carb ice experience

I've probably fired up the Cub 500 times, and every time until the
most recent one went much the same way, just as my instructor taught
me: when you do the mag check (1500 rpm in the Cub) your last check is
pull carb heat on, and watch the rpms drop a bit. Then, if they climb
back to 1500, you know you had a bit of carb ice and that it has
melted, and you will be especially cautious thereafter to avoid icing.

But the other day, first cold day, I had quite a different experience.
The engine may have been running rough when I taxied--hard to know
with earphones, but I had a feel it was rough. Did the mag check.
Pulled carb heat on. Whoom! Rpms went up to 1700.

Now what was the difference between that experience and the ordinary
one where the rpms drop, then rise back to 1500?

Thanks!


-- all the best, Dan Ford

email: usenet AT danford DOT net

Warbird's Forum: www.warbirdforum.com
Piper Cub Forum: www.pipercubforum.com
the blog: www.danford.net
In Search of Lost Time: www.readingproust.com

If I rememberf correctly (I fly injected engines now) when you pull the
carb heat on you are adding warm air to the air intake, the engine rpms
drop because its less efficient. The drop in RPM does not mean you have
carb ice, it means the carb heat is working.

If the RPMs went up right away you probably melted away exisitng carb
ice.

On Sun, 23 Oct 2005 06:39:40 -0400, Cub Driver usenet AT danford DOT
net wrote in ::

Did the mag check.
Pulled carb heat on. Whoom! Rpms went up to 1700.

What was the temperature/dew point spread at the time? Was the air
mass wet?

"Cub Driver" usenet AT danford DOT net wrote in message
...

I've probably fired up the Cub 500 times, and every time until the
most recent one went much the same way, just as my instructor taught
me: when you do the mag check (1500 rpm in the Cub) your last check is
pull carb heat on, and watch the rpms drop a bit. Then, if they climb
back to 1500, you know you had a bit of carb ice and that it has
melted, and you will be especially cautious thereafter to avoid icing.

But the other day, first cold day, I had quite a different experience.
The engine may have been running rough when I taxied--hard to know
with earphones, but I had a feel it was rough. Did the mag check.
Pulled carb heat on. Whoom! Rpms went up to 1700.

Now what was the difference between that experience and the ordinary
one where the rpms drop, then rise back to 1500?

Thanks!


-- all the best, Dan Ford

Check the idle mixture. When you pull carb heat on you are putting hot,
less dense, air through the carbuerator. If you were initially running lean
applying carb heat will improve the fuel/air mixture. Cold day, dense air,
more fuel required.

Allen

Dan
Application of carb heat causes a change in rpm's because it changes
the fuel/air ratio.....NOT because there is carb ice. If carb ice is
present, the heated air will hopefully melt the ice and it will then
cause the engine top stutter a little as it goes thru the combustion
process.
If you have carb ice conditions, you can run with full heat on to avoid
the ice formation. However, on the ground, this will by-pass any
filters and you can suck dust into your carb with the dirty air and a
potential for serious engine problems.
In severe low temps, sometimes it requires full carb heat just to get a
close to useable air density for decent fuel/air ratios.
Another trick to stick in your pilots bag of magic is to use the primer
knob if your engine gets starved for carb ice and stops. The primer
system on most carbureted engines bypasses the carb and goes directly
to the intake manifold. Your fuel schematic should show you that. I've
used it a couple of times to keep the engine runing in severe
conditions around the country. The engine will run for a few seconds
then die again so you keep stroking the primer knob to keep it running
until hopefully the heat will come up enough to become effective again.
The addition of a carb heat gauge is worthwhile for most aircraft
(normal aspirated). A periodic carb heat check in the air doesn't hurt
either.
Discuss the situation with your local mechanic and he can explain it
better. If he can't, find a more experienced mechanic!
Cheers and safe flying

Cub Driver wrote:

Now what was the difference between that experience and the ordinary
one where the rpms drop, then rise back to 1500?

Sounds like you had a lean mixture for some reason. When you hit the heat, the
mix got richer.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

wrote:

Another trick to stick in your pilots bag of magic is to use the primer
knob if your engine gets starved for carb ice and stops. The primer
system on most carbureted engines bypasses the carb and goes directly
to the intake manifold. Your fuel schematic should show you that.

The problem with this is that carb ice blocks the flow of air into the engine.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

Cub Driver wrote:

Now what was the difference between that experience and the ordinary
one where the rpms drop, then rise back to 1500?

The Selway Kid just mentioned that carb heat application also bypasses the air
filter in most aircraft. IMO, the most likely problem here is a clogged filter.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.

On cold mornings, using summer mogas (which will have a lower vapor
pressure), the fuel doesn't vaporize as easily in the manifold.
Applying carb heat can sometimes improve this and the RPM will rise a
bit. I use the heat on my A-65 on such mornings.


Dan

"George Patterson" wrote in message
news:7lO6f.4165$tl5.718@trnddc02...
wrote:

Another trick to stick in your pilots bag of magic is to use the primer
knob if your engine gets starved for carb ice and stops. The primer
system on most carbureted engines bypasses the carb and goes directly
to the intake manifold. Your fuel schematic should show you that.

The problem with this is that carb ice blocks the flow of air into the
engine.

George Patterson
Drink is the curse of the land. It makes you quarrel with your
neighbor.
It makes you shoot at your landlord. And it makes you miss him.

It can. But depnding on the design of the carburetor, it can sometimes block
the fuel first. I've seen ice block a booster venturi which shut down the
fuel, but left plenty of room for air to flow...

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.