Mixture setting before start

midlifeflyer

midlifeflyer

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Question for the mechanics: What does the mixture control do when the engine is off? Fore a practical example, is there a real difference between the following start-up procedures in an simple fuel-injected 172:

Start #1 (from the POH)
1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- IDLE CUT OFF.
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- ADVANCE to obtain 3 to 5 GPH fuel flow, then return to IDLE CUT OFF position.
7. Ignition Switch -- START (release when engine starts).
8. Mixture -- ADVANCE smoothly to RICH when engine fires.

Start #2 (modified; mods in red)
1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- FULL RICH
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- 3 to 5 GPH fuel flow, then IDLE CUT OFF position.
7. Ignition Switch -- START (release when engine starts).
8. Mixture -- ADVANCE smoothly to RICH when engine fires
 
Not a mechanic, but I know a thing or two about engines.

I am assuming you are referring to a 172 with an injected Lycoming. Basically the mixture control in the idle cut-off position will prevent any pressure from getting to the fuel distributor (spider) and thus injectors. In a cold start, this really doesn't matter because there is no pressure. In a hot start, it can matter because the Lycoming fuel system tends to retain fuel pressure that you'll see if you push the mixture forward. This can change the amount of fuel that ends up in the engine and thus change how well (or if at all) the engine starts.

I also have my preferred hot and cold start techniques for injected Lycomings which I have posted and can share again if you'd like.
 
Always interested in good input, @Ted, so share away :)

In this case it's not a "what's the best way to start the engine" question. Much simpler (or simpleton :D). More about shortcuts.

A number of fuel injected types have similar cold start sequences. The mixture starts at cutoff and moves to full rich for the boost prime. I was just wondering whether I can skip that first idle and just put the mixture at full rich right at the beginning of the start-up sequence.

And yeah, I should have mentioned normal cold start. Hot starts change everything with much more variety.
 
midlifeflyer said:
A number of fuel injected types have similar cold start sequences. The mixture starts at cutoff and moves to full rich for the boost prime. I was just wondering whether I can skip that first idle and just put the mixture at full rich right at the beginning of the start-up sequence.
Click to expand...

For cold start, absolutely you can shove the mixture full rich at the beginning of the startup sequence. When I got in the Aztec for a cold start I'd shove all the levers full forward when I got in the plane. It doesn't hurt anything or change the procedure.

My start procedures for Lycomings are below. I'm only talking about the engine stuff, I'll let you add the appropriate electrical power bits for your particular aircraft.

Cold Start:

1) Everything full forward
2) Fuel pump ON for 3-5 seconds. Exact time depends on the aircraft.
3) Fuel pump OFF
4) Throttle cracked, mixture to idle cut-off
5) Crank until it starts, push mixture full rich once it starts (or probably less than full rich for high elevations)

For really cold starts, at step 4 leave the mixture full rich. Sometimes you can use the trick of priming while you're starting to get extra fuel in for those super cold starts.

Hot Start:

1) Fuel pump ON
2) Throttle forward. Mixture forward 3-5 seconds, flood the engine a bit.
3) Mixture idle cut-off
4) Crank until it starts. When it starts, use your 3 hand maneuver to push the mixture forward and pull the throttle back. Works really well once you get the hang of it.
 
midlifeflyer said:
I was just wondering whether I can skip that first idle and just put the mixture at full rich right at the beginning of the start-up sequence.
Click to expand...
Maybe Ted can elaborate on the 'why', but starting with the mixture full rich is exactly what the POH calls for in the Continental powers Baron.
 
Fearless Tower said:
Maybe Ted can elaborate on the 'why', but starting with the mixture full rich is exactly what the POH calls for in the Continental powers Baron.
Click to expand...

It really just has to do with the differences in the Continental fuel system. I don't know all of the exact whys, but I think it has to do with the relative locations of the fuel system components on Continentals vs. Lycomings (i.e. less prone to boil the fuel). I think it also has to do with how the throttle and mixture levers act as orifices within the system.

I generally use different techniques on Lycomings vs. Continentals because of the fuel system differences.
 
