local mechanic here says to lean aggressively. :dunno:
 
azure said:
Nope. The mags have even been overhauled since then, and this behavior is still very much present. It does appear to be a mixture setting/DA issue since it does not happen during the winter, the only question is why it didn't happen when the engine was new, during the first summer I owned the plane which was 2010.

Are your symptoms new? How long have you been flying the plane?
Click to expand...

Just started happening. Engine is 900 since overhaul, mags are 300 since new. I've owned the plane for 4 years now, not noticed anything like this before. Thanks for the update, every bit of info is useful when troubleshooting!

It is interesting that you still have the symptoms.
 
CMongoose said:
Just started happening. Engine is 900 since overhaul, mags are 300 since new. I've owned the plane for 4 years now, not noticed anything like this before. Thanks for the update, every bit of info is useful when troubleshooting!

It is interesting that you still have the symptoms.
Click to expand...
Yeah, when it's a new symptom I would be a bit concerned too, and was. Has there been any recent mx on the fuel system or mixture setting? How much of a drop are you seeing, and is it just on one mag or on both? Does leaning reduce the drop?

In any case, let us know what (if anything) it turns out to be.
 
azure said:
Yep, I didn't say he did. That's how it usually happens, someone wakes a thread and someone replies to a long-ago post thinking it's new. Every post has a data stamp, it would help if people would check it before replying.
Click to expand...
Really sorry for ruining your week, with a single post.
 
azure said:
Yeah, when it's a new symptom I would be a bit concerned too, and was. Has there been any recent mx on the fuel system or mixture setting? How much of a drop are you seeing, and is it just on one mag or on both? Does leaning reduce the drop?

In any case, let us know what (if anything) it turns out to be.
Click to expand...

The mixture setting was adjusted last year, as it was running a little rich. Mag timing was done at the same time. My Dukes fuel pump failed earlier this year, and I'm wondering whether some debris may have moved downstream into the spider or injectors.

Run up is smooth and good if I lean the mixture, 60rpm both sides. There is a 25rpm rise to Idle Cut Off so it's not running rich. The Lycoming SB posted above says that's an indication of a "Propeller Load Condition", I presume they are implying that the prop is not sitting at full high, so that's a possibility to check.

Spark Plugs are all new. There's a slight vibration when running, but it's possible that I'm just noticing it more now that I'm looking for it.

The other oddity is on the mag check, most notably on the left mag, EGT1 and 2 rise much faster than the other two. When I lean out, they all rise and peak around the same point though. That's what has me wondering about the spider and injectors, wondering if there's some uneven fuel delivery going on.
 
azure said:
In any case, let us know what (if anything) it turns out to be.
Click to expand...

For me, it turned out to be the injector pump, in one way or another. First thought was a leaking center body seal, but they tested for that and no joy. Finally, since we've changed out every other part over the past few years we installed an overhauled injector pump. That fixed the issue.

The possible root cause was one of two items. Some internal damage inside the pump, but NOT a leaking center body seal, or when installing the pump the mechanic noticed that the two nuts holding the top of the pump on were not as tight as they should have been.

The scientist in me wants to know which of the two was the root cause, the economist in me would prefer not to know exactly because of the potential cost difference between the two :). Either way, the issue is now fixed!
 
You must log in or register to reply here.
Back
Top