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Carb Heat


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Yesterday it was about 20 degrees F and I went flying. I seemed to notice some engine roughness so I pulled my carb heat. It seemed to provide a little smoother engine and I picked up a little RPM on the tach. Does anyone else who flies in cold weather notice improvement to engine running by using carb heat? I didn't seem to notice this last winter. The only thing new between last winter and this winter is I've added the K&N filter. I have a 2006 CTSW.

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Hi Dick,

 

Make sure the vent tubes that come off the carbs and go through the airbox are centered so the holes in that tubing are fully in the airbox and one side or the other is not on the outside of the airbox. The 20F shouldn't have had anything to do with the roughness if the engine was up to temp.

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Hi Roger. The balance tubes were centered. It wasn't the slight roughness that I'm thinking about but it's the fact that the engine picked up some RPM's when I pulled the carb heat. This is normally a sign that there's some ice in the carbs on a conventional engine. With our CTSW's, pulling carb heat just opens a trap door to let ambient engine compartment air into the airbox so it's not that we're really adding a lot of heat to the inet air. Maybe I had some extra water in the 10% alcohol Mo-gas fuel I'm using and this gave me some fuel line icing? We're in for about 3 months of freezing weather now so I'll have plenty of times to investigate this. :-(

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It wasn't the slight roughness that I'm thinking about but it's the fact that the engine picked up some RPM's when I pulled the carb heat. This is normally a sign that there's some ice in the carbs on a conventional engine.

 

Well, sort of.

 

If there is carb ice, applying carb heat will initially cause an rpm loss and possibly near total power loss as the ice is melted and ingested by the engine. Any gain in rpm will only occur after all the ice is gone. SOP is also to always use FULL carb heat when you need it - partial application may in fact make things worse.

 

I would take the rough engine kinda seriously - it is almost certainly not just carb ice. And remember, carb ice is more related to humidity that absolute temp. In fact, very cold air holds less moisture so I believe carb ice is less likely when its very cold.

 

Once I flew my Grumman Tiger with my family from from S FL to CT for XMAS with relatives. On startup to fly home, the engine seemed a bit rough initially. I chalked it up to the plane not being used to cold weather, and it had smoothed out by the time I did the runup.

 

Then, about 10 minutes after takeoff all hell broke loose -RPM loss, backfiring, vibration, you name it. This at about 6,000' over Long Island Sound. I declared an emergency and headed to the nearest airport. I had enough power to arrive over it with sufficient altitude and circled down with partial power for an uneventful landing. Upon deplaning, there was oil covering the nosewheel fairing, and I found this under the cowling:

 

5250133958_8ef94615de_z.jpg

 

A frozen exhaust valve had caused the pushrod to bend. I still have the pushrod and the tube as a souvenir.

 

My point? Engines rarely fail with no warning at all - so we should take those warnings, even subtle ones, seriously.

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In most cases turning on the carb heat allows warmer less dense air into the cylinders. If there was a normal mixture control like traditional aircraft engines then this decrease in air pressure will lead to a richer mixture which depending on your phase of flight and throttle setting would normally lead to lower rpm readings. In the case of the Bing carbs your mixture is attempting to compensate for and pressure changes but there is also in inevitable lag with this. In the case of our carbs, when we apply carb heat, the initial inrush of warm air leads to higher internal cylinder temps which increases power temporarily but this rpm increase should again drop after the mixture has stabilized.

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Dick, one of the things you said was you added a K&N filter. If you had to much of the K&N goo on the filter It might not let enough air in on these cold dry days. Pulling the carb heat by-passes the filter allowing more air to the engine. This could account for the RPM rise when pulling carb heat. Tom

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Thanks for the thoughts on this. I didn't get a good chance to delve into this and will have to fly some more in cold weather and see what happens with/without carb heat applied. Might have some more info to discuss with you guys. My K&N filter is installed straight out of the box so it has had the "goo" applied by the factory. Not to say that the factory might have overapplied it? I figured that our situation of just having a trap door on the airbox and the Bing compensating carbs would add a little curve to the standard application of carb heat and Jeremy's thoughts on this are enlightening. In this situation, I recall that initial application of carb heat caused an increase in RPM's and this seemed to stay and not go away but I need to revisit this and give things a longer time than I did originally.

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  • 9 months later...

Is there anything to look for then? SLightly lower EGT due to the enriching action? I haven't noticed much there either. Maybe would have to wait 2 minutes or something. It feels sort of like the door close buttons on elevators. They are just there to make people feel better.

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Nothing will happen with rpm or anything else. You get hotter air into the airbox, but the carbs are so hot you won't see any difference. Our engine is so tightly cowled and hot inside I don't know if carb ice could even form. After you shut down your engine put your hand in the oil check door on the carb. It's hot.