Ted DuPuis said:
It really just has to do with the differences in the Continental fuel system. I don't know all of the exact whys, but I think it has to do with the relative locations of the fuel system components on Continentals vs. Lycomings (i.e. less prone to boil the fuel). I think it also has to do with how the throttle and mixture levers act as orifices within the system.

I generally use different techniques on Lycomings vs. Continentals because of the fuel system differences.
Click to expand...
I figured it was something like that.
 
Fearless Tower said:
Maybe Ted can elaborate on the 'why', but starting with the mixture full rich is exactly what the POH calls for in the Continental powers Baron.
Click to expand...
Same-same on the TSIO-360 and its required for priming. Full Rich is fine when cold. Full Rich is not so fine when hot.
 
Clark1961 said:
Same-same on the TSIO-360 and its required for priming. Full Rich is fine when cold. Full Rich is not so fine when hot.
Click to expand...

Interesting. On the Continentals I've flown usually you can do hot starts full rich, too. Sometimes pulling it back an inch or so does help it start easier.
 
Ted DuPuis said:
Interesting. On the Continentals I've flown usually you can do hot starts full rich, too. Sometimes pulling it back an inch or so does help it start easier.
Click to expand...
Hot start: pressurize fuel system, throttle open, mixture closed, start cranking, open mixture slowly, when engine fires open mixture a hair more while retarding throttle and using third hand to continue cranking until engine is running. Note that having a third hand is an optional item installed at owners expense.
 
Ted DuPuis said:
Interesting. On the Continentals I've flown usually you can do hot starts full rich, too. Sometimes pulling it back an inch or so does help it start easier.
Click to expand...

That was one thing I liked about the Baron. Easy hot starts. You just ran the boost pumps for 30 seconds or so with the mixture in cutout and then followed the normal cold start procedure
 
Fearless Tower said:
That was one thing I liked about the Baron. Easy hot starts. You just ran the boost pumps for 30 seconds or so with the mixture in cutout and then followed the normal cold start procedure
Click to expand...
I used to always do that...until ted showed me his way...which has worked perfectly on every Continental I've tried. Added benefit of not having to sit there running the pump so much.

I just shove everything full forward, hit the boost pump for a couple seconds, then start cranking and slowly pulling the throttle back at the same time. Eventually, it'll light, and you just need to yank the throttle back so that you don't look like a doctor in a cirrus starting at 2700 RPM.

I've had by far more consistent starts doing it this way and it doesn't take me any longer than a normal cold start.
 
And @jesse beat me to my Continental hot start procedure. :)
 
Don't know if this applies to the smaller engines but the big Continental IO550s work like this:

Cold
Mixture Full Rich
Throttle Max then back to quarter
High Boost Prime 2 seconds
Boost On
Ignition On (warm up at 1000 rpm or less)

Hot Start
Mixture Full Rich
Throttle to quarter
High Boost Prime 2 seconds
Ignition On Then Boost On just as engine catches
 
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Clark1961 said:
Hot start: pressurize fuel system, throttle open, mixture closed, start cranking, open mixture slowly, when engine fires open mixture a hair more while retarding throttle and using third hand to continue cranking until engine is running. Note that having a third hand is an optional item installed at owners expense.
Click to expand...

^this^
 
citizen5000 said:
Don't know if this applies to the smaller engines but the big Continental IO550s work like this:

Cold
Mixture Full Rich
Throttle Max then back to quarter
High Boost Prime 2 seconds
Boost On
Ignition On (warm up at 1000 rpm or less)

Hot Start
Mixture Full Rich
Throttle to quarter
High Boost Prime 2 seconds
Ignition On Then Boost On just as engine catches
Click to expand...

That's one method. My methods are different and work on 550s quite well too.
 
midlifeflyer said:
Question for the mechanics: What does the mixture control do when the engine is off? Fore a practical example, is there a real difference between the following start-up procedures in an simple fuel-injected 172:

Start #1 (from the POH)
1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- IDLE CUT OFF.
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- ADVANCE to obtain 3 to 5 GPH fuel flow, then return to IDLE CUT OFF position.
7. Ignition Switch -- START (release when engine starts).
8. Mixture -- ADVANCE smoothly to RICH when engine fires.

