Piper cherokee engine roughness??

J

jmarine225

Pre-takeoff checklist
Joined
Jan 19, 2020
Messages
286
Display Name
Jmarine225
Good evening. Been a LONG day here. Curious on your inputs on this one as I’ve received one opinion so far which may be correct. Flew my Cherokee 140B down to the beach for the day. It was a hazy hot humid day. Departed around 7pm and climbing to 6,500 the sky was really hazy but smooth. Upon reaching my assigned altitude I began to reduce power to 24-2500 rpm’s when the plane began shaking as the engine was running very rough. I checked the rpm and it was now down to 2000-2100. I began to descend and turn towards a field when the engine recovered and was operating normally the remainder of the flight and taxi to the fbo. Post engine inspection did not reveal anything. I did a run up on the ground to 2100 with no issues. All gauges operating in the green the entire flight. Any thoughts??
 
That’s the consensus on it. I had my young daughters with me and was close to sunset so I didn’t want to take any chances and went to the field. Now I’m like, damn, never thought of carb ice in a Cherokee.

What to do from here now. Do I go back to the plane, run it up and if all is good depart again, based on probability of carb ice? Or have a mechanic check it out yet there is no mechanic on the field.

This is my first time I’ve ever had an inflight issue besides vacuum pump which was easily detected.
 
I have been there, my mechanic and I were not able to find anything wrong.
Then I was able to prove that carb heat fixed the roughness every time. No need for a mechanic! Lol
Now I know when it is likely and stay ahead of it by using carb heat before it can happen.
I say go fly it and be ready with carb heat when needed.
 
What was the OAT on the ground and at altitude? What was the dew point spread?

Go back to the plane, and SOLO, run it up with a carb heat test, and if all is good takeoff for a short flight. If all remains good, go back, pick up the kids, and go home.
 

Attachments

Last edited:
I agree that Carb Ice is the likely culprit

However; a bad muffler may cause this as well.

With a cold engine slap the bottom of the muffler with an open palm.

If there is any “ clunking” further investigation is needed.

Inserting a yardstick up the tailpipe to check if the core is moving

would be my next step. I have a couple bad ones here and when the

core obstructs the outlet the engine loses power. There doesn't appear to

be any consistent pattern when it occurs.

I’ll second Dom on SOLO.
 
Magman said:
I’ll second Dom on SOLO.
Click to expand...
Thirded. You're a test pilot until you can be assured that the issue is resolved. Suggest that you may also want to run carb heat just a little longer during the run-up than you normally would just to check the drop, to ensure you get rid of any ice which may have formed during taxi. As long as you're not in blowing dust and gravel, holding carb heat open for a slow count of ten shouldn't hurt anything and will help you know that you're not taking off with some ice already half-formed in the carb.
 
Appreciate everyone’s insight. I grabbed a rental car and drove my daughters home that night. Was a win for them because they love rental cars. Haha.

So apparently Signature ACY no longer has maintenance on the field. Anyone know of a mechanic who would cover over just to check the carb, throttle etc to. To be sure it’s all good, no issues
 
With all due respect, I don't think you need a mechanic, I think you need to take a look at the chart that Domenick attached above, or find another. I'd bet lunch you were in a range where carb ice was likely in a descent. Next step, again not trying to be a jerk, review procedure on rough engine in your aircraft, which probably includes carb heat as one of the first steps. And...carb heat is easy to check on the ground at runup and should be done at every runup.

Anyone that thinks you can't get carb ice in a Cherokee is wrong.

Very happy you brought you, the plane, and the daughters in safe.
 
