Tiny drop with carb heat applied

RyanB

RyanB

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A very long story short, my A&P installed a new Marvel carb on the Archer. Everything seems to be in good running order at first glance, although when I apply carb heat, I hardly get any drop at all. It drops, but a very tiny drop. The mechanic looked at it while I exercised the carb heat arm and said that it appears to be functioning correctly. Is this normal or does the carb need an adjustment?
 
RyanB said:
A very long story short, my A&P installed a new Marvel carb on the Archer. Everything seems to be in good running order at first glance, although when I apply carb heat, I hardly get any drop at all. It drops, but a very tiny drop. The mechanic looked at it while I exercised the carb heat arm and said that it appears to be functioning correctly. Is this normal or does the carb need an adjustment?
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The C85 with a Stromberg that I have on my Fly Baby also has a minor drop at normal test RPMs. Do a test run to a higher RPM, or test it in flight.

A portion of the drop is due to restriction of the airflow, not merely the higher temperature. If a setup doesn't restrict the airflow as much, it won't have as much drop.

Ron Wanttaja
 
RyanB said:
Is this normal or does the carb need an adjustment?
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FYI: The carb should be adjusted/set on its own parameters. I've found the level of carb heat RPM drop was dependent on various conditions like OAT, density, flight status, etc. Are you comparing this to your previous carb?
 
Bell206 said:
FYI: The carb should be adjusted/set on its own parameters. I've found the level of carb heat RPM drop was dependent on various conditions like OAT, density, flight status, etc. Are you comparing this to your previous carb?
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Yeah previous carb. If I had to put a numeric value on it, it doesn’t appear to drop more than 25rpm at most, it hardly moves the needle.

Since there is a drop and the mechanic doesn’t see any problems with the control arm while exercising, should I go ahead and fly it and get things warmed up? This is after new mags, harnesses, plugs and hoses have been installed, so it hasn’t actually been flown since.
 
RyanB said:
his is after new mags, harnesses, plugs and hoses have been installed, so it hasn’t actually been flown since.
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Technically checking it on the ground won't give a good response nor is it recommended. Since it is a concern perhaps pick a day when the probability of carb ice is low and take it for a spin. Given you've replaced everything that increases performance it may just be your previous carb, plugs, wires, etc were at a lesser state than thought. Regardless, if your comfortable with the aircraft and your APIA gave you a thumbs up don't see why you shouldn't see what you got.
 
Not familiar with the air box on an Archer, but are there felt seals on the sides of the diverter? the carb heat rpm drop on my Comanche increased significantly after those were replaced.
 
If the drop is much less than with the previous carb, I'd suspect that something hasn't been done right. 25 RPM on a Lycoming is too small. It sounds like the heat hose to the airbox might be loose or torn or completely off at one end. The flapper arm might be moving, but the flapper itself, or even the shaft, might not be. Or it's misrigged and the flapper isn't reaching full travel.

Carb ice causes an awful lot of accidents because pilots don't understand it, and some even think it's just a winter thing. It's serious, even in a Lyc, and I'd be looking hard to see what isn't right. If it was my airplane, I'd want to KNOW that it's all OK.
 
On my experimental (I know apples and lug nuts) the carb heat drop will be less if I have the mixture leaned (as I do for all ground ops). I run a Marvel MA-3SPA.

YMMV ...
 
Has your fuel flow changed when operating at full rich?
you might trying doing a Carb heat check with it at full Rich and then another with it leaned to Peak.

Brian
 
My guess is it’s not a big deal to correct . So here are some thoughts. Keep in mind that Carb Heat decreases air density which has the effect of richening Mixture.

1. With engine warmed up Mixture should be checked.

At Run-up RPM ( 2000) slowly leaning Mixture should result in a small
( 10-20 RPM) rise. Best checked with an optical Tach. This tells you the Mixture
Is slightly richer than Best Power. Zero Rise is too Lean. No real adjustment
here though. Leaning the Mixture with Carb Heat ON should provide a larger
rise than in OFF.

Repeat this in the Idle Range ( 600 -1000 RPM). You should get a rise of about
25 RPM. Less = Lean and More = Rich. The Carb Idle Mixture can be adjusted
via a Knob on the Carb. Effect at higher RPM is minimal though.

