check idle w/ carb heat?

GeorgeC

GeorgeC

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GeorgeC
Some checklists have you set 1700 rpm, apply carb heat, verify rpm loss, remove carb heat, verify rpm recovery.

I was trained to verify that the engine runs smoothly at idle with the carb heat applied, presumably to simulate the engine configuration at landing.

Is it worth doing two checks, one at 1700, and another at idle? I tend to combine the two; apply carb heat at 1700, verify rpm loss, go to idle, verify smooth idle, carb heat off, back to 1000 rpm...
 
Make sure it will idle with both carb heat on and one mag only is another check.
 
I check carb heat at idle even though POH doesn't call for it. Figured if it idles during all previous checks, then proceeds to fail during the next, just may be worth taking the time to figure out what changed.

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GeorgeC said:
Some checklists have you set 1700 rpm, apply carb heat, verify rpm loss, remove carb heat, verify rpm recovery.

I was trained to verify that the engine runs smoothly at idle with the carb heat applied, presumably to simulate the engine configuration at landing.

Is it worth doing two checks, one at 1700, and another at idle? I tend to combine the two; apply carb heat at 1700, verify rpm loss, go to idle, verify smooth idle, carb heat off, back to 1000 rpm...
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All the time-honored carb heat check does is confirm that the cable from the carb heat knob to the carburetor has not been broken or is not frozen in place. It is a check of the mechanism, nothing more.

Bob
 
Then why look for an RPM loss? A carb heat check is supposed to verify that you can direct warmed air into your carb.
 
Stewartb said:
Then why look for an RPM loss? A carb heat check is supposed to verify that you can direct warmed air into your carb.
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I think his point is that the actual RMP for the carb heat check is irrelevant. Does not matter if it is 500 or 1700 RMP. The RMP drop IS the verification that the mechanical linkage is indeed working correctly and doing it at 1700 is just customary but not at all necessary.
 
You won't see any important rpm change at idle and may not be making enough heat at idle to check it effectively anyway. The reason we check carb heat is to assure we can melt ice, not to assure we can move a knob. You're verifying that moving the knob produces the appropriate response.
 
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I do both. The check at 1700revs is to insure that the heat is actually working. The slight drop in RPM is due to the mixture being enriched from the heat application, so it does serve a purpose.
 
The check at 1700 looks for the RPM drop to prove it's working, and you should also be looking for any rise in RPM while the heat is on. That takes a few seconds, and if you just pull-and-push you might not get the warning that conditions today are ripe for carb ice.
 
There is a small rpm drop at idle. I check at idle and per POH before take off
 
My carb heat has never been out at idle. But then I don't pull throttle to full idle when landing. And I never touch down with carb heat on.
 
Cherokees don't "normally" land with carb heat on... unlike Cessnas.

Idle check isn't a bad idea, but often when taxiing I'm at idle anyway so that covers that bit.

How often do you check your mag grounding before shutting down? I don't have mags on one plane I fly, but I sometimes check it on the ones with mags
 
I've never seen that in the POH in any airplane I've flown. But it still seems like a pretty good idea, might start doing it now.
 
The O-300 powered Cessna 170 (rather prone to carb ice), suggests LEAVING carb heat on after the runup/carb heat check.

One thing after noting the RPM drop on pulling the carb heat is to check that it isn't rising again on its own after a second (indication you've iced up on the ground).
 
It is called Low Idle and it can be an indicator of something not being right. If you have Carb Heat, do it.
In most birds you may idle around 6-700 and with CH should go down about 100 RPM.
 
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Stewartb said:
You won't see any important rpm change at idle and may not be making enough heat at idle to check it effectively anyway. The reason we check carb heat is to assure we can melt ice, not to assure we can move a knob. You're verifying that moving the knob produces the appropriate response.
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Agreed. I also believe there is another reason for the carb heat check prior to takeoff, which is to make sure that any ice that may have formed during warm up and taxi is eliminated prior to takeoff.

Ryanb said:
Do you turn it off just prior to touchdown?
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I close the carb heat on short final prior to landing. Especially when landing on dirt, grass, gravel, etc. The induction air is unfiltered when the carb heat is applied so I'd rather not suck a bunch of dirt in the engine if I can avoid it.
 
As a student I was instructed to set the throttle at 1700 rpm, then pull carb heat and check slight rpm loss. Then throttle to tide, check engine still running. Then push carb heat in, check for slight rpm increase.

Now I ask, what is a carburetor..???:lol::lol::lol:
 
Jim Case said:
I've never seen that in the POH in any airplane I've flown. But it still seems like a pretty good idea, might start doing it now.
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Apparently you haven't flown very many types then.
mondtster said:
I close the carb heat on short final prior to landing. Especially when landing on dirt, grass, gravel, etc. The induction air is unfiltered when the carb heat is applied so I'd rather not suck a bunch of dirt in the engine if I can avoid it.
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Good practice!
 
I check at idle to make sure the engine doesn't stop when I have low rpm and carb heat.
I find it is an essential check in a lot of older planes, especially anything you have to hand prop to start.
Nothing will energize your day like pulling the the carb heat on, pulling the throttle all the way back, and suddenly find you are flying a glider with no starter.
 
Although not in the POH or checklist, my old crusty CFI ingrained in me to check carb heat both at runup rpm and at idle.
 
My older Cessna 150 was prone to carb icing.

If it was suspected, the carb heat would remain on during the first part of the takeoff roll, just to heat blast the carb good.
 
kgruber said:
My older Cessna 150 was prone to carb icing.

If it was suspected, the carb heat would remain on during the first part of the takeoff roll, just to heat blast the carb good.
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When I crashed my Luscombe, I had a long discussion with the NTSB. The investigator told me they think carb ice causes a lot of accidents on takeoff, but it's hard to prove as the evidence melts. He advised me to check carb heat, then keep the carb heat on all the way to centerline, then turn it off right before takeoff. I don't do this, but it was interesting to hear him talk about it.

Now if you have a Luscombe 8A with only a fuselage fuel tank, by AD you have to takeoff with carb heat on to limit your climb rate! Have fun sucking all of that dirt into your engine on a grass runway!
 
If you see vapor trails coming off prop on runup it means high moisture. Use carb heat to melt potential ice.
 
coloradobluesky said:
If you see vapor trails coming off prop on runup it means high moisture. Use carb heat to melt potential ice.
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Something like this? :)
10529018453_a8b543f094_b.jpg
 
That's every morning in the south. Washes the windows I guess.
 
As I said earlier I pull carb heat at idle, something else I do is check the mags at idle and lean aggressively during taxi.
 
I generally follow the same procedure as the OP. I also like to get the carb heat back off ASAP so I am not sucking in unfiltered air.
 
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