Another thing to look at is a leak in your primer line or it wasn’t fully locked
 
jmarine225 said:
Good evening. Been a LONG day here. Curious on your inputs on this one as I’ve received one opinion so far which may be correct. Flew my Cherokee 140B down to the beach for the day. It was a hazy hot humid day. Departed around 7pm and climbing to 6,500 the sky was really hazy but smooth. Upon reaching my assigned altitude I began to reduce power to 24-2500 rpm’s when the plane began shaking as the engine was running very rough. I checked the rpm and it was now down to 2000-2100. I began to descend and turn towards a field when the engine recovered and was operating normally the remainder of the flight and taxi to the fbo. Post engine inspection did not reveal anything. I did a run up on the ground to 2100 with no issues. All gauges operating in the green the entire flight. Any thoughts??
Click to expand...
That exact same thing happened to me one time in a 140 and I did the same thing- precautionary landing. Run ups were fine after that and I flew it another 150 plus hours with no incident. Consensus was temporarily stuck valve (it had a new cylinder) or carb ice.
 
Carb ice has caused a lot of accidents. By some estimates, it is the most common cause of power failures. It is not well understood by some pilots, and their instructors are sometimes no smarter. Carb ice can happen on a 100-degree day, too. The temperature of the air passing though the carb drops by as much as 70°F as its pressure is reduced in the venturi or around the edge of the throttle plate (pressure drop causes a temperature drop, basic physics) and fuel evaporating in the airflow drops the temperature further as the fuel converts from liquid to vapor, absorbing heat from the air in the process.

So it's not just a wintertime thing. And if the engine starts losing power, pull that carb heat immediately, since if the engine keeps dying, the exhaust system cools off and now you haven't got the heat to clear the ice. The carb heat will increase the RPM drop and roughness as the ice melts, and then it will clear up. Too many guys push the carb heat off when things get worse, and that, too, causes accidents.

When you're going flying, you should know the atmospheric properties in play at the time, right? Two of the things in a METAR are the temp/dewpoint numbers, and when they are close together, carb ice is a real possibility. One should not be caught unawares. The closest airport to me has this METAR at the moment:

1784400374377.png

In the red we see that the temperature is 29°C and the dewpoint is 21. That is unusually close for this country; it's been very humid for weeks now. Let's check a carb ice chart:

1784400744972.png

I've drawn in the lines for the temp and dewpoint. We find ourselves at risk of serious icing especially at descent power. The partially-closed throttle plate causes the air to accelerate tremendously between the plate's edge and the carb bore wall, and that's where the ice will form. It starts to choke off the airflow and the engine starts to lose power.

Some engine/airframe installations are worse than others. Some engines are worse than others. Use the chart in your POH, as it will be made for your airplane and might be somewhat different than this one.

There are some dangerous myths out there. One is "Cherokees don't get carb ice." Another is "Lycomings don't ice up much." Both are dangerous. I have had carb ice in both cases. If you have a carb, you can get carb ice. Period.
 
Last edited:
Dan Thomas said:
Carb ice has caused a lot of accidents. By some estimates, it is the most common cause of power failures. It is not well understood by some pilots, and their instructors are sometimes no smarter. Carb ice can happen on a 100-degree day, too. The temperature of the air passing though the carb drops by as much as 70°F as its pressure is reduced in the venturi or around the edge of the throttle plate (pressure drop causes a temperature drop, basic physics) and fuel evaporating in the airflow drops the temperature further as the fuel converts from liquid to vapor, absorbing heat from the air in the process.

So it's not just a wintertime thing. And if the engine starts losing power, pull that carb heat immediately, since if the engine keeps dying, the exhaust system cools off and now you haven't got the heat to clear the ice. The carb heat will increase the RPM drop and roughness as the ice melts, and then it will clear up. Too many guys push the carb heat off when things get worse, and that, too, causes accidents.

When you're going flying, you should know the atmospheric properties in play at the time, right? Two of the things in a METAR are the temp/dewpoint numbers, and when they are close together, carb ice is a real possibility. One should not be caught unawares. The closest airport to me has this METAR at the moment:

View attachment 153578

In the red we see that the temperature is 29°C and the dewpoint is 21. That is unusually close for this country; it's been very humid for weeks now. Let's check a carb ice chart:

View attachment 153579

I've drawn in the lines for the temp and dewpoint. We find ourselves at risk of serious icing especially at descent power. The partially-closed throttle plate causes the air to accelerate tremendously between the plate's edge and the carb bore wall, and that's where the ice will form. It starts to choke off the airflow and the engine starts to lose power.

