Minor loss of power in flight

ircphoenix

ircphoenix

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ircphoenix
Alright party people,

Last night I took the girlfriend and one of the childrens up for a night flight.

The plane (Cherokee 140, O-320) didn't want to start up as easily as normal. Runup was normal. On climb out I thought I heard the RPMs drop a bit then come back up. Sometimes I chalk these things up to my imagination or head positioning causing the headset seal to break somewhat. As I'm flying I definitely hear the engine drop and resume... maybe 25-50 RPM. Not enough to notice anything on the RPM gauge. This occurs every minute or so. We get about 10 minutes out, and I don't feel comfortable continuing the flight, so I turn around and park it. The "issue" persisted all the way to the ground.

Now if it were just me, I would have continued on, leaned it out and run it hard. The plane had been sitting for two weeks, so I think it could have been plugs that were gunked up. It was SKC and 15/6, so I don't think it could have been carb ice. Gonna have the mechanic pull plugs this week and take a look. I was told after I landed that the battery was depleted earlier in the day and it had to be charged, so that's what I attribute the rough start to.

Any ideas?
 
What did the fuel pressure gauge say?
 
I had done my sump prior to flight and I didn't see any water... that's interesting with the 150 tank. I wonder if the Cherokee is similar. It was pretty minor, but consistent enough... and anything not firing at 100% is going to make me pretty squeamish when I have the girlfriend and her 10 year old on board.
 
Also how many hours on your mags?
 
Slicks or bendix?

Normal looking EGTs on runup? No major difference between L and R?
 
James331 said:
Slicks or bendix?

Normal looking EGTs on runup? No major difference between L and R?
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We're delving into specifics that I don't immediately recall but I *believe* they're Slicks. And EGTs were good during runup on both sides. There was about a 75 rpm drop for each side. Made sure I warmed the engine prior to taxi and run up as well.
 
Is the aircraft hangared? We've been getting so much rain here lately...if stored outside, maybe a little water got past the fuel caps.
 
SoCal RV Flyer said:
Is the aircraft hangared? We've been getting so much rain here lately...if stored outside, maybe a little water got past the fuel caps.
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It's outside. Hasn't flown after the rain till last night. Might be why it didn't want to turn over right away... and if the tanks have the same issue that the 152 does with the sump, then I can certainly see that being the problem even though my sump samples from the wings and engine checked out.
 
mulligan said:
Glad you landed safe.
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Me too! I would have turned back much sooner had I felt things were scary... but it was purely precautionary, and I was over Long Beach and Los Alamitos (if I have an emergency, I'm using it. Sorry Reserves) so I felt like I had time to decide if I was going to continue on or not.
 
Just ran by the airport on my way home from work. Sumped both tanks and the engine sump. I'd assume the flying would have moved any water towards the sumps. No water in the right tank or engine sump (which I used) and nothing in the left tank which I didn't use. *shrug*
 
ircphoenix said:
Just ran by the airport on my way home from work. Sumped both tanks and the engine sump. I'd assume the flying would have moved any water towards the sumps. No water in the right tank or engine sump (which I used) and nothing in the left tank which I didn't use. *shrug*
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:thumbsup:

Always best to start with the simplest and least expensive possibilities, and just keep systematically eliminating one at a time.
 
ircphoenix said:
It was SKC and 15/6, so I don't think it could have been carb ice.
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Why not. That is in the icing range — green zone on the following chart:

4720697_image0048fb0.gif


Did you even try the carb heat?
 
dmspilot said:
Why not. That is in the icing range — green zone on the following chart:

4720697_image0048fb0.gif


Did you even try the carb heat?
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I did not try carb heat. The O-320 in the Cherokee 140 isn't known for making ice with the way the carb is set up. I guess it's entirely possible. But the fact that it was dipping almost imperceptibly every so often as opposed to getting increasingly worse led me to believe it wasn't the carb. I don't have a whole lot of experience with carb icing though.
 
I can't think of any reason not to pull carb heat in that situation.

Might be a wise move to make that part of your default rough engine memory items.
 
