Carburetor heat problems...

OK, now that I have the mags redone, new spark plugs and all...and another problem has surfaced.

When I apply carb heat on initial runup I get a 400 rpm drop and the engine runs really rough. I have noticed this over the course of the last few months. This happens when the engine is cold, but gets even worse when it is warm. I noticed this this morning when I was leaving ANE for Duluth, but the runup was otherwise uneventful.

I decided to pull the carb heat on when I was in cruise and the engine ran extremely rough, and it didn't get better with time (I have experienced carb ice in the past with other carburated aircraft, with the rpm dropping, then rising...this time no rise was noted).

Of note was an experience when flying back from Ely and the Fall Colors Tour....I was paged to call the ER while in flight, so I landed at Cloquet and didn't dare shut down the engine, so I pulled onto the grass to speak with the ER physicians about my patient. After completing that call I checked for traffic and pulled onto the runway, gave er full throttle and the engine was running so rough I aborted the takeoff, ran up the engine (wouldn't go up to 1700 rpm), leaned it out to clear the plugs (I thought) and tried the runup again. Again, really rough running engine, couldn't get up to even 1400 rpm, so I taxied onto the ramp and looked things over and discovered my carb heat was full on. Felt stupid (I had left the carb heat on upon landing), pushed in the carb heat and ran it up. Ran perfectly well, I flew home without problems.

I inspected the lever and cable from the carb heat control and the arm which moves the carb heat flap and it moves full travel.

Any suggestions or explanations? I am going to ask Larry Cassem to take a look this weekend, and am a bit nervous about the flight home tomorrow...will leave the danged carb heat lever alone until I get this figured out.

Thanks.

Randy

Something come loose blocking the scat tubing - collapsed tubing, something loose and blocking in the muffler shroud?

CARB HEAT PROBLEM

Had very similar problem recently and found two halves of carb were loose. Happened on long ferry flight. Got progressively worse, Started with 75 RPM drop and went to 250. Also idled rough. Perfect now.

On a related note, the pawl on my carburetour heat bellcrank is no longer engaging the detents on the heat box. It creeps on fairly rapidly. Ideas? Bend the pawl? Clickstop carb heat cable/knob? If so what length?

Not to hijack Randy's worthy thread....

Thanks. cubscout

if I read your thing right,

your muffler can is broke inside the shroud on right side.. assuming normal cub style muffler.....

would be my first guess, since you say it runs worse than normal with carb heat on( you are sending burnt gases to carb with carb heat on, not hot fresh air)

gonna go reread your post....

reread your post,

yup, your muffler is cracked or broken on carb heat side,

fix it before you fly I would think,

DO NOT TURN ON BACKSEAT HEAT if so equipped....

at least pull shroud open and find out whats up on that right half...

How cool is it - - -

That MKS Mike is one of those who share so generously!

One of the main reasons I hang out here is to learn stuff, and you guys are great providers.

cubscout said:

On a related note, the pawl on my carburetour heat bellcrank is no longer engaging the detents on the heat box. It creeps on fairly rapidly. Ideas? ....
Thanks. cubscout

Click to expand...

pull carb heat on...

now kink carb heat cable and shroud slightly...

when you push cable in it will be held snug now.... no joke!

Randy,

You may want to grab the dome on the end of your round air filter and see if you can force movement back and forth. If so, your carb halves are starting to separate and need the be re-tightened.

Good luck,

Jerry

had the same problem once...it was caused by a crack in the exhaust pipe feeding the carburetor with exhaust gas.

Post deleted,
Randy,

My bad, my posting was supposed to be a PM. My public apologies for the strong opinion that was supposed to be private. Sorry buddy.

Jeff

Wow what a place! Oct 1, 2:30 first post with symptoms, By 3:30 2 probable answers with Mikes being 90% the right one I bet. Lets see here. Possible carbon monoxide leak into cabin, possible partial loss of power leading to unintended landing with what consequences?, pilot "stuck" at symptom level continueing to fly (this is not a criticism of non mechanic pilot, but bet he came up to speed after Mikes post). All I can say is wow!! Also someone should keep track of these wins.