Some engine/airframe installations are worse than others. Some engines are worse than others. Use the chart in your POH, as it will be made for your airplane and might be somewhat different than this one.

There are some dangerous myths out there. One is "Cherokees don't get carb ice." Another is "Lycomings don't ice up much." Both are dangerous. I have had carb ice in both cases. If you have a carb, you can get carb ice. Period.
Click to expand...
Upon checking my notes the temp/ dew point was 25/18. Plottin that into the chart puts me into a bad spot with cruise power, which is where it began to develop roughness. Appreciate all the insight on the Cherokee carb ice issues. Again,after flying for over 10 years I was always told will never happen in a Cherokee and no need to use the carb heat. Good learning lesson for me.
 
TylerSC said:
That exact same thing happened to me one time in a 140 and I did the same thing- precautionary landing. Run ups were fine after that and I flew it another 150 plus hours with no incident. Consensus was temporarily stuck valve (it had a new cylinder) or carb ice.
Click to expand...
Appreciate the info. Curious though. Was there an incident after the 150 plus hours?
 
jmarine225 said:
That’s the consensus on it. I had my young daughters with me and was close to sunset so I didn’t want to take any chances and went to the field. Now I’m like, damn, never thought of carb ice in a Cherokee.

What to do from here now. Do I go back to the plane, run it up and if all is good depart again, based on probability of carb ice? Or have a mechanic check it out yet there is no mechanic on the field.

This is my first time I’ve ever had an inflight issue besides vacuum pump which was easily detected.
Click to expand...

You did the right thing by landing at the nearest field, but I am curious, did you follow the emergency checklist items? - which should have included carb heat, magnetos, mixture etc.. It's possible you could have identified and resolved the issue right away, and continued on.
 
This is NOT normal for a Cherokee Carb Heat Muff!

It DID fly this way though!
 
Thanks for the help. Picked up the aircraft and flew her home today. Outside temp was 85. During runup, carb heat is all over the place. 500 drop back to 100 then 300 bouncing all over. Not sure if it’s from the heat and radiant heat. Departed reached cruise at 6500 and same thing again upon reducing throttle. Engine began running really rough. Added carb heat for a bit didnt help much. Adjusted the mixture and had to lean it out quite a bit. Engine ran smooth then for entire flight until I was descending and richening the mixture. Had to lean it out a bit again. So at this point I believe something with mixture at the least, or throttle and mixture?
 
jmarine225 said:
Thanks for the help. Picked up the aircraft and flew her home today. Outside temp was 85. During runup, carb heat is all over the place. 500 drop back to 100 then 300 bouncing all over. Not sure if it’s from the heat and radiant heat. Departed reached cruise at 6500 and same thing again upon reducing throttle. Engine began running really rough. Added carb heat for a bit didnt help much. Adjusted the mixture and had to lean it out quite a bit. Engine ran smooth then for entire flight until I was descending and richening the mixture. Had to lean it out a bit again. So at this point I believe something with mixture at the least, or throttle and mixture?
Click to expand...
See my post above. If you have a leak in your primer line or if the primer valve itself isn't seated or leaks this is the behavior you'll see. It's running overly rich due to extra fuel when that happens and you'll see stumbles at lower power settings but runs OK at higher.
 
Had my base mechanic check this out. Ground run, compression check and fuel lines all good. Carb heat check was good. Anyone have any other suggestions on this one?
 
Reread the dozen posts above that mention carburetor icing. Your Monday post indicates you didn't consult a chart during or after that flight that identifies likely icing conditions. Did you? If not, you've missed the opportunity to identify the most likely explanation for the engine roughness.

Until you perform a flight that mimics the atmospheric conditions, your initial altitude, follows the same descent profile as the problematic flights, and you apply carb heat as recommended by the many sources available, your mechanic, and the readers of this forum are just throwing darts.
 