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  • 4 years later...

I know that this thread has 5 year but I hope in all that time someone found any reason for it.

Today I flew about 1.5h quite nice weather, 14°C.

I changed few months ago my oem filter to a K&N. Exactly same facts that original question. I wanted to test any rpm drop when heating on so at 5000 rpm, 210km/h, i set the autopilot, hold altitude and pulled carb heat, my surprise was exactly the same happened, heat off, 4990 rpm, heat on 5010 rpm and the most weird thing, smoother engine.

I did it 4 times, always same results.

Any idea about happens?

KN filter has original oil, no extra oil added.

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Adding carb heat will richen the engine mixture with the hot air. The rpm drop you saw is normal for our CT's. It could be a tad more or less. It isn't an exact science. 

 

14c + 46F. You went from a cold air intake to hot air and this affects the running. Cold air is better and causes the engine to be a little leaner versus hot air. Nothing to worry about. Maybe a carb sync is in your immediate future.

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Thank you Roger.

 

I balanced carbs few hours ago, about 10h. But it was summer and quite hot, not sure if it affects. I am affraid mix is should be a little bit richer. That could explain my usual high egts. My Dynon shows an alarm over 800°C (it is not going higher than that) sometimes. If I apply more throttle, more fuel, it goes down a little to green arc.

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800C = 1472F and that's too low for an EGT alarm. 1616F is the max. Set your low yellow alarm for 1520F and the red around 1580F - 1600F. Yellow arcs are just to get your attention and make you keep an eye on a specific temp or pressure. It doesn't mean there is any harm being done.

Normal Rotax EGT's depending on the engine, plane setup and where the probes are located on the exhaust pipe will all be a tad different from plane to plane. The normal EGT's for what's in the field is around 1300F - 1475F. These are normal. I would rather have a 1450F over a 1325F. The lower one is too rich and cool if everything was setup right. That said this also depends on if your EGT probe is in the correct position and of course where your needle clip is on the needle. It should be in the third one down from the top. The EGT probe should be 100mm or 4" out from the exhaust port. Too close causes higher reported temps and farther away cooler reported temps. The EGT's vary with throttle position. The carbs are setup to run cooler with high throttle settings and they should run hotter down around 4800F ish. The richer settings for full throttle is to help stave off detonation issues on take-off and full throttle scenarios. There are other things that can affect the EGT, but they are usually more minor compared to the above items. Outside air temps can affect them too.

 

Bottom line is 1475F (800c) isn't worth losing any sleep over as it is normal and you just need to reset the alarm parameters on the Dynon. 

 

p.s.

If I ever saw my EGT temps climb up over 1520F I would want to know why. Seeing 1500F is no big shake.

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I have an analog tach and I can't see any drop in RPM with carb heat, I'm guessing the instrument just doesn't have the precision the Dynon does, 100rpm is about 1/16" movement on my tach, LOL.  I did confirm my carb heat door is opening, I guess air under the cowl is just not that much warmer than the airbox air to cause a big drop.  In a "traditional" airplane engine that only makes 2650rpm it would probably be a more noticeable thing.  

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How did you notice carb ice? Engine dropping revs and running unbalanced?

 

Last year I had a problem, flying 7ºC raining a little , cloudy, humidity nearly 100%, I didnt use carb heat and after 1h, engine started to do really weird things, about 2 seconds misfire, later fine again, 20minutes later, same happened, It repeated 4 or 5 times, I landed in the nearest field and talk to my mecanics, they asked me to check I had water in the bowls and there it was, a drop of water there. Some people thinks ice in carb when melted went to bowl and others think I had water in the fuel. (I have a gascolator and check it before every flight, never found water.

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  • 5 years later...

Last week I experienced exactly the same as ctspain described above: a very short "misfire" of 2 sec, slight power drop after 1hr inflight during light rain 2°C OAT. This repeated after 20'. Landed, checked carb bowl and spark plugs. But they seemed to be clean and ok. Took off to finish my trip and EXACTLY the same happened: after 1hr and then again 20' after that. This time no rain anymore but hazy and 5°. 

I still had the paper air filter. Which became probably soaked and maybe some water went trough?? I checked the dried version of this filter and tried to blew trough it. almost impossible. So this could be an issue as well. I think he even took some air from the carb heat automatically as the carb heat door doesn't close fully. Would the carb heat door (partially)open automatically with negative pressure when the paper filter would block all air?

I was wandering if there are some other similar cases ?

Thanks!

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On 4/1/2021 at 4:19 PM, Roger Lee said:

If the air filter restricts the air flow then it will richen the mixture and burn poorly. Most of us has switched to a K&N air filter. That said do not over oil the K&N or it can cause issues too.

I don't understand the statement about the lean condition with less air from restricted airflow.  

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