Engine start with mixture rich?

[PinionZero1]

I’ve asked a lot of people why we start the Lycoming IO-360-A1A with mixture cutoff after priming. 

 I started the other day with keeping the mixture rich and it fired up easier than when I start it in the cutoff position.

Any Mooniacs out there have a logical explanation why Lycoming has this procedure?

[Ragsf15e]

39 minutes ago, PinionZero1 said:

I’ve asked a lot of people why we start the Lycoming IO-360-A1A with mixture cutoff after priming. 

 I started the other day with keeping the mixture rich and it fired up easier than when I start it in the cutoff position.

Any Mooniacs out there have a logical explanation why Lycoming has this procedure?

Dunno, but I had my F for ~10 years and it started great, but I was still able to learn a new technique from someone on this board… maybe @Shadrach or @M20F?

When priming, both mixture and throttle all the way in (which I already knew), the pull mixture to cutoff, THEN turn off boost pump.  The fuel pressure will stay elevated. Start the engine, and when you push in the mixture after it fires, it’s a smoother transition to normal idle as the fuel pressure is already up.

Now back to your question… my Continental TSIO-360mb starts with mixture rich…

[Fritz1]

Starting with mixture cutoff will eventually resolve a flooded engine, everybody has their own way of doing things, I start the Bravo with mixture rich, 4-5 seconds priming injection at full throttle which actuates the boost pump, pull throttle to idle, 1.5 turns on the throttle, if she does not fire after one revolution, hit the boost pump and she will fire, idle at 1000, I rather underprime than overprime, quick shot with the boost pump into the cranking engine resolves any underpriming

[M20F]

When cold I prime liberally (maybe 1-2 seconds with green PSI) then mixture off, slight crack on throttle, crank, 1-2 blades, advance mixture when fires. 
 
When hot.  Half throttle, no prime, mixture off, crank, 3-4 blades, fires, advance throttle, if it seems like it will die hit boost pump, retard throttle to idle.  This method also works for a flood start, just more blades spinning till start. 
 
I haven’t had a starting issue in probably 10yrs and whenever I have really had one (always hot) I just flood it.  If it isn’t starting cold you probably aren’t priming enough or something else is wrong.  The IO360 is bueno when cold, it can be naughty when hot (but a flood start will always fix that).

I have a Skytec starter which helps.  

[M20F]

9 hours ago, PinionZero1 said:

I’ve asked a lot of people why we start the Lycoming IO-360-A1A with mixture cutoff after priming. 

 I started the other day with keeping the mixture rich and it fired up easier than when I start it in the cutoff position.

Any Mooniacs out there have a logical explanation why Lycoming has this procedure?

My guess is you are not priming enough which is why it starts easier with mixture forward.  Try priming 1-2 seconds more, then start with idle cutoff.  
 
In the end though if what you are doing is consistent and repeatable, just keep doing it.  

[GeeBee]

The logical reason is the Lycoming has a diaphragm fuel pump which delivers the same volume of fuel each blade swing, regardless of RPM. So at cranking RPM you get a lot of fuel, especially if the mixture is rich causes a lot of fuel through the servo. If the engine does not catch, say due to weak spark, you can quickly become flooded. This raises the prospect of a cranking fire.  A Continental has a rotary pump which is not very efficient at cranking RPM as it relies on ejection force. Thus until it gets to idle RPM it starts primarily on prime fuel. Lycoming wants you to do the same thing, start on prime fuel then switch over to pumped fuel but it is done differently because of different ways the fuel is pumped.

The above posters are correct. If it starts better mixture rich you likely are not priming enough using the Lycoming procedure. Usually with Lycomings if I am unfamiliar with the airplane, I usually over prime a little bit and advanced the throttle as I crank until the airflow meets the amount of fuel in the intake. That usually also cuts down on the possibility of an intake back fire. 

[PinionZero1]

3 hours ago, GeeBee said:

The logical reason is the Lycoming has a diaphragm fuel pump which delivers the same volume of fuel each blade swing, regardless of RPM. So at cranking RPM you get a lot of fuel, especially if the mixture is rich causes a lot of fuel through the servo. If the engine does not catch, say due to weak spark, you can quickly become flooded. This raises the prospect of a cranking fire.  A Continental has a rotary pump which is not very efficient at cranking RPM as it relies on ejection force. Thus until it gets to idle RPM it starts primarily on prime fuel. Lycoming wants you to do the same thing, start on prime fuel then switch over to pumped fuel but it is done differently because of different ways the fuel is pumped.

The above posters are correct. If it starts better mixture rich you likely are not priming enough using the Lycoming procedure. Usually with Lycomings if I am unfamiliar with the airplane, I usually over prime a little bit and advanced the throttle as I crank until the airflow meets the amount of fuel in the intake. That usually also cuts down on the possibility of an intake back fire. 

Thanks GeeBee… this is the answer I am looking for!!!  Appreciate it.

[Shadrach]

On 6/14/2025 at 12:58 PM, PinionZero1 said:

Thanks GeeBee… this is the answer I am looking for!!!  Appreciate it.

In the cold dense air of winter, an IO360 can be started almost like a car. Mixture rich, throttle cracked and turn the key.  However, that procedure can flood the engine before it fires if conditions are not ideal.  It matters not what technique you use as long as a combustible mix of air and fuel is delivered to the cylinders.  For cold starts, the prime, idle cut-off and crank with the throttle cracked method has always achieved consistent results for me. The engine often starts on the first blade.

[MikeOH]

3 minutes ago, Shadrach said:

The prime, idle cut-off with throttle cracked method for cold starts has always achieved consistent results for me. The engine often starts on the first blade.

That's two of us!

[Shadrach]

Another benefit to the prime, idle cu-off, and crank method is that it tells you that the system is holding pressure.

I once scrubbed a trip because the fuel pressure would not hold. What I discovered was a very slight leak in the line between the servo and the firewall/fuel pressure gauge. It was under the fire sleeve, so not immediately obvious. We found it by smell.  Caught it by noting that pressure was not holding consistently after prime

[PinionZero1]

All great info… thanks for your time to respond.