3393RP said:
Reread the dozen posts above that mention carburetor icing. Your Monday post indicates you didn't consult a chart during or after that flight that identifies likely icing conditions. Did you? If not, you've missed the opportunity to identify the most likely explanation for the engine roughness.

Until you perform a flight that mimics the atmospheric conditions, your initial altitude, follows the same descent profile as the problematic flights, and you apply carb heat as recommended by the many sources available, your mechanic, and the readers of this forum are just throwing darts.
Click to expand...
Sorry, I thought I made it clear in follow up posts. I again, flew the aircraft, climbed to the same altitude as upon leveling g off experienced the same roughness. Carb heat in did little to nothing to help. Actually leaning the mixture quite a bit had the engine running smoothly until my decent at approximately 3000 where I had the lecture back to full rich, it began stumbling again and had to lean it out.
 
I sounded a bit like a dick in my earlier post, and I apologize for that.

The improvement upon leaning is an indication your carburetor may be allowing fuel to enter the cylinders in excess of normal operating requirements, or, as @Pugs mentioned, it could be a leaking primer. Check the primer if you haven't already, and if you have, check it again. Look for torn or excessively worn o-rings. The repair kit is less than ten bucks. If you cant find an entry in your engine logbook stating the o-rings have been changed in the last five years, change them, and go fly the airplane. I'm not a proponent of shotgun parts changing, but the low cost to install the primer repair kit makes it a worthwhile effort.

If that doesn't fix the problem, it may be necessary to remove the carburetor and check it for proper operation. Discuss all of this with your mechanic again. I hope you'll find a resolution.
 
Also, does power output seem normal to you before the engine gets rough?

Long shot, but another way to have a rich mixture is with an intake airflow restriction. It would have to be downstream of the carb heat box since it seems to be the same with or without heat.
 
Did you check the integrity of the carb heat cable and connection to the flapper valve? When you say "Carb heat was all over the place on runup" - does that mean that when you activated carb heat, the rpms fluctuated vs stabilizing at a lower RPM? Another thing to check would be the integrity of the tubing from the carb heat valve to the intake. Might be a tear or something causing a leak.
I'd find a cause before flying again.
 
Thanks for all of the input. The mechanic checked out the fuel lines, primer, carb heat, plugs, compression check and everything was okay he stated. He stated if it continues the carb may have to be pulled and sent out for overhaul.
 
The issue does seem to be related to mixture. It may not seem intuitive, but check the induction tubing for leakage. Having an air leak can cause havoc with engine mixture. Ensure all the band clamps are tight. You may even want to pull the soft tubes and visually inspect for places that have not sealed to the metal.
 
3393RP said:
as @Pugs mentioned, it could be a leaking primer. Check the primer if you haven't already, and if you have, check it again. Look for torn or excessively worn o-rings. The repair kit is less than ten bucks. If you cant find an entry in your engine logbook stating the o-rings have been changed in the last five years, change them, and go fly the airplane. I'm not a proponent of shotgun parts changing, but the low cost to install the primer repair kit makes it a worthwhile effort.
Click to expand...
Those O-rings only keep the fuel in the primer and the air out of it. They have nothing to do with fuel passing too easily through the primer to the engine. That is controlled by a spring-loaded needle that seats in the outlet in the bottom of the primer bore when the plunger is fully in and locked. If the spring in that plunger was broken, the needle wouldn't seat properly, or if there was debris under the needle, extra fuel could be drawn into the intake. Those two things need checking.

In the carb, there are multiple things that could be faulty. Bad accelerator pump springs. A loose mixture valve sleeve. A main metering jet coming loose. A mixture control lever loose on the valve shaft. An overhaul would likely be a waste of money; all it needs is a carb tech to open it and check for those defects.