Start #2 (modified; mods in red)
1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- FULL RICH
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- 3 to 5 GPH fuel flow, then IDLE CUT OFF position.
7. Ignition Switch -- START (release when engine starts).
8. Mixture -- ADVANCE smoothly to RICH when engine fires
Click to expand...

How is the second one a short cut? They both have 8 steps.

Oh, and if this is a 172, fuel pump off is missing.
 
Have used Ted's for Lycosaurs and Clark's for Connies.

Interested to try Ted's on Connie's, but never really had problems with either one of the others.

And of course my boring carb'd Connie is just "set mixture an inch richer than where we'd cruise it at this altitude and start it... if it's cold, one shot of prime... really cold, two."

When I first joined the 182 co-ownership everyone was starting it full rich like the book says and it started but always had to be immediately leaned at this altitude. Mechanic one day when we were doing multiple starts to run up for a test of some vacuum stuff, reaches over, pulls the mixture out about halfway and turns the key, and it's instantly running in one blade. We learned something that day. Now everyone starts it leaned and it starts soooooo easy. Obviously it's because of our altitude sitting on the ground, but the book doesn't mention it, even though it is intuitively obvious after you realize it works better.

Probably relates to the simplicity of the system and a lot of starts, but that old O-470 is the easiest starting of anything I've found. Just turns over and runs, hot or cold.

Note: The Lycosaur TO-360-E1A6D on the Turbo Seminole likes the Connie method better. Both cold and hot. Odd, but it does. Hot if you keep the mixture at idle cutoff and crank with the throttle cracked, and move the mixture immediately forward as soon as you hear one cylinder hit, it starts like a dream. Usually four or five blades. Vrooom.
 
denverpilot said:
When I first joined the 182 co-ownership everyone was starting it full rich like the book says and it started but always had to be immediately leaned at this altitude. Mechanic one day when we were doing multiple starts to run up for a test of some vacuum stuff, reaches over, pulls the mixture out about halfway and turns the key, and it's instantly running in one blade. We learned something that day. Now everyone starts it leaned and it starts soooooo easy. Obviously it's because of our altitude sitting on the ground, but the book doesn't mention it, even though it is intuitively obvious after you realize it works better.
Click to expand...
So much of "the book" assumes near sea level altitudes.
 
midlifeflyer said:
So much of "the book" assumes near sea level altitudes.
Click to expand...

I only get worried when I see transients with fuel pouring out of the bottom of the cowl and they're cranking, and cranking, and cranking... haha.

If I see that and can safely get there, I'll always run over and signal them to stop with the cut signal and then let 'em know they have a pool of fuel under the nose -- that's always a fun conversation starter -- and offer some advice to get it started and gently make sure they understand their POH leaning procedure for whatever they're flying... try to be nice about it. They often have passengers. No need to embarrass them too much.

It's just common courtesy around here. Can't really expect everyone to "get it". LOL. It's better than watching them light their airplane on fire, I figure. :)

Usually at least one per summer.
 
denverpilot said:
I only get worried when I see transients with fuel pouring out of the bottom of the cowl and they're cranking, and cranking, and cranking... haha.

If I see that and can safely get there, I'll always run over and signal them to stop with the cut signal and then let 'em know they have a pool of fuel under the nose -- that's always a fun conversation starter -- and offer some advice to get it started and gently make sure they understand their POH leaning procedure for whatever they're flying... try to be nice about it. They often have passengers. No need to embarrass them too much.

It's just common courtesy around here. Can't really expect everyone to "get it". LOL. It's better than watching them light their airplane on fire, I figure. :)

Usually at least one per summer.
Click to expand...

It's not just up there. It happens everywhere. A lot of people just don't know how to start airplanes. Honestly, biggest argument for electronic engine controls in my opinion.
 
Ted DuPuis said:
It really just has to do with the differences in the Continental fuel system. I don't know all of the exact whys, but I think it has to do with the relative locations of the fuel system components on Continentals vs. Lycomings (i.e. less prone to boil the fuel). I think it also has to do with how the throttle and mixture levers act as orifices within the system.

I generally use different techniques on Lycomings vs. Continentals because of the fuel system differences.
Click to expand...

