Stewartb
Final Approach
Start with carb heat? Is that a thing?
Mixture full rich for landing?
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Sinistar
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Brad
Actually I think my previous mechanic meant to pull on carb heat once it was started when it is really cold and idling. I don't think he meant to actually start it with carb heat. So I never tried it. He did lots of Continentals. One nice thing for us is that we were always pre-heated (Tannis) so it wasn't like we were trying to start it at -10F and warm it up. Perhaps that what meant, a really cold start.
It is crazy how much the mixture knob comes out during ground idle during summer vs winter. In summer it can be almost all the way out. In winter, even pre-heated lucky to pull it out much at all.
Stewartb
Final Approach
The best trick for cold starts with cold cylinders is to prime, then leave the primer out and charged to feed fuel to the reluctant engine.
chemgeek
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You use the mixture to adjust the air:fuel ratio to a desired optimum, for simple engines typically best economy or best power. If you run full rich above 5000 MSL or so, you are almost certainly not at best power mixture. You can verify this by leaning in level flight and watching rpm rise. Running full rich will put you in very uncertain fuel planning territory (POH fuel consumption tables usually assumes proper leaning), and will result in prodigious carbon buildup, which could worst case interfere with proper spark plug function. Do your cylinders a favor and run them properly leaned at cruise or at altitude. Lean agressively during taxi, too. Your spark plugs will look squeaky clean and you won't get as much lead or carbon buildup. And you will save a boatload on fuel.
Dan Thomas
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Mixture control in today's cars is automatic, and many people are too young to remember the hassles we had with driving over high mountain passes in pre-ECU cars. The car's power was reduced not only by the drop in air density, but by the rich mixture that resulted. In airplanes we have a manual means of adjusting it to get the best and safest performance, and a pilot needs to understand why.
The carburetor feeds fuel into the airflow based on the air's velocity through it, not on that air's density. It's velocity that causes the pressure drop that sucks the fuel into the airflow in the venturi. At altitude, or on hot days, the density reduces but the fuel flow doesn't change much, so the mixture gets richer, and if it gets rich enough the engine will quit. Fuel injection systems are calibrated to deliver fuel based on throttle position and RPM (Continental), or throttle position and airflow (Lycoming/RSA). They also need manual mixture control.
The carburetor feeds fuel into the airflow based on the air's velocity through it, not on that air's density. It's velocity that causes the pressure drop that sucks the fuel into the airflow in the venturi. At altitude, or on hot days, the density reduces but the fuel flow doesn't change much, so the mixture gets richer, and if it gets rich enough the engine will quit. Fuel injection systems are calibrated to deliver fuel based on throttle position and RPM (Continental), or throttle position and airflow (Lycoming/RSA). They also need manual mixture control.