Engine stumbles when carb heat disapplied

[photofly]

A question for the petrol-heads:

I'm merrily flying along in the skies over Ontario this afternoon in a C172, O-320 powered. I have the throttle set for 2300rpm, mixture leaned to peak rpm, as recommended in the POH.

I apply carb heat and the rpm drops off 100-150rpm, with no significant roughness.

After 20 seconds, I close the carb-heat control, smoothly but not particularly slowly. The engine falters; rpm drops to 1700 or so, before picking right back up to 2300.

This is a repeatable occurrence.

Any suggestions as to why?

[CamAero]

Check to see that the air filter is clean, not partially plugged.

Carb Heat is unfiltered air.

[Heliian]

Does it behave the same when the mixture is enrichened?

[Ramjet555]

Now, Can you explain WHY that was NOT Normal?

[photofly]

Does it behave the same when the mixture is enrichened?

No; it was something to do with the leaned mixture.

[Cat Driver]

I apply carb heat and the rpm drops off 100-150rpm, with no significant roughness.

Which means the engine was not over leaned. ( Excessively leaned )

[DonutHole]

I check anything associated with the flapper valve in the carb box, seals rigging and bushing.

What happens if you run the carb heat half on half off (not asking you to
Do this by trying... Theoretically speaking only)

[DanWEC]

Possibly the carb or float system is a little dirty or in need of a tune.

You know that applying carb heat creates a richer mixture because of the reduced air density of hot air.
When you remove the carb heat, the air mass flowing through the venturi is suddenly increased, and Bernoulli says to draw more fuel through the jet.
It's possible the main jet is a little constricted, and/or the needle valve and float linkage is a little slow to react, creating a temporary lean condition. Could also be a low float bowl level.
It's sortof the same kind of condition that an accelerator pump alleviates.

Now, this is all assuming the Marvel Schebler or whatever on an 0-320 is the same as any of the 27 cars and bikes I've had... if not, then this is just a bunch of garbage.

[Colonel Sanders]

a temporary lean condition

That would be my guess too.

Because it doesn't happen with rich mixture, if
I understand correctly, I would suspect that
perhaps due to the less-than-perfect fuel distribution
associated with a carbureted intake system, one
cylinder is maybe going a little leaner than the rest.

A 4 Cyl EGT would tell the story.

[photofly]

I thought sensible use of carb heat was relatively uncontroversial, until I did some searching online, last night. It seems it gets people just as worked up as any other topic in aviation.

[Colonel Sanders]

I wrote this for my students:

http://www.pittspecials.com/articles/CarbHeat.htm

[Heliian]

Sounds like a carb issue. When you move the carb heat off, the air density increases rapidly and the fuel can't keep up. Could be in need of a cleaning.

[iflyforpie]

I'd look at a possible restriction in the air box or hoses that is blocking the carb heat air. That RPM drop seems a bit high and would explain why it stumbles when you disapply it.

Carbs meter fuel based on pressure, not density. Specifically, it is how fast the air is going through the venturi, not how much. 'Mass airflow' means nothing to a Marvel Schebler carb which is why we have a mixture control.

Really, there should be minimal pressure difference between having carb heat on and carb heat off... the filter being a restriction with carb off but the various turns of the shroud, box, flap, and hoses restricting air with carb heat on. If the pressure is the same, the fuel flow is the same so the carb doesn't need to 'catch up'. The only difference would be the density which would make the mixture a bit richer with carb heat off.

If there is a restriction when the carb heat is on, then the fuel flow will go down because airflow through the venturi isn't as fast. When you take the carb heat off, now there is more airflow (based on a possible restriction, not density) and the fuel has to catch up only using the main metering jet and main nozzle, possibly causing the engine to stumble.

[CamAero]

I'd look at a possible restriction in the air box or hoses that is blocking the carb heat air. That RPM drop seems a bit high and would explain why it stumbles when you disapply it.

Carbs meter fuel based on pressure, not density. Specifically, it is how fast the air is going through the venturi, not how much. 'Mass airflow' means nothing to a Marvel Schebler carb which is why we have a mixture control.

Really, there should be minimal pressure difference between having carb heat on and carb heat off... the filter being a restriction with carb off but the various turns of the shroud, box, flap, and hoses restricting air with carb heat on. If the pressure is the same, the fuel flow is the same so the carb doesn't need to 'catch up'. The only difference would be the density which would make the mixture a bit richer with carb heat off.

If there is a restriction when the carb heat is on, then the fuel flow will go down because airflow through the venturi isn't as fast. When you take the carb heat off, now there is more airflow (based on a possible restriction, not density) and the fuel has to catch up only using the main metering jet and main nozzle, possibly causing the engine to stumble.

