idle mixture procedure details?

[13brv3]

Greetings,

I've been working toward setting the idle mixture on my O-360, and I have a question about the exact procedure.

According to the manual "...move the cockpit mixture control lever with a smooth steady pull toward idle cut-off..." My question is how long should this "smooth steady" motion take?

If I pull the lever at a rate of 3-5 second from rich to cutoff, I get about a 50 rpm increase near the lean end. If I pull it much slower, or hunt for the spot with max rpm, then I get maybe 150 rpm increase. Is it right the way it is, or am I way too rich still

FWIW, I haven't spent much time messing with this because it's a new engine, and I need to put some hours on it before spending too much time with ground runs like this. Mostly, I'm checking it every time I shut down the engine from a test run.

Thanks,
Rusty (receive airworthiness certificate today, first flight is imminent)

[Mel]

Rusty,
First of all, congratulations on getting your certificate. Second, don't worry too much about idle mixture at this point. It will change as the engine get broken in. Once you are above idle the idle mixture circuit is negligible.

[13brv3]

Thanks for the comments Mel. I didn't realize the mixture would change as the engine breaks in, but at some point I'll still have to do this, so hopefully someone can give me some advice on the proper method. I can call Aerosport if necessary, but I figured this was a common procedure that would generate plenty of quick answers here.

Thanks,
Rusty

[mahlon_r]

Rusty,
Shoot for a total 50 rpm rise by slowllllllllllllllllllllly pulling the mixture control.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk.?

[13brv3]

Thanks Mahlon. As always, I appreciate your participation in these forums, even for folks like me who bought from the competition (purely for the warranty).

Thanks,
Rusty (might be a good day for a first flight...)

[brianwallis]

For Carb engines... I set the idle around 700 somewhere... I slowly pull the mixture back and I aim for about a 30 rpm rise....
For Injected engines... I'm at 1000 rpm and then look for a 30 rpm rise.
Good Luck//./ ps Listen to EVERYTHING MEL SAYS... and you will have to change the mixture (I'm thinking about 12 clicks) once it's broken in.
Brian Wallis
AP/IA

[13brv3]

Thanks for the comments Brian. Does the mixture go richer, or leaner when it breaks in? I'll probably adjust it at least in the ballpark, then watch it for a while until I figure the engine is done breaking in. Then I'll set it for 30-50 rpm rise as you and others have suggested.

Cheers,
Rusty

[piacibello franco]

I was reading Van's Final Inspection and test flight chapter and noted the part about this rpm rise on mixture cutoff. I am flying a low time RV7a and I watched for that rise when I shutdown, but did not observe anything. Perhaps I am pulling the mixture out too quickly. The engine is a Superior XP360, carbureted.

Why is the RPM supposed to rise? What should I look for, and should I be seeing this rise on a brand new engine? (Not broken in yet, 40 hrs)

[edneff]

I had the same question. Anybody know why the RPM is supposed to increase as you go to idle cut-off on the mixture???

[Kevin Horton]

Originally Posted by edneff

I had the same question. Anybody know why the RPM is supposed to increase as you go to idle cut-off on the mixture???

I believe it is because the idle mixture is supposed to be richer than the mixture for best power. So, as you lean the mixture out, you come closer and closer to the mixture for best power, and the power increases, which leads to an increase in rpm. The amount of rpm rise is a good measure of how far on the rich side the mixture is set.

Note: I am not 100% certain of the above info, but it fits with established theory as I understand it.