Edit / Disclaimer
While my initial problems were generally fixed, there are other mixture issues that people have written about in posts below.
OLD:
This has been a persisting issue since release.
All prop planes have mixture incorrectly modeled.
Mixture controls the fuel to air ratio of an airplane. As you increase mixture, the fuel flow should therefore increase.
When you increase the mixture in MSFS props, the fuel flow increases until about 91%, and then once you pass 91% there’s a sharp decline in fuel flow.
This is responsible for so many of the “performance” related issues with the airplanes - people are departing at sea level airports with full mixture and can barely climb in the C172. You can fix this by leaning the mixture slightly and watching the fuel flow rise dramatically. This is completely unrealistic.
EDIT: I understand now that the fuel flow might drop if the engine is being flooded with fuel at higher altitudes, but this is happening at sea level, which is very much not how it should work.
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Completely agree with you. I have reported this bug. However, the mixture seems to do what it’s supposed to after the patch. Fuel flow decreases as you lean the engine, but you still have to lean the engine before takeoff to get optimal engine power, so there’s still issues with the leaning of piston engines. Also, turboprop engines are still flawed. I’m hoping to see another patch in 2-3 weeks time to adress these issues.
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I tried it post-patch and it’s still broken. Have to power mixture to 91% to get maximum fuel flow (not maximum power - maximum fuel flow).
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Same here. 100 rpm dies out. But 91 makes the engine run fine. Glad not the only one.
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This thread died. This is probably my biggest issue with airplanes right now in the sim.
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Seems there is a bit of confusion about what the mixture control actually does. It’s pretty simple, especially in the older models like the C152 and C172: it controls the flow of fuel. As you pull the control toward you, the amount of fuel being supplied to the engine decreases. At sea level and up to about 6,000 feet, in real life you run at “Full Rich.” Flight Sim does a poor job modeling this.
Here’s the Wiki to explain things in detail, if you’re interested. 
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I don’t see any confusion in this thread, and I’d assume Asobo knows what they’re doing… I think it’s just bug in the sim. And wanted to add: mixture specifically exists to adjust the fuel to air ratio, or the makeup of the combustion mixture. As you climb, air particles are less dense, so you need to decrease the fuel in that mixture to maintain the most efficient ratio for combustion. The throttle controls total flow of the fuel-air mixture, and therefore the total fuel flow. Decreasing the mixture will also reduce the fuel flow, but the throttle controls the total amount of the fuel-air mixture that goes into cylinders (more mixture = larger explosions).
And point taken, but in the 172 3,000 ft MSL is when you lean the mixture 
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Sounds like you’ve got your PPL…
Student pilot who is required to know all of this stuff for my progress checks / tests 
Getting there, but COVID threw a road block.
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What a bummer about COVID! Glad to hear you’re getting your “ticket” though… it remains one of the best (and most fun) things I’ve ever done. Good luck with your journey; it lasts a lifetime. Cheers!
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Yep mixture behaves just like in FSX. Just as bad I should say.
I could not even start my engine at a 6500ft airfield with mixture on full rich.
Plus, for any altitude the perfect setting seems to be 50%.
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Thanks so much! Very excited! What types of planes do you fly?
I know, right? If you have the SR22 the mixture control is good for one thing: cutoff. They’ll fix these things I’m sure. The “stock” aircraft are still leaps and bounds better than the ones that were released with FSX, so I’m happy about that. The DA62 is especially nice, and the G1000 suite in that A/C seems to work as it should.
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Hi,
well that’s something new, I flew the DR400 today but I used ctrl+e this time, I was surprised to see that my mixture was set at max 91%. Had no problems in the 1hr flight.
A few days ago I had 2x total loss in fuel pressure.
This was when I started the aircraft up manually with mixture set to 100%. I did not fly high and kept mixture at 100% then the issues started.
I took my first “introductory” flight in a Piper PA28-161 Warrior. Cost $5.00
I soloed in a brand new Piper Tomahawk, which still had the “new airplane” smell. rolf!
I then joined a local flying club and became part owner of a C150, C172, another Piper Warrior, and a C182.
I’ve also spent a lot of time upside down in a Citabria. 
However, of all the aircraft I’ve flown, the little 1965 C150E remains my absolute favorite. It’s like the Volkswagen Beetle of aircraft. You can’t help but smile when you fly it.
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I would like to revive this thread, as this is a very real issue (problem) when compared to real life flying. The sim requires leaning above and beyond what is even remotely realistic, both in terms of density altitude it wants you to start leaning (where it seems to start around 4k’, note DA not PA), and as far as how aggressive you have to lean. (Which is to say, it’s far too much compared to reality. The amount of leaning it wants would cause a cutoff IRL.)
When I used to fly for real, we hardly bothered even looking at the mixture control until about 6,000’ DA (and frequently higher), and even then only a minor adjustment towards lean was plenty. I never once encountered the complete inability to climb in a 172 IRL due to a mixture that was “too rich” as compared to what I’ve experienced in the sim. It DOES make me pay way more attention to the mixture, but it’s not accurate.
So, I’m bumping the thread and adding my vote.
Regards,
KevyKevTPA
PP, ASEL, IA
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Leaning your aircraft at runup (runup to check mags is not on any checklist that I’ve seen) when you are flying out of a strip at 292’ is pretty silly.
“By the book” you start at 3000’ but as you said, below 6500’ (VFR) I never even touched the mixture in either the C172 or the C150. Of course in the C150, by the time you reached 6500’ you needed to start your letdown.
Totally agree!
Ain’t that the truth! Never flown a 150, but I learned in 152s during summer in Florida, where the SL field probably started at a DA of 2,000-2,500’. I was chunky, and so was my CFI, and we always started with full fuel, so we were overweight to begin with. I’m not sure we ever even made it that high anyway!
I don’t bother with runups in the sim as I have most failures turned off, anyway. (Cheating, I know, but I’m simming for fun, not to scare the ■■■■ out of myself.) I have, however, started going through a full range of motion check on the flight controls sometime between startup and takeoff, as it seems to make them smoother and less quirky. Not sure if that’s real or in my imagination, though.
Yea… she may have been “low and slow” but that little 150 was (and still remains) my favorite aircraft. Did all my X-country work for my PPL in her. No electric flaps; pulled them into position with a handle that looked like a parking brake. All in at 40 degrees, flying into a 15 knot headwind you probably could have run faster! 
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the mixture only doing cutoff in the sr22 is the same in the g58 baron…no actual fuel mixture control…just run and cutoff essentially…if you turn on the fuel pump, you go from 21gph at 2700rpm to 35gph…adjusting the mixture only varies between these two numbers
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