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Help/insight please on adjusting idle speed/mixture

walkman

Well Known Member
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.

leaning.png


Thank you
 
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mech

Find someone that knows what they are doing to adjust it. Not something the inexperienced should be tinkering with.
 
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.
 
Its fuel injection. Responses to the tune of "pay someone else to do it" are neither enlightening or helpful.

Thanks
 
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.
 
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:p
 
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.
 
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
 
yes

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.
 
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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.
 
My advice (warning: I am a carb guy)


1) view the EAA video on adjusting idle mixture.
2) I believe your idle setting is too rich. Lean it a 1/4 turn (or 1/2 turn to start) via the idle mixture screw on the throttle body/servo (not the red knob) and repeat the test in the video. Keep doing this until you get the recommended behavior.

You are likely getting the large drop as you lean because your idle is very rich. I am guessing that you don't get much resolution with the mixture knob on the idle circuit (just a guess, I am a carb guy)

If you struggle much in getting it to behave, be sure to heed the previous posters advice about checking for induction leaks.

If you search, you will find solid descriptions of the test and interpretation for adjusting idle mixture.

Larry
 
Sure sounds like the barrel valve is scored and leaking air, no amount of fiddling as you've already done will get it to idle right.
 
It is a very simple procedure. If you can't dial it in easily per RSA instructions, there is something else wrong as others are suggesting.

I chased mine for some time and could never get it right. We tested for all the usual suspects, returned the servo (nothing wrong), you name it. It turned out to be a slightly sticky exhaust valve that was VERY difficult to diagnose.
At normal operating RPM's, like in climb or cruise, the combustion cycle is so fast that "minor" problems, like a slightly sticking valve, may not show themselves. However, at idle RPM, things are slowed down and those same problems can manifest themselves into a more obvious issue.

Nobody would have expected a slightly sticky valve to be something to look for as it is so unusual. I had multiple mechanic friends helping me. I found the issue almost by accident. I could not imagine what hiring a mechanic would have cost. (and I seriously doubt they would have found the issue.)
 
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:p

Now that's a thought out and helpful reply.
 
I know this is kind of a stupid question, but you've checked to be sure the throttle arm is moving from stop to stop, right?

I ask that because I was sure mine was when I installed the Van's throttle cable but it turns out it wasn't because the standard arm that came with the servo was actually too long so I swapped it out for a smaller one from Precision and all the problems were solved.
 
Got some good pointers from Don at AFP yesterday. Great guy, spent 30 minutes on the phone with me for $0, and seems very knowledgeable. I'm really looking forward to the FI seminar this fall.

One of his first comments when I mentioned that I'd gone back to the RSA installation and operation manual and adjusted the starting length of the connection to 2.5" nominal as recommended in the manual, was to throw the manual away, its half nonsense.

Per his recommendation I'll be setting the starting point for the idle stop by setting a .006" gap between throttle plate and bore, and setting the starting point for the idle mixture by adjusting the linkage until I get a fuel flow (disconnecting the line before the flow divider) of 1.2 gph with throttle at idle, full rich, boost pump on. That is my starting point to start adjusting.

Don seems to think it is a possibility there may be another problem as well, but that I'm likely not too lean, but way too rich. Hopefully more info this weekend.

Thanks for the constructive insights.
 
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I have spent my time with Don on the phone as well. He straightened me out on idle adjustment during my phase one. I'm not typically a "faithful" guy when it comes to purchases...but Airflow gets my fuel delivery business. Don and his people are a valuable resource.
 
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