Re: [COUPERS-TECH] Leaning with Stromberg carb

[Sydney Cohen]

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Dave, make sure that (1) you have the autogas STC and (2) the auto fuel has no alcohol in it.  Take a graduated cylinder or a tall, thin glass jar, put in exactly 20 oz. of the auto gas and 2 oz. of water.  The water will go to the bottom.  Use a Sharpie pen to mark the water level.  Put your hand or a tight cover on the top and shake it vigorously.  Let it settle out and check the water level.  If it is now higher, alcohol is present in the auto fuel.  Don't use it.  If the water level is the same, no alcohol is present.  Toss this sample, since it's now contaminated with water, and use the rest.

Syd
 

David Douglas Winters wrote:

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OK guys,

 

This lead-on-the-plugs thing is looking more disturbing all the time.

The previous owner of my ‘coupe advised me to run it on half automobile high-octane and half 100LL to avoid deposits.

 

Any opinions on this practice?

 

Dave Winters

2797H

 

-----Original Message-----
From: John Cooper [mailto:[EMAIL PROTECTED]]
Sent: Monday, December 19, 2005 12:06 PM
To: tech@ercoupers.com
Subject: Re: [COUPERS-TECH] Leaning with Stromberg carb

 

 
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At 02:11 PM 12/16/2005, WRB wrote:

> There are many possible causes of preignition, very few of which would
> be attributed to poor maintenance practices.  Deposits accumulate,
> magneto distributor caps deteriorate, moisture gets into them, etc. 
> However, the percentages are irrelevant.  It only matters if the plane
> you are flying is right or not.

I strongly disagree.  Most of us have a professional set our timing.

I made no mention of timing.

 

More obfuscation.

LOL

 "Use of leaded fuels" has not been a historical
problem of significance in engines such as ours. 

No, but it is now.  The C75/85 was certified on 73 octane fuel which was lead free.  80 had a MAXIMUM lead content 25% of that found in 100LL.  Lead fouling of plugs and lead deposits in the combustion chambers are real problems today if you use 100LL.

You are suggesting that well maintained engines
properly operated are just as likely to have significant cylinder
deposits as poorly maintained engines operated by the less informed and
proficient, and that's neither logical nor credible.

If that's what you think I said, you misunderstood me.

You're right.  We were all taught that.  This discussion was not a "one
size fits all", though.  If there is a genuine danger in such careful
and minor application of carb heat in the manner described for the
purpose described, what is it?

I thought I did. Use of partial carb heat reduces the amount of heat energy available should carb ice develop, sometimes to the point where it is insufficient. Also, I quote from the Ercoupe Owner's Manual: "Unless icing conditions in the carburetor are severe, do not cruise with the heat on."

> It is difficult to control ...

Are you suggesting our simple push-pull knob moves untouched after
adjustment, or that we need a vernier?

No, that the temperature of the incoming air is difficult to control precisely.  Full carb heat is a known quantity.  Partial carb heat is not.  Under some conditions partial carb heat can actually cause the formation of carb ice.  Also, at high power levels, carb heat lowers the detonation margin.

> I thought this related back to leaning on the ground, hence the
> concern about unfiltered air.

Sorry.  I assumed it obvious you don't lean on the ground to clear
plugs at run up in a dust storm.

Leaning is not the point.  We're talking about carb heat.  Use of carb heat on the ground should be avoided due to the fact that it allows unfiltered air into the intake.  The prop provides the dust storm.

>>  "...the mixture should be full rich unless the engine is rough due
>> to a too-rich mixture."
>
> Duh!  However, if you have a Stromberg And you have to lean And you're
> below 5000' DA, something else is happening.  Best look into it!
>
>
>> Per the Ercoupe Instruction Manual, p. 6 states:  "For average
>> operations
>> below 5000 ft. altitude, the carburetor should be left in the
>> full-rich
>> position.  When at part throttle, the fuel consumption may be
>> improved by
>> leaning the mixture, but in no case should the control be moved back
>> far
>> enough to decrease the engine rpm."
>
> Duh again.  And again, if you're below 5000' DA and leaning DOESN'T
> decrease RPM, something else is going on.  Best look into it.

You either didn't read the second sentence, or you disagree with Erco.

You may think that's what I said, but again, you misunderstood me.  The Erco text is somewhat ambiguous, too boot.  Please clarify the operations below 5000' that are not "average" where you think leaning will be beneficial.

>> It would not require "any sort of a full throttle enrichment device"
>> to
>> achieve extra richness at full throttle if the large holes in said
>> plates
>> were sized in design to deliver excess fuel at full throttle.
>
> Those holes regulate the amount of air (not fuel) at (approximately
> atmospheric) pressure vented to the float chamber.

Did I suggest otherwise?

You said: "...if the large holes in said plates were sized in design to deliver excess fuel at full throttle." As I stated, the holes do not deliver fuel.  For a given mixture control setting, the leaning effect increases as the throttle is opened, so sizing of the holes could not cause a richer mixture at full throttle. Large holes do nothing; smaller ones increase the leaning effect.

If the holes are of a size when mixture is at "Full Rich" to deliver
more fuel to the engine than it can convert to power (as a "safety
factor" at take-off), they will do so.

Again, the leaning effect of the mixture control increases with increased airflow through the venturi.  At idle there is no effect and the full effect (moderated by the mixture control setting) is obtained at full power. There simply is no way that a richer mixture could be delivered at full throttle than at lower throttle settings.

The cable controls are entirely separate.  If you refer to what happens
inside after leaning when the throttle is moved, more information would
seem necessary for clarity.

By coupled I mean that the leaning effect of a given mixture control setting varies with changes in the throttle setting (air flow).  If you thought I was referring to a mechanical connection between the two controls then you misunderstood what I said.

Since this is taking on the tone of a religious discussion rather than a technical one, this is my final post on the subject.

The Stromberg NA-S3A1 carburetor does not have an "economizer" or full throttle enrichment feature.  If you don't believe me, I refer you to an article in the January, 2006 issue of Light {Plane Maintenance which states "The NA-S2 and NA-S3 Models...are not fitted with an economizer, accelerator pump or mixture control.  (Note: the NA-S3A1 does have a mixture control.)"

>From the Continental A & C Series Operator's Manual, Ground Running; Warmup "Leave Mixture at 'Full Rich'.  (See 'Ground Operations at High Altitude Airports', Section V for exceptions.)" And Ground Operations at High Altitude Airports states "Prior to takeoff from fields above 5000 feet elevation, the mixture control should be leaned to give maximum RPM at full throttle, static runup."

>From the section on Cruise: "At altitudes of more than 5000' above sea level adjust mixture control for best rich power by moving toward 'lean' position until maximum RPM is obtained with fixed throttle.  Return control toward 'Full Rich' position until RPM drops just perceptibly.... Readjust the fuel-air mixture for each change in throttle setting or altitude."

(Here's the kicker.)  "CAUTION . . . Do not lean the air fuel mixture, unless such adjustment results in a higher RPM.  Excessively lean mixtures cause over-heating and may result in damage to the engine."

Finally, from the section on Descending and Landing "The mixture control must be in 'Full Rich' position during descent.

And finally, I leave you with this, from the Ercoupe 415 D flight manual :"Flight: Adjust mixture control for best RPM (above 5000 feet.)"

But what do they (Continental, Erco, LPM) know?

;)
John Cooper
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865

John Cooper
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865

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