RPM drop and "popping" sound on mag check, C-172

Hi there,

Googling past threads yielded nothing definitive so here goes:

On runup today, noted the following:
1. RPM to 1700, switched to right mag, dropped ~150 rpm

2. Increased RPM to 2000, leaned back until I got a 100rpm drop and
held for one minute to clear it. We've had to do this pretty regularly
lately*

3. During the lean-out I started hearing a regular popping or clicking
noise, clearly electrical in nature, about 1-2x/second on the intercom

4. After leaning out, throttle back to 1700, check right mag. RPM drop
maybe 50-75, popping sound disappears

5. Switch to left mag, popping sound returns, rpm drops 200

6. Repeated leaning out at RPM, same result, i.e., left mag still
dropping too much for me.

I'm going to call the shop Monday to have the plugs checked, is there
anything else this sequence of events suggests to anyone? Airplane in
question is a 1979 C172N, engine tach time about 1800. We do have one
cylinder that tested 63/80 at annual back in March.

* We do lean for taxi, but probably not enough. It has required a
burn-off more often than not the last few months, but one or two
cycles has always cleared it up successfully.

TIA,
-cwk.

I recently had these symptoms, and it was a bad spark plug wire. It was
intermittently sparking between the center conductor and the sheathing
braid, rather than at the plug electrode. Easily found with a cable tester,
most mechanics should have one -- something like this:
http://www.aircraftspruce.com/catalo...abletester.php


"C Kingsbury" wrote in message
om...
Hi there,

Googling past threads yielded nothing definitive so here goes:

On runup today, noted the following:
1. RPM to 1700, switched to right mag, dropped ~150 rpm

2. Increased RPM to 2000, leaned back until I got a 100rpm drop and
held for one minute to clear it. We've had to do this pretty regularly
lately*

3. During the lean-out I started hearing a regular popping or clicking
noise, clearly electrical in nature, about 1-2x/second on the intercom

4. After leaning out, throttle back to 1700, check right mag. RPM drop
maybe 50-75, popping sound disappears

5. Switch to left mag, popping sound returns, rpm drops 200

6. Repeated leaning out at RPM, same result, i.e., left mag still
dropping too much for me.

I'm going to call the shop Monday to have the plugs checked, is there
anything else this sequence of events suggests to anyone? Airplane in
question is a 1979 C172N, engine tach time about 1800. We do have one
cylinder that tested 63/80 at annual back in March.

* We do lean for taxi, but probably not enough. It has required a
burn-off more often than not the last few months, but one or two
cycles has always cleared it up successfully.

TIA,
-cwk.

* We do lean for taxi, but probably not enough. It has required a
burn-off more often than not the last few months, but one or two
cycles has always cleared it up successfully.

Probably not the case here but carburetor ice can build on a long taxi just
like when landing. Might pull heat and see if it does anything.

(C Kingsbury) wrote in message . com...



I'm going to call the shop Monday to have the plugs checked, is there
anything else this sequence of events suggests to anyone? Airplane in
question is a 1979 C172N, engine tach time about 1800.

Have the P lead connections checked for looseness, check any RF
filters for loose connections, have the indivisual plug leads check
for high voltage leakage and have the mag output voltage checked too.
A weak or degraded spark will not promote full combustion in the
cylinder and will lead to rapid fouling.

If they don't have the test equipment, swap mags and check. If the
problem moves, it's a mag problem. If it goes away, it was a loose
wire or connection.

Craig C.

C Kingsbury wrote:

I'm going to call the shop Monday to have the plugs checked, is there
anything else this sequence of events suggests to anyone?

Plug cables.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

I agree on plug wires.
On a high humidity night, open up the cowl and start the engine.
You can see the glow and sparks.

G.R. Patterson III wrote:

C Kingsbury wrote:

I'm going to call the shop Monday to have the plugs checked, is there
anything else this sequence of events suggests to anyone?


Plug cables.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

john smith wrote in message news:yeh1d.2344

Make sure the plug gaps don't get too large as this will electrically
stress the insulation systems in the plug wires as well as in the
magnetos.

A friend had a premature mag failure from this by going too long
between plug regappings.


1700 hrs without ever a rough mag.......

Make sure the plug gaps don't get too large as this will electrically
stress the insulation systems in the plug wires as well as in the
magnetos.


And if you can hit it with the hose while it's running, you can soon tell
what it will do in a good storm!

When the insulation gets rubbed or dirty or cracked, it creates a weak point
where the high voltage will arc over rather than fire the plug. Engines
operated in bush strips will have more dirt build up, and should be kept
clean for safer operation.

Same with a car or bike engine, you want them to keep running when it starts
to rain. Especially if like me you live 22 miles out of town, and have 7
creek crossings that when it rains heavy, it goes up to "F___, that's to
deep to drive through!"

Peter