Going on for months? Wow. I must take mechanical problems more seriously than some of you. If I had an indicator like that I may fly it home, after I ruled out impending failure, but that would be the last flight before the problem was addressed. How far does a plane need to deviate from standard run-up parameters before you ground it? I mean that as a general question to the forum, not as an attack on Randy.

Stewart

StewartB said:

Going on for months? Wow. I must take mechanical problems more seriously than some of you. If I had an indicator like that I may fly it home, after I ruled out impending failure, but that would be the last flight before the problem was addressed. How far does a plane need to deviate from standard run-up parameters before you ground it? I mean that as a general question to the forum, not as an attack on Randy.

Stewart

Click to expand...

........

Um, like a change in starting characteristics...............
......

Not offended by comments. Taking these to heart, though.

The carb heat findings were only slightly perceptible, beginning a monyh or two ago. Had a slightly lower drop than usual when doing the sunup, not enough to alarm me. I notified my mechanic, he came out and checked it out and he could find nothing wrong, didn't think it to be significant.

It became a more significant drop on the trip up to the Fall Colors Tour, when the mag problem and starting problem, and It was mentioned to the mechanic who removed the mags, plugs. He also felt it not significant. My flight yesterday led to the current thread.

I appreciate the suggestion to have another set of eyes look this over. I feel I have acted appropriately in seeking help from my mechanic when a noticeable change happened. I accept the criticism that I should get a second opinion if not comfortable with the first.

I am having this looked at tomorrow, and wanted to provide the mechanic any help you fellows could offer, hence this posting.

Thanks.

Randy

Randy,

Sounds to me as if you've done precisely the right things.

Often, these failures manifest themselves gradually, over a period of time. If this is indeed a bad muffler, I could certainly see how what you describe could take a while to really get your attention.

Also, when talking to mechanics, the PILOT has the advantage that he's seeing the airplane's operating parameters on every flight, whereas the mechanic only sees a snapshot of the conditions.

So, since a drop on an individual mag is acceptable up to -175 rpm, and no more than 50 difference, it would be really easy for your mag drop to start off at -90 each side, and as the muffler fails, gradually creep up to -150 a side.

That would/should alert the pilot, but a mechanic looks at it, and it's within normal operating parameters--all is well.

It's not until these things get totally out of whack that they really become apparent enough to diagnose and fix. Sounds like you are there now.

Exhaust leaks can be pretty hard to detect quickly. I was once exposed to CO poisoning in a C-206, which made me MUCH more conscious of exhaust leaks/problems, and it does sound like that may be what you have.

Take care,

MTV

I had the same problem it was 4 loose screws holding the float bowl on the carb. If leaning the mixture helps with the rpm drop that is where I would look. Just take a light and look for gas stain on side of carb. For me it was the only place not covered in oil that was the clue :lol:
Denny

A friend had a similar issue one thing he found was the carb heat cable was moving a little and making contact with a p lead shorting out one mag, also had a problem with the carb.

updates?????

Update.

Darrel Starr was waiting for word of my safe return from Duluth, but in the meantime had checked with the IA who had done my last few annuals (who also does Darrel's), running the scenario by him...particularly Mike's comments that this was a leaky muffler/exhaust problem. The IA recalled specifically swinging my engine, disassembling my the shroud and muffler and finding it in excellent shape. Further, Darrel called the IA who had performed the removal and replacement of the mags a few days previously (also the IA who had installed Darrel's engine), and it was Larry's best guess that the problem would lie in separation of the two halves of the carburetor.

I called Darrel immediately after returning from Duluth and he asked me to stay on the phone with him while I checked out Larry's diagnosis. He asked me to grasp the air filter dome and see if it would wiggle...it did. He asked me to then open the cowl and see if the two halves of the carburetor moved when I wiggled the dome...the upper one did not...it was solidly attached to the engine case. The lower half did move, and it was very clear that there was a gap between the two halves, about 0.05 mm in thickness. Darrel and I felt a great relief in having confirmed Larry's diagnosis (as well as Stearman600 and cubflier)! We speculate that this slowly developing problem contributed to my inability to quantify and detect a problem in the making. It was slow, insidious and I simply failed to notice the changes in rpm drop over an extended period of time.