In that same vein but opposite, I was picturing, say that the air filter is partially plugged; When carb heat is off, the engine has stabilized and adjusted to the restricted flow, with a lower than atmospheric pressure between the filter and the throttle plate. When carb heat is applied, the 'vacuum effect' of the partially plugged filter is lost and that area between the throttle plate and the carb-heat-intake goes back to atmospheric pressure. The engine stabilizes. When the carb heat is selected OFF again, maybe the resulting momentary low pressure in the induction system causes a reduction of air flow through the venturi and consequently reduced fuel flow, just for a second or two?

[black hole]

I think that your problem might be temperature related.From your post nothing was said about what the OAT was at the time. On extreme cold days appling power at take off will often cause the same stumble. Aviation fuel does not atomize as well in the cold.

Not ruling out a mechanical problem----Just a thought.


BH

[crazy_aviator]

One other problem that can be encountered is airflow disruption ( causing stumbling) as the flapper moves from the closed to the open ( carb air off) position. some systems have an inherent fault when there is a combination of temperature OR other restriction ( ice still being thawed, as often happens when pilots dont leave the heat on enough,,,,on another vein, wisdom in dealing with carb icing issues will teach a pilot that partial heat in SPECIFIC installations and specific atmospheric conditions is superior to just full on / off applications. BUT alas, due to general ignorance/ liability, we teach either full on or nothing

[photofly]

I tried it again yesterday and today and couldn't reproduce the effect. I guess it was just one of those things, or else whatever was loose in the carb has shaken itself back into place

[CpnCrunch]

I'm experiencing something very similar to this on a 150. Mixture is leaned, engine running fine, carb heat applied, then engine runs rough when carb heat removed. Always happens when temp is just below freezing. First time I just thought it was carb ice, so I left the carb heat on. This time even after leaving the carb heat on for a few minutes (with engine running fine), as soon as we turned off the carb heat it ran rough again and rpm dropped about 100rpm. Couldn't possibly have been carb icing, as it would have melted. I tried richening the mixture and that cured the problem - engine ran perfectly fine with carb heat off for the rest of the flight.

It's weird that the engine was running fine with the mixture leaned before carb heat, but it runs rough when carb heat is removed. I'm not sure if the problem will go away if you leave it for a while (like happened with photofly). I don't like a rough running engine, so I've never left it more than about 10 seconds before either turning carb heat back on or richening the mixture.

Just wondering if photofly or anyone else ever figured this out.

[hoptwoit]

Had an occurrence similar to this on a 172 with O 320. Because it had to do with the application or carb heat We got so focused looking at all the issues listed above that at first we missed the body to bowl screws were a little loose. There is even an SB for it http://msacarbs.com/pdf/SB-17_O%20081210.pdf

Just one more thing to check.

[CpnCrunch]

Excellent, thanks for the tip. We'll be sure to check it out.

[Colonel Sanders]

I suspect it might have something to do with the
carb heat flapper improving the fuel distribution.

Perhaps what happens, when you take the carb
heat off, is that one cylinder goes lean, due to the
suboptimal fuel/air distribution typical of carburetors.

You know, the same reason you can't lean a carbureted
engine LOP. At least, I never can - it goes rough first,
because one cylinder is leaner than the rest.

PS Remember that when it's cold, you've already
leaned the engine out.

[CpnCrunch]

One time when it happened it did sound like it was only running on 3 cylinders - lots of vibration and rpm drop which cleared up immediately after reapplying carb heat. It's just odd that it runs fine after leaning the engine, but the problem starts only after applying carb heat and removing it.

[Colonel Sanders]

Sure sounds like fuel distribution to me.

For some reason - I don't know why - applying
carb heat will often improve the fuel distribution.

No idea why. Should have paid more attention
to my undergrad fluid dynamics course, I guess.

PF: Physician, heal thyself! I am sure you recall
more about curl and divergence than I do.

[CpnCrunch]

Ok, thanks. I think I'll take a quick look just to make sure the carb isn't about to fall off, and if everything looks fine (and it runs okay after enrichening), assume that it is just quirky design flaw.

[FenderManDan]

The same thing happened to me today. Just in cruise leaned as usual opened up the carb heat to check it for 10sec and then closed it. The engine stumbled to 1700 rpm and came back up to 2400 within the couple of seconds. Now i did leaned it roughly more to keep the temps up but not go LOP, i think. Perhaps cold temps pushed me over to be too lean for a brief moment.

C172 with o320. OAT -21c