One final warning: As I have posted here many times, 90% of performance problems are electrical, and in that engine, that means ignition. If the mags are weak, varying mixture will cause misfiring and roughness and RPM drops. That spark needs to be hot, meaning that the points and condensers need to be in decent shape, and the internal E-gap setting must be right. When the throttle is pulled back, the mixture normally tends to lean out. Full throttle is richer, carb heat is richer, and really rich mixtures can also be hard to ignite with a weak spark.

So the question should be asked: When were those mags last off for internal inspection? When were the plug leads last tested? How old are those spark plugs, and are they the old-style Champions? Terrible plugs.
 
Domenick said:
The issue does seem to be related to mixture. It may not seem intuitive, but check the induction tubing for leakage. Having an air leak can cause havoc with engine mixture. Ensure all the band clamps are tight. You may even want to pull the soft tubes and visually inspect for places that have not sealed to the metal.
Click to expand...
Except that the engine smartens up when it is leaned. Leaking induction makes the engine run lean already.
 
Dan Thomas said:
Except that the engine smartens up when it is leaned. Leaking induction makes the engine run lean already.
Click to expand...
Which was why I labeled the suggestion not intuitive. I'd still check it. Weird/inexplicable stuff happens and it's an easy check.
 
Dan Thomas said:
Those O-rings only keep the fuel in the primer and the air out of it. They have nothing to do with fuel passing too easily through the primer to the engine. That is controlled by a spring-loaded needle that seats in the outlet in the bottom of the primer bore when the plunger is fully in and locked. If the spring in that plunger was broken, the needle wouldn't seat properly, or if there was debris under the needle, extra fuel could be drawn into the intake. Those two things need checking.

In the carb, there are multiple things that could be faulty. Bad accelerator pump springs. A loose mixture valve sleeve. A main metering jet coming loose. A mixture control lever loose on the valve shaft. An overhaul would likely be a waste of money; all it needs is a carb tech to open it and check for those defects.

One final warning: As I have posted here many times, 90% of performance problems are electrical, and in that engine, that means ignition. If the mags are weak, varying mixture will cause misfiring and roughness and RPM drops. That spark needs to be hot, meaning that the points and condensers need to be in decent shape, and the internal E-gap setting must be right. When the throttle is pulled back, the mixture normally tends to lean out. Full throttle is richer, carb heat is richer, and really rich mixtures can also be hard to ignite with a weak spark.

So the question should be asked: When were those mags last off for internal inspection? When were the plug leads last tested? How old are those spark plugs, and are they the old-style Champions? Terrible plugs.
Click to expand...
Mags were installed in the past few years. Have maybe 150 hours on them at most.
 
Hang 4 said:
Did you check the integrity of the carb heat cable and connection to the flapper valve? When you say "Carb heat was all over the place on runup" - does that mean that when you activated carb heat, the rpms fluctuated vs stabilizing at a lower RPM? Another thing to check would be the integrity of the tubing from the carb heat valve to the intake. Might be a tear or something causing a leak.
I'd find a cause before flying again.
Click to expand...
Mechanics took a look and said there does t appear to be any issues with it. I can only assume they checked the cable, flapper, blockages etc. they ran it up and carb heat was stable on the ground during run up.
 
I had the same thing happen on my pa-28 140, turned out the carburetor bowl screws had come lose and there was a gap between the gasket and bowl, just dumb ass luck I found it…….
 
Garee said:
I had the same thing happen on my pa-28 140, turned out the carburetor bowl screws had come lose and there was a gap between the gasket and bowl, just dumb ass luck I found it…….
Click to expand...
Thanks for this input. Was this found by yourself or a mechanic? I had a mechanic look at the aircraft but not sure exactly if he checked this. If found by yourself I’m curious if it’s something I’d be able to locate and repair.
 
Found it myself. Yep, I just tightened the screws back up snug
 
It’s been 20 years but I think that’s exactly what happened
 
Even if you are not mechanically inclined, it's a good idea to look at everything up close ever once in a while like during oil changes. They call me mr obvious.

Interesting that it needs to go to a carb specialist if it's leaking.
I would think the OP would smell fuel sometimes if it were leaking?
 
You must log in or register to reply here.
Back
Top