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  #1  
Old 09-08-2014, 09:05 AM
walkman's Avatar
walkman walkman is offline
 
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Default Help/insight please on adjusting idle speed/mixture

I spent a few hours yesterday trying to make sure my idle speed and mixture were set correctly. Starting with the various RSA manuals I removed the idle mixture linkage, set to the 2.5" nominal with the adjuster centered (it was 2.75"), and started to fool around with it. I ended up having to lengthen the link again slightly, and fool around a lot with the idle stop. This graph represents the best I was able to get it adjusted.

With the throttle on the idle stop, mixture full rich, it idles at 700 ±20 with the occasional "pop" in the exhaust. Occasional means very 3-5 seconds.

As I slowly pull the mixture back I get a corresponding slow drop in idle speed and an increase in the popping coupled with an increase in the variance of RPM. Idle speed drops off to 550 ±50. During this decrease it feels to me like the engine is too lean.

Then, right before ICO, I get an RPM rise to about 1,020 RPM, the RPM variance drops to ±10, the engine smooths out and popping stops. A minute further reduction in mixture and the engine shuts off.

My questions are this:

First, is this normal? I would have expected steady RPM until the rise, not a slow decline.

Second, the RPM rise at the end is about 300 above the initial idle speed, and 500 above the RPM just before the rise. This is much more than the RSA recommended amount (various sources put this at 10-50 RPM). Again, is this normal? I can lean it out a bit more, but it already feels too lean. Leaning it further makes the idle very rough indeed, with lots of popping.

https://www.dropbox.com/s/mpip4mbeu0...aning.png?dl=0

Thank you
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Last edited by walkman : 09-08-2014 at 09:12 AM.
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  #2  
Old 09-08-2014, 01:10 PM
paul mosher
 
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Default mech

Find someone that knows what they are doing to adjust it. Not something the inexperienced should be tinkering with.
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  #3  
Old 09-08-2014, 02:05 PM
esco esco is offline
 
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Default Not normal

Concur w/ recommendation to find someone experienced w/ your engine & carb combination to save time and frustration. You can do it yourself... but a recip-experienced A&P may be able to both teach you why it's not working right, and enable you to do it yourself in the future.

For my O-320, 300rpm rise is not "normal" per Lycoming.
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  #4  
Old 09-08-2014, 04:34 PM
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Default

Its fuel injection. Responses to the tune of "pay someone else to do it" are neither enlightening or helpful.

Thanks
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  #5  
Old 09-08-2014, 05:03 PM
Mike H Mike H is offline
 
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Sounds like you are transitioning to idle cut off too slowly to get an accurate indication of the true idle mixture. If you move the mixture lever too slowly you effectively lean the engine for a period of operation prior to flame out which is not giving you an indication of the idle mixture setting. When I check idle mixture I always run the engine at mag check RPM for 15-20 seconds followed by a fairly rapid return to idle, then once the idle has stabilized I move the mixture rapidly to ICO.
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  #6  
Old 09-08-2014, 05:07 PM
Mike H Mike H is offline
 
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Quote:
Originally Posted by walkman View Post
Its fuel injection. Responses to the tune of "pay someone else to do it" are neither enlightening or helpful.

Thanks
Hang around a little longer and someone will surely start paraphrasing Mr. Busch and tell you that you are wasting time and money trying to maintain your aircraft and you should just look the other way, leave it alone and keep flying it. Besides all A&P mechanics do is damage airplanes
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  #7  
Old 09-08-2014, 05:27 PM
RVDan RVDan is offline
 
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Default

Ensure the mixture control (red knob) is to the full rich stop. IMO, I would try to lean the mixture until you get about a 50 RPM rise. You may have to readjust idle speed in the process. If you can't get the desired result, there may be another problem to be looked at. I have had cases where the Mixture was adjusted near limits of travel and would say that is very possible. BTW, the idle mixture has nothing to do with mixture at speeds much over 1000 rpm or so.
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  #8  
Old 09-08-2014, 05:40 PM
C-FAH Q's Avatar
C-FAH Q C-FAH Q is offline
 
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Location: Ontario, Canada
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Default Ico

First I think your idle speed is to fast. Screw the adjusting bolt out till it idles at about 600rpm. Second your idle mixtue is to rich. Find your mixture linkage, if it has two blocks with a thumb wheel in between them, look for a little arrow that has a "R"(rich) and turn it the other way. At 600 rpm just before it goes to idle cut off you should only see about a 60 rpm rise. This may take a few starts and stops to get right.
Hope this helps
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  #9  
Old 09-08-2014, 08:24 PM
paul mosher
 
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Default yes

Quote:
Originally Posted by walkman View Post
Its fuel injection. Responses to the tune of "pay someone else to do it" are neither enlightening or helpful.

Thanks
And most of the free info is worth exactly what you pay for it. If you want to cheap out on a very critical engine component, go ahead it's your life.

Last edited by paul mosher : 09-08-2014 at 08:30 PM.
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  #10  
Old 09-08-2014, 09:47 PM
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ColoRv ColoRv is offline
 
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Default

Quote:
Originally Posted by paul mosher View Post
And most of the free info is worth exactly what you pay for it. If you want to cheap out on a very critical engine component, go ahead it's your life.
A bit over dramatic I think. Adjusting engine idle and idle mixture is hardly so complicated as to require a scientist to do it for you. If one can't turn a screw without supervision, and those pilots are certainly out there, then by all means call an "expert", but the manuals are written in third grade English....not hard to follow.

To the OP, be sure you've run the high mp/low mp test ala Mike Busch to check for induction leaks before you go through too much brain damage. Induction leaks do affect idle and you will chase your tail a bit if you have leaks. Once I corrected mine, the idle mixture adjustment was a piece of cake.
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