I thought the difference on the injected engines is that Continental has a fuel return line to the tank and Lycoming doesn't. On a hot Continental start opening the mixture and running the electric boost pump will circulate cold fuel from the tank through the heat soaked fuel lines in the engine compartment and eliminate any vapour issues before cranking the starter(s).

To the OP, as others have noted you can use either sequence and it will work, but I personally find it more logical to group related actions. My cold start sequence for my Aztec Lycomings is the similar to your "Start #1" sequence but I reverse the order of 5 and 6 - Mixture open, fuel boost pump on until fuel flow indication, then fuel pump off (missing in your sequence), mixture to idle cut off. These 4 things are related, to prime the cold engine, and it seems logical to group them together in the flow order I use.
 
GRG55 said:
To the OP, as others have noted you can use either sequence and it will work, but I personally find it more logical to group related actions. My cold start sequence for my Aztec Lycomings is the similar to your "Start #1" sequence but I reverse the order of 5 and 6 - Mixture open, fuel boost pump on until fuel flow indication, then fuel pump off (missing in your sequence), mixture to idle cut off. These 4 things are related, to prime the cold engine, and it seems logical to group them together in the flow order I use.
Click to expand...
Agreed.

I was using the step-by-step from the POH (and failed to copy one) solely to have an example of what I was asking. My personal checklists are significantly condensed and grouped into my flow patterns.

So, for example, my actual cold start procedure begins with a flow of the power quadrant, setting mixture, prop, throttle (and whatever else might, or might not, be there) in the priming position before I hit the master.
 
GRG55 said:
I thought the difference on the injected engines is that Continental has a fuel return line to the tank and Lycoming doesn't. On a hot Continental start opening the mixture and running the electric boost pump will circulate cold fuel from the tank through the heat soaked fuel lines in the engine compartment and eliminate any vapour issues before cranking the starter(s).
Click to expand...

A lot of people think that. The return line does allow you to do the option of running the boost pump for 30-60 seconds and then starting, but note that the hot start technique I use on Continentals doesn't make use of that (and by Jesse's observations, my hot start method is more reliable).

Really, Continental and Lycoming fuel systems are just completely different. Continental basically has a fuel pump which provides pressure based off of RPM and sometimes also manifold pressure/ambient air pressure (depends on the pump). Then you have a "fuel controller" which is effectively two orifices, one based on throttle and one based on mixture. The Lycoming system has a fuel pump that provides a regulated pressure to a fuel servo, which is basically the air flow meter of a pressure carburetor. The airflow through the venturi creates a pressure that moves a diaphragm within the servo, and that's what changes fuel flow. In fact, Lycoming fuel injection systems have something in the original design descriptions that describe them as a "Pressure Carburetor with multi-point distribution and particularly good resistance to icing" or something like that.

My personal opinion is the Lycoming fuel systems are better than the Continentals. They're much simpler from a plumbing perspective (especially compared to a turbo Continental) and I find that because they're airflow based, they don't do weird things like have too low of fuel flow at part throttle climb settings on naturally aspirated birds. They can be harder to hot start but I never had issues once I got the tricks worked out.
 
Ted DuPuis said:
And @jesse beat me to my Continental hot start procedure. :)
Click to expand...

Yes. When it comes to hot starting a fuel injected Continental there must be dozens of theories out there. There are videos on it and the Continental lit has their own take on it.

One of the techniques is called the 'three hands method' involves 'sliding' the mixture from cutoff to full as the engine catches.

The method I listed comes from the mechanic that did our annual a month ago (TSIO550K engine). He gave us a 15 minute description of the relationship between the mixture, throttle and boost pumps. Then said his method only requires two hands and to try it out. So far it seems to work consistently well.

Also, if the hot engine doesn't start after three turns (according to the mechanic) then the method changes again because the engine is flooded and hot and that's a new level of nightmare.
 
citizen5000 said:
Yes. When it comes to hot starting a fuel injected Continental there must be dozens of theories out there. There are videos on it and the Continental lit has their own take on it.

One of the techniques is called the 'three hands method' involves 'sliding' the mixture from cutoff to full as the engine catches.