2. Verify rigging with Cowl off. I think you want the filter removed on an Archer as
well.

When activating Carb Heat the stopping action should take place in the Airbox
to assure full travel. The Control should not hit stops in the cockpit .

In COLD ; observe Flapper position. If Flapper does not go full travel and seal
off the inlet you will already have some Carb Heat ON. Hence little drop in
HOT.

In HOT; the Flapper should again go full travel and seal off inlet. In it does not
you will not get full heat and little drop.


it is possible the Carb Float Level is set too low; which would result in a leaner Mixture. I’ve seen engines so Lean that applying Carb Heat would give an RPM increase. Suggest you and your Tech check the preceding before blaming the Carb though.
 
My first annual I got it back and had the same thing. Everyone told me it was fine, repeatedly. That it was likely due to being bitter cold out, go fly it try it when the engines warmer, etc. Flying a known ice maker I insisted my IA take a third look, I wouldn’t fly it. This was hard as I was a newbie owner and all the old salts said it wasn’t really an issue.

sure enough they had rewired the carb heat flapper door and it wasn’t closing fully- it looked like it at first glance but upon closer inspection it wasn’t quite seated closed. A minor re adjust and it was back to a drop. My insistence paid off…

id encourage you to be just as insistent till yo know it’s right. Maintenance induced problems are very real, it’s not distrust of our mechanics- just simple statistics. Don’t be a statistic make sure it’s right before flight.
 
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Huckster79 said:
My first annual I got it babk and had the same thing. Everyone told me it was fine, repeatedly. That it was likely die to bring bitter cold out, go fly it try it when the engines warmer, etc. flying a known ice maker I insisted my IA take a third look, I wouldn’t fly it. This was hard as I was a newbie owner and all the old salts said it wasn’t really an issue.

sure enough they had rewired the carb heat flapper door and it wasn’t closing fully- it looked like it at first glance but upon closer inspection it wasn’t quite seated closed. A minor re adjust and it was back to a drop. My insistence paid off…

id encourage you to be just as insistent till yo know it’s right. Maintenance induced problems are very real, it’s not distrust of our mechanics- just simple statistics. Don’t be a statistic make sure it’s right before flight.
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It appears the problem solved itself once it got cowled up. I assume it had something to do with airflow while being de-cowled. The carb heat drops well within spec now!
 
RyanB said:
It appears the problem solved itself once it got cowled up. I assume it had something to do with airflow while being de-cowled. The carb heat drops well within spec now!
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fantastic!
 
Bill said:
*shakes head* Should have gone with the DC carb.
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And what is a DC carb? Is it something STC'd to replace the Marvel-designed carbs? Never heard of anything like that.

The Marvel carbs were built by Marvel, by Facet, by Precision Airmotive, and now by Marvel again, and by Volare (Tempest). Every one of them has managed to introduce defects of one sort or another resulting in numerous service bulletins and ADs. The whole design, even with no defects, is clunky, and they don't run as well as my ancient Bendix-Stromberg carb. In the 1990s an AD mandated the replacement of the two-piece venturi with a single-piece affair; the two-piece had a habit of working loose until it came free and got sucked into the engine. The alternative to the AD was to inspect that two-piece every 100 hours to make sure it was secure. The single-piece had flat-bottomed legs on it that disrupted the airflow through it, messing up the atomization and homogenization, so another AD was issued to change the fuel nozzle to fix that. In some airplanes the problem got worse, so yet another AD, or a revision, allowed one to reinstall the two-piece venturi and old nozzle and inspect it every 100 hours. But most of those old parts had been trashed.

Other ADs dealt with fuel bowl floats. Lots of problems there. Replacing the "obsolete" soldered brass float with plastic floats made of various polymers resulted in lots of problems with dissolving or sinking floats. The real reason was to save money, as the brass float was labor-intensive to make. Saving money no matter how much it cost.

So yeah, a completely new design to cover the various MA-3 and MA-4 models would be welcome, as long as it improves things and doesn't just introduce a new set of problems. I don't see that happening. There are too many models and settings, and the new carb would have to be a plug-and-play affair, complicating the redesign. Expensive. Owners are are already complaining about the cost of parts.
 
Dan Thomas said:
And what is a DC carb? Is it something STC'd to replace the Marvel-designed carbs? Never heard of anything like that.
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It’s just a joke about the comic series (Marvel and DC). :)
 
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