Engine stumbles when carb heat disapplied

[Rookie50]

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.

I have read this as well, applying to my 0540. Something like Running with just a touch of carb heat -- I think when it's cold OAT...improves the fuel atomization....

[CpnCrunch]

Update: I checked the carb and it is fine. No staining, the bowl is not loose and all screws are tight. However I did notice a kink in the hot air hose that goes into the carb heat box:

http://i.imgur.com/lHG6Y6j.jpg

http://i.imgur.com/UjfCJ3t.jpg

The owner of the plane is going to talk to his AME to see if it's an issue.

[cgzro]

Yeah that looks a bit scary!
Although it does not seem explain your symptoms but it would tend to collapse with carb heat applied and restrict the intake with obvious results.
definitely should be fixed.

[CpnCrunch]

At first I thought it wouldn't explain the symptoms. However thinking about it a bit more (and re-reading this thread), I'm thinking that if the airflow is restricted when carb heat is applied then there could be a lean-cut when carb heat is removed (and airflow increases through the carb).

Also I'm wondering if having the cabin heat on makes it worse, as I think cabin heat was on during the times this has happened. Perhaps having cabin heat on diverts more of the airflow away from the airbox.

Anyway, we're going to get it looked at before anyone flies again.

[AirFrame]

The owner of the plane is going to talk to his AME to see if it's an issue.

That hose looks pristine compared to some of the airplanes I learned to fly in. Since it runs smoothly with carb heat on, it's unlikely to be part of your problem though.

[CpnCrunch]

Ok, I just did a little experiment on a flight tonight and I think I've solved the problem. I leaned the mixture slightly lean of peak rpm (yes I know this is bad) and applied carb heat. After removing carb heat, the rpm dropped. This happened no matter whether cabin heat was on or off. Richening the mixture a bit solved the problem. Also, we have fixed the hose, so that is not an issue.

So basically the rpm will just drop after removing carb heat if the mixture is leaned too much. You don't want it to be lean of peak anyway, so it's probably a feature rather than a bug.

As to why the rpm drops after applying and removing carb heat without touching the mixture: who knows, and I'm not sure it really matters. It could be the ram-air of the airscoop causing a lean-cut due to higher dynamic air pressure (even with the air filter) compared to the hot air, or something along those lines.

[Colonel Sanders]

People like to make stuff complicated. Reminds
me of a guy I know, he's eight years into his PhD
in Physics. With his 4 years undergrad (no masters)
he's 12 years into his university education, so far.

Anyways, what I do when I jump into an airplane,
and it doesn't matter if it's carbureted or fuel-injected,
fixed-pitch or constant-speed prop, is to lean for
max airspeed.

I know. Too simple. Won't get you a PhD in Physics.

You want a PhD in Physics? Write a paper on LOP,
addressing fuel-injected with and without GAMI
injectors, and carburetors. Don't forget a chapter
on running your Briggs & Stratton lawnmower
engine LOP.

Meanwhile 99% of PPL's don't touch the red knob,
and bottom spark plugs everywhere are fouling so
that people can complain about excessive RPM
drop during mag check during runup.