The method I listed comes from the mechanic that did our annual a month ago (TSIO550K engine). He gave us a 15 minute description of the relationship between the mixture, throttle and boost pumps. Then said his method only requires two hands and to try it out. So far it seems to work consistently well.

Also, if the hot engine doesn't start after three turns (according to the mechanic) then the method changes again because the engine is flooded and hot and that's a new level of nightmare.
Click to expand...

So I realized that I didn't state my hot start method for Continentals here (at least I don't think I did). Here it is:

- Everything full forward
- Prime like a cold start (3-5 seconds)
- Hit the starter. While cranking, pull the throttle back (at a rate of about 5 seconds from full throttle to idle)
- It'll catch at a certain point, usually about 1/3 throttle. Then adjust the throttle appropriately

Works the first time, 99% of the time.
 
Hot starts for the Continental IO-550-N in the SR22 we pulled throttle and mixture full back (idle/cut-off), run the pump for 60 seconds (to clear vaporized lines), pump off, throttle slightly open, mixture full rich and start. Works great.
 
wayne said:
Hot starts for the Continental IO-550-N in the SR22 we pulled throttle and mixture full back (idle/cut-off), run the pump for 60 seconds (to clear vaporized lines), pump off, throttle slightly open, mixture full rich and start. Works great.
Click to expand...

Doing this in the SR22T dumps fuel on the ground. But this is one technique often cited.
 
Last edited:
denverpilot said:
I only get worried when I see transients with fuel pouring out of the bottom of the cowl and they're cranking, and cranking, and cranking... haha.

If I see that and can safely get there, I'll always run over and signal them to stop with the cut signal and then let 'em know they have a pool of fuel under the nose -- that's always a fun conversation starter -- and offer some advice to get it started and gently make sure they understand their POH leaning procedure for whatever they're flying... try to be nice about it. They often have passengers. No need to embarrass them too much.

It's just common courtesy around here. Can't really expect everyone to "get it". LOL. It's better than watching them light their airplane on fire, I figure. :)

Usually at least one per summer.
Click to expand...


I had this happen with a FI arrow at sea level to me. Was SUPER embarrassed.
 
citizen5000 said:
Don't know if this applies to the smaller engines but the big Continental IO550s work like this:

Cold
Mixture Full Rich
Throttle Max then back to quarter
High Boost Prime 2 seconds
Boost On
Ignition On (warm up at 1000 rpm or less)

Hot Start
Mixture Full Rich
Throttle to quarter
High Boost Prime 2 seconds
Ignition On Then Boost On just as engine catches
Click to expand...
This works for me too. Except for a cold start I just prime until I see the GPH peak (usually like 5 seconds) then bring the throttle back to a quarter inch. For the hot start, no prime, throttle almost full forward. Boost pump when engine catches then bring throttle back for 1000rpm
 
wayne said:
Hot starts for the Continental IO-550-N in the SR22 we pulled throttle and mixture full back (idle/cut-off), run the pump for 60 seconds (to clear vaporized lines), pump off, throttle slightly open, mixture full rich and start. Works great.
Click to expand...

So how does the pump clear vaporized lines with the mixture at idle cut off? Is the Continental return to tank loop upstream of the mixture control?

I've never owned a plane with a Continental engine so find the differences from Lycoming mildly intriguing.
 
GRG55 said:
So how does the pump clear vaporized lines with the mixture at idle cut off? Is the Continental return to tank loop upstream of the mixture control?

I've never owned a plane with a Continental engine so find the differences from Lycoming mildly intriguing.
Click to expand...
The return line starts in the fuel control valve so it's at the mixture control. The cutaways I've seen show the return port upstream of the port which sends fuel to the spyder. I've never actually seen the internals of the fuel control.
 
There is a return line, where I don't know. I can tell you that after some frustrating mixed results I was shown that method and had no issues with hot starts since.
 
Do IO Lycomings have fuel return lines to a header tank?

Do any factory IO systems use purge valves?

My new exp has Airflow Performane mecanical injection (no fuel return) and uses a zero leakage servo so no purge valve. I've been advised to get in the habit of turning off the fuel valve at shutdown because zero leakage isn't perfect. I never had to do that in Continentals.
 
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