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Idle Mixture Adjustment

ChrisMcC

Member
I have an AeroSport Power IO-375 with dual Pmags, horizontal Silverhawk FI, and 30 hrs TTAF. After every flight the sniffle valve was spitting fuel, and AeroSport said this is not normal. They recommended adjusting the idle mixture. I also wasn't getting much of a rise on shutdown, so...

I leaned the idle mixture 2 1/2 turns and I now have a 100 RPM rise on shutdown... still too rich, right?

After the above idle mixture adjustment, at idle (taxi and short final) the engine is producing loud backfires... too lean, right?

What do I do now?
 
Per the book, you want a 50RPM rise at ICO. More than 50RPM indicates that the idle mixture is too rich.

Check for induction leaks -- manifold pressure line, stuck sniffle valve, gaskets, etc.

Cheers!
 
Yes, the backfires are likely from too lean, assuming they are afterfires in the exhaust and not actual backfires in the initake. 2 1/2 turns is a lot. To make it easier, try best MAP. Adjust idle mixture to achieve the lowest MAP that you can get at a given RPM (750 is good). This is the optimum idle mixture for your setup. Each time you adjust the mixture, you need to reset the idle level to the RPM you started at and are using for this exercise, as RPMs will increase as you get closer to optimum. Engine should be warmed up for this.

You will need a helper for this approach though. I chock the plane and use my kids or my wife to read the map and reset RPM while I adjust with engine running. I imagine you could also make changes, restart, check, etc. by yourself.
 
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You will need a helper for this approach though. I chock the plane and use my kids or my wife to read the map and reset RPM while I adjust with engine running.

Did this give anyone else the heebies?

IMHO, this is unnecessary and dangerous, not to mention the trauma to the wife or kid should the unthinkable happen here.

What's the hurry? Test it, shut off the engine, adjust, then get away from the prop and retest. Repeat as needed.
 
...the engine is producing loud backfires... too lean, right?
Not so fast there, my friend. Have someone look at your exhaust while the engine is backfiring. If there's black smoke coming out of the stacks, your mixture is too rich. Or, you can just try leaning the mixture at idle and see if the backfiring goes away.
 
Not suggesting that I'm right, but in my 30 plus years of working on airplanes, adjusting mixture with the engine running is fairly common, and is what I do.
As the lawyers say: "Do not try this at home"
 
Idle mixture adjust depends upon outside air temperature, and altitude. This Aviation Consumer article doesn't discuss altitude but otherwise has good information: Carburetor Idle Issues: Check the Mixture First. All I can find regarding altitude is this from AvWeb:
If you fly from a high altitude airport, you might want the idle mixture richer to compensate for flying to airports at lower elevations. Temperature changes may require the idle mixture set slightly rich, colder temperatures require a richer mixture.
 
Idle mixture adjust depends upon outside air temperature, and altitude. This Aviation Consumer article doesn't discuss altitude but otherwise has good information: Carburetor Idle Issues: Check the Mixture First. All I can find regarding altitude is this from AvWeb:

You are correct. Warmer temps require less fuel than cold temps and higher altitudes require less fuel than lower altitudes. The key is to either adjust them accordingly or find a compromise. I tend to set my mixture around 50 degrees. A tad rich for summer and a tad lean for winter. In search of compromise, its best to err on the rich side. Engine runs ok when too rich, but not so much when too lean

Larry
 
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Not so fast there, my friend. Have someone look at your exhaust while the engine is backfiring. If there's black smoke coming out of the stacks, your mixture is too rich. Or, you can just try leaning the mixture at idle and see if the backfiring goes away.

If the backfire occurred AFTER aggressively leaning his engine (2.5 turns) how could you attribute the backfiring to an overly rich condition? After fires are common with a too lean condition. backfires and afterfires are not a common symptom of a too rich condition.

Larry
 
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Did this give anyone else the heebies?

IMHO, this is unnecessary and dangerous, not to mention the trauma to the wife or kid should the unthinkable happen here.

What's the hurry? Test it, shut off the engine, adjust, then get away from the prop and retest. Repeat as needed.

And what do you say to all the EFII users that need to stand at a running engine to time it? Rip it all out?

Your welcome to your opinion, but please don't knock mine. We all approach risk a different way and often reach different conclusions. Many people would call you nuts for flying in an airplane that you, or even more scary someone you don't know, built in a garage.

Larry
 
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And what do you say to all the EFII users that need to stand at a running engine to time it? Rip it all out?

Larry

I would say that might be necessary risk, whereas idle mixture adjustment on an injection throttle body with the engine running is an UNnecessary risk, since it can be easily done another way with considerably less risk.

Given the option to accomplish a task and reduce or eliminate a risk, I'd choose that option. But that's just me...I'd rather reduce or eliminate risks than tempt fate unnecessarily.
 
Thank you all for your replies. Maybe I wasn't terribly clear:

(1) the 100 RPM rise would indicate an overly rich idle mixture. Yes, ~50 is normal.

(2) the after fires at idle would indicate an overly lean idle mixture.

So, it's hard to know what directly to go. I will try to MAP technique. Great suggestion. I will also return the idle mixture closer to where it was, and investigate the sniffle valve issue.
 
To elaborate on my previous statement that too rich can cause backfiring, I have evidence. This video was taken at an airport with a 8,000' density altitude, after leaning for max RPM during the runup. It wasn't lean enough and the engine backfired and died right there on the runway. The black smoke is evidence of too rich a mixture. I leaned aggressively and got her running again, then took off.

https://www.dropbox.com/s/ay7prqm4i68uvz2/Engine quit at Centennial.mp4?dl=0
 
backfire/afterfire?

In my own humble opinion an easy thing to check (just to cover your bases) is looking at your exhaust for leakage at the gasket flanges. Do a quick look with a mirror and a flashlight just to be able to cross that off your list of possible causes.
I live under the downwind leg for 35 at KPLU and hear a lot of snap, crackle and pop noises.....usually from gaskets leaking.
Later today I'll be adjusting idle on my own plane.
good luck
danny
 
On leaks...

Unfortunately, the Southwest Airlines Flu Virus, Variant 1 (SWAFV1) knocked me out for a few days, but during my convalescence I did a virtual walk around of my engine (IO-360-A1B6) looking for possible manifold leaks -- I think I've identified a few that I should share:

1. Bad sniffle valve -- check ball gets stuck, and the "suck" doesn't close it cleanly and lets air in.
2. Manifold pressure tap -- 45? AN fitting on #3 Jug (YMMV), to a hose, to another AN fitting on to the transducer manifold, to another hose to EI system. If anything along this path leaks it's going to let air in. Use Loctite 565 on those NPT fittings.
3. Gaskets around the intake runners -- top and bottom.
4. Gasket between FI Servo and Intake plenum.
5. Then the more gross errors - cracked intakes, cracked intake plenum, etc.

Personally, I'm going to revisit #2 on my setup as I had discovered that one of the tygon hoses had popped off the Plasma III.

Cheers!
 
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