engine "missing" in flight

murphey

murphey

Touchdown! Greaser!
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murphey
No, it's still attached to the cherokee...

It was supposed to be a short, 30 min flight for breakfast. I got about 10 min out and was not happy with how the engine was running. Seemed to be "missing" with frequent regularity. On the ground (runup), no problems with the mags. Just cleaned and regapped the plugs 3 flights ago. Any suggestions what could be happening?
 
Most troubleshooting technique tells you if something was just changed/done/etc to check there.
 
Id check the spark plug wires, sometimes if they go back in wrong the helo-coil will ground against the spark plug wall and cause that missing....a dropped spark plug could also cause that.
 
Unfortunately, "runups" on the ground are relatively pointless if you want to find a problem that isn't very, very basic. Have you done a high-power in-flight mag check? I would recommend that because you'll get some good data, such as:

taters said:
Id check the spark plug wires, sometimes if they go back in wrong the helo-coil will ground against the spark plug wall and cause that missing....a dropped spark plug could also cause that.
Click to expand...
It could be something with the wires or (less likely IMO) fuel contamination. If you do the check described above and everything runs smoothly at that point, you might take that as an indication that the problem is intermittent and therefore could be related to things like contamination or wiring.

-Felix
 
How high was the humidity?

I didn't realize I had an encounter with carb ice until well afterwards. In my case it was a ton of sputtering roughness when I pulled the power and prop pitch back on takeoff.
 
mikea said:
How high was the humidity?

I didn't realize I had an encounter with carb ice until well afterwards. In my case it was a ton of sputtering roughness when I pulled the power and prop pitch back on takeoff.
Click to expand...

I live in Colorado. What's humidity?

Oh, right - Tstorms and snow...

But seriously, wasn't roughness. I fiddled with power while in the air and nothing changed. I didn't try the in-air mag check - that'll be tomorrow's project.
 
Alright, enlighten me. What would be the difference between an in air mag check and a ground check at 2500 instead of 2000 rpm? (serious question):confused:

Other than the fact that if you do have a problem it may be more difficult to get back to the hangar/fbo with the in air check...
 
wpierce said:
Alright, enlighten me. What would be the difference between an in air mag check and a ground check at 2500 instead of 2000 rpm? (serious question):confused:

Other than the fact that if you do have a problem it may be more difficult to get back to the hangar/fbo with the in air check...
Click to expand...
The higher the power output, the more strain is placed on the ignition system. There is a huge significant difference in power between 2000 and 2500 rpm for most engines. Also, most ground checks are done when the engine is running full-rich, which makes the F/A mixture easier to ignite, too.

It's ideal to do the mag check at as high power as you're comfortable with, with a F/A mixture that is as lean as possible (because that is going to put the most strain on your mags, spark plus, etc.). You can certainly run full power and 100 dfLOP in some engines on the ground, but it's probably (don't want to say definitely, but I can't imagine an engine where this is a good idea :)) not advisable given that there isn't much cooling airflow....

This is why doing an on-ground mag check is not very useful. You're most likely not going to find any but the most severe problems, and you would find those anyways as soon as you power up on the runway.

-Felix
 
Pilot Complaint: Engine missing.

Mechanic's Logbook entry: After thorough search, engine found on front of aircraft.

{;-)

Jim
 
weirdjim said:
Pilot Complaint: Engine missing.

Mechanic's Logbook entry: After thorough search, engine found on front of aircraft.

{;-)

Jim
Click to expand...

tsk tsk tsk...I already pre-empted that joke in the original posting...More originality, please.
 
...I enjoyed it!
 
We had a similar problem on the 6. The runup would be just fine, but at cruise at 9500' the engine would run rough on the left mag. Ended up getting the mag rebuilt and that seems to have taken care of the roughness. Now however, we are having problems starting on the left mag. Annual is in Sept, so we're going to go through the entire left ignition system.
The ignition system is pirty dang simple. You have plugs, wires and mags, that's it. So you can only blame one of those (OK, maybe you could blame a flaky mag switch)
We're gonna try replacing the wires, and pulling the mag again to see if there's anything obvious (before springing for a new mag)
 
COFlyBoy said:
We had a similar problem on the 6. The runup would be just fine, but at cruise at 9500' the engine would run rough on the left mag. Ended up getting the mag rebuilt and that seems to have taken care of the roughness. Now however, we are having problems starting on the left mag. Annual is in Sept, so we're going to go through the entire left ignition system.
The ignition system is pirty dang simple. You have plugs, wires and mags, that's it. So you can only blame one of those (OK, maybe you could blame a flaky mag switch)
We're gonna try replacing the wires, and pulling the mag again to see if there's anything obvious (before springing for a new mag)
Click to expand...

You could also blame worn P-leads. Those are the wires between the mag and ignition switch. They can chafe and fray and suffer intermittent shorting to whatever they're chafing on, the shielding can fray and cause shorting at the ends, and so forth.

Most of the time it's burned contact points, a worn points cam or rubbing block, oil inside the mag, or a worn distributor rotor or bearings or carbon brush. Once in a while the capacitor will be defective.

If the points burn or the cam wears, the mag's E-gap internal timing changes and the spark gets weak. E-gap is pretty critical. If you find that the timing of the mag to the crankshaft is shifting, the internal timing is probably changing. If the timing gets advanced, the points are wearing. If it's retarded, the cam or rubbing block is wearing.

Dan
 
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