I spoke to Larry, who will remove the carb Monday morning and carry over to Tim at Bolduc Aviation to give the carb a good going over and to replace the gasket between the two carb halves and replace the washers which are supposed to help hold the two halves together.

It is Larry's feeling that the weak point of this system lies in these washers, which are designed to prevent the bolts holding the two halves together from backing out. They are unique in that they consist of a series of small tabs which are to be bent up once the bolt is torqued into place, acting as a safety wire, as it were, but without the safety wire. Larry has seen this scenario in the past, and thinks it is likely due to inappropriate reuse of these washers following disassembly and reassembly of the two halves without using a new washer. As the metal is bent back and forth a time or two it weakens, can flatten out and the head of the bolt can back out of the assembly, hence allowing the halves to separate partially...in THIS case enough to permit air to suck in through the separation, leading to the symptoms. Darrel speculates that this problem may be worsened by deterioration of the gasket, as well. When I looked at the washers I could clearly see that Larry's hypothesis is likely, in that the tabs had flattened out.

It is my expectation that Larry will have the carb off, have the carb inspected, replace the gasket, replace the washers and reinstall the carb by Monday afternoon, when I can then get out to the airport to fly the airplane. I will see if Darrel Starr will be kind enough to take some photos of the carb and the flattened washers so you can see what we found.

I took several things from this experience.

I need to be more aware of progressively small changes in the performance of my aircraft. While inexcusable, it is explainable given the slow rotation of the bolt heads out of the housing which holds the two halves of the carb together. I think I was distracted by the problem with the hot starts, which turned out to be a magneto problem.

I also came away with a greater appreciation for the mechanics who examine and maintain my super cub. These two fellows are experienced, are skilled and they both happen to know that my life depends upon their skills, their expertise and their professionalism. They also happen to know that I use my aircraft to provide medical care for others, and I think this makes them even more vigilant, if that is possible. They are not perfect, nor am I, but they are willing to accept the fact that while we live in an imperfect world it is important to strive for perfection.

I am embarrassed, frankly, to have questioned to some degree the work that these two fellows provided me in the past. How could they have missed these things while doing the annuals, the maintenance, the repairs, while experts on this site and elsewhere seemingly easily provided confident diagnosis? How indeed. It is easy to have 20/20 vision in hindsight, or from hundreds of miles away. I think it is important to understand that what we offer here are possible explanations, potential answers, potential solutions. It is pretty easy to be an armchair quarterback.

Having written these things, though, I gotta tell you, you people who stick your neck out to offer possible answers and solutions on this website are pretty danged awesome. While I took some heat for my inattention to warning signs, I cannot begin to tell you how much I appreciate the feedback and ideas that were offered here. How reassuring it is to be able to sit down to my computer and present symptoms, findings and to get help in synthesizing a list of differential diagnoses, and treatment plans!

Oh, that the world of medicine was as open to airing our dilemmas as you folks are here. I thank you, kindly, for your words and wisdom and diversity of thought.

Randy

cool, waiting for results then,...

i may learn something new to worry about...

i never really care about the loose carb halfs at all, unless leaking badly.... just figured standard cub stuff.....

I heard that you offered a big prize for the one who guessed it right. If you need my address...

Seriously Randy, just so you know, this is common problem that can sneak up on anyone. When it happened to me it got real bad all of a sudden. Now part of my preflight is to try to shake my airfilter dome.

Glad you figured it out.

Jerry

hmm. re-read all posts and then wandered off in netland to see what I could learn about updraft aircraft carbs. Now I can't ever imagine where the two halves of a carb starting to seperate is a good thing and taken on it's own the loosening bolts would cause all kinds of mischief BUT what is the connection to the carb heat? Did pulling carb heat on "cock" the loose carb halves apart to one side changing float setting or allow air unmetered where it doesn't belong ? Or?? The fact that a couple of posters had had the same symptoms before and found the loose bolts is fine but what is the theory behind the "rough" running and the connection to the carb heat? Just curious.