One mag is probably intermittent and may have a bad coil. I had that on an old Aeronca Sedan on floats once. The engine rpm varied maybe 50-75 rpm in flight. A local tractor magneto repair facility fixed it to get me home. Back then a new coil cost $2. That was over 50 years ago......
 
Carb ice, should be the first thing to try when that happens, put it on and leave it on, if it is carb ice when you turn on carb heat chance are it will make the engine run worse for a short while as the ice melts.
 
The perils of training in an IO and buying an O I suppose.

Wouldn't carb ice cause the problem consistently instead of intermittently though?
 
Not necessarily - little icy tendrils forming, and periodically breaking and going into the intake, "cough," all normal again.
 
Its probably either fuel or ignition. Carb ice is possible for fuel, but unlikely at full throttle. Fuel pump possible, could be water in gas. Magneto is most likely suspect, but it could be a wire. Give it a compression test, check the plugs, pull a magneto and inspect it, maybe replace or rebuild it. Could be a lot of things. Intermittent makes it hard to troubleshoot. Do a full throttle runup somewhere there is no gravel or other planes. Stuck valve is possible.
 
Something to think about; There was a Tiger come in last week. Claimed it would run rough intermitently. Opened the cowl and it jumped right out at me. 2 Plug wires had either come loose, or had never been tightened. My guess is were finger tight and worked loose.
 
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bluerooster said:
Something to think about; There was a Tiger come in last week. Claimed it would run rough intermitently. Opened the cowl and it jumped right out at me. 2 Plug wires had either come loose, or had never been tightened. My guess is were finger tight and worked loose.
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Any info is good info. Gives me somewhere to start. Thanks!!!
 
I know this isn't going to help much but I recently changed the sump valve in my right wing. After draining all the fuel from the sump, I unscrewed the valve and got quite a bit of residual fuel still in the tank. This only made me realize that even though you sump the tanks, you may not be sumping the lowest part of the tank. o_O
 
Was the issue present regardless of whether the electric fuel pump was on or not?
 
My Cherokee 180 when I first got it would drop like 50 rpm in flight. I would add carb heat and nothing changed. It would dip rpm and come back. You could hear it drop and come back. Then a few months later on a XC flight with my wife I lost the left mag. Didn't know we lost it until we tried to head home. Plane refused to start! After a go around with the on field maintenance and a weeks time (I have a thread about it and the hatred I had toward the airport), they rebuilt it and never had the problem since. That next annual I sent my right mag off to MagnetoER in bald knob, AR and she purrs like a kitten.

Point Im trying to make...look and see how long its been since mags were serviced. If over 500 hours send them in. Mine had 900 on them.
 
As though I haven't had enough bills with this plane. A few months without something breaking would be LOVELY.
 
Timbeck2 said:
I know this isn't going to help much but I recently changed the sump valve in my right wing. After draining all the fuel from the sump, I unscrewed the valve and got quite a bit of residual fuel still in the tank. This only made me realize that even though you sump the tanks, you may not be sumping the lowest part of the tank. o_O
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Yes. Every aircraft I've worked on has required to be leveled to be completely sumped, and you'd have to go in with rags if you wanted to sop it all up.
 
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Alright so... mags were R&R'd for 500 hour inspection. That was a little less than 2 years ago and 110 hours ago. "Repaired, inspected, and tested in accordance with Slick service manual as per 500 hr insp."

Someone else flew my plane yesterday to Santa Barbara and experienced no problems. So who freaking knows. Maybe it was my imagination... 20 times in 20 minutes.
 
ircphoenix said:
As though I haven't had enough bills with this plane. A few months without something breaking would be LOVELY.
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I'm sure quite a few of us can identify with that sentiment. :)

These old airplanes need attention, and most often its a matter of dealing with the systems and steadily bringing each one up to reliable service. My problem is every time I must replace something inside the cowling on one side I usually don't wait long to do the same on the other: 24 new spark plugs, new ignition harnesses, new starters, replaced a number of the EGT probes, now working on replacing/rebuilding the 4 magnetos. Still more economical than a Cirrus ;)
 
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