Randy, Like your comment about monday morning quarterbacks, I had a rpm drop once and no one ever figured it out but upon doing a overhaul found a couple of things that may have been the problem. It did go away after the rebuild. I know of a guy that had a problem with an exhaust crack that ran fine on run up but about the time he got into the air the engine would quit. Took them a few day's to find that and it was one of the last things they looked at.

qsmx440 said:

hmm. re-read all posts and then wandered off in netland to see what I could learn about updraft aircraft carbs. Now I can't ever imagine where the two halves of a carb starting to seperate is a good thing and taken on it's own the loosening bolts would cause all kinds of mischief BUT what is the connection to the carb heat? Did pulling carb heat on "cock" the loose carb halves apart to one side changing float setting or allow air unmetered where it doesn't belong ? Or?? The fact that a couple of posters had had the same symptoms before and found the loose bolts is fine but what is the theory behind the "rough" running and the connection to the carb heat? Just curious.

Click to expand...

Applying carb heat admits warmer air than ambient to the carb.The warm air is not as dense so it richens the mixture even more.

Bill

Randy, that same thing happened to me 3 or 4 years ago at New Holstein. I can't remember all the symptoms, but it didn't run good. The little tabs that hold the carb. screws were't holding. The bottom half was loose and I mean loose . As I remember Behindpropellers helped me get things back together, but it took a while to figure it out. Now when I check the oil, that's one of the things I look at.

Tim

The older carburetors are safety wired and they do the same thing. I think the carburetor bowl gasket shrinks. I would not reuse the carburetor lock tabs or any lock tab for that matter. I had to replace the leather seal around the accelerator pump shaft recently and bought the lock tabs from A/C Spruce and they weren't expensive.

Great to know the symptoms of this problem.

I have found the "amazing shrinking bowl gasket trick" mostly on aircraft that go from a steady diet of mogas back to 100LL. For those of you that run lots of mogas you may also see that your old school type gaskolator bowl gaskets swell up making it hard to re-install after inspecting the gaskolator screen. I've also had problems with the seals in the fuel tank sight gauges when switching between mogas and 100LL.

Jason

Applying carb heat admits warmer air than ambient to the carb.The warm air is not as dense so it richens the mixture even more.

Bill[/quote]

Thats one of my thoughts that prompted my original question. Another poster said excess air was allowed in (if I remember the quote correctly). That makes sense actually and that would (if so) cause the engine to run leaner (think manifold leak) but if so carb heat should (enrichen) improve the gas / air ratio.? The reason for my nit picking on this is it's best to really understand a problem to declare it solved and for future diagnosis of like problems. It would be interesting to look at the plugs after a few minutes of running. I guess we can leave this thread and just declare the two halves "loose and that ain't the way they were meant to be" and consider it done but inquiring minds (at least mine) want to know. Rich or lean and why if lean wouldn't carb heat improve it? And Randy, the reasons in my case for monday morning quarterbacking and all the questions are 1. this can be life and death possibly for you and if a few feathers get ruffled getting at the cause so be it. It definently isn't personal on this end, 2. I have extensive professional motorcycle carb experience but almost zero aircraft carb knowledge and I'm going to have one of these lycomings on my experimental and be resposible for it and this thread might expose me to some of the inner workings of the updraft carb. 3. "It ain't over till it's over". Been a troubleshooter for 40 years (first mechanics and then board level RF electronics) and been fooled many times by the obvious solution. It's kind of funny how the more troubleshooting experience you get the more suspicious you get of the solution until it's been tested for a while. One solution/confidence builder from this thread is that several people have said they solved the same problem by fixing the loose carb. 4. Inside my head, niggling at the back of my brain, ugh won't go away "why does enrichening" ie applying carb heat, make it run "leaner? Please someone explain it to me so I canmove on to some other puzzle . dave

Dave, no sweat for me regarding ruffled feathers. I want people around me to feel free to offer criticism and concerns, both professionally and personally.

Regarding life and death, I flew with the windows open all the way home Friday! Steady, fresh, crisp air. Chose my route carefully, too. I don't subscribe to the motto "all's well that end's well."

Randy