Idle Cut-off Question

[Lizard Lips]

Just finished my RV-14A and am about 3 hours into my Phase 1 fly-off. Installed is a Lycoming IO-390 and I have begun having a problem with the idle cut off. After a brief run, or flight, the engine will not shut down when the mixture is pulled all the way out, supposedly in idle cut-off position. I don't like to kill the engine by shutting off the mags so my only option is to put the fuel selector in the OFF position and wait for it to run out of gas.

I assume I need to adjust the mixture control a few turns. Is there a procedure for setting the mixture to the "sweet" spot of idle cut off?

Joe
Longview, Texas

RB-14A First Flight Completed 05/31/2020
Working on Phase One
Arion Lightning, LS-1 Flown regularly
2020 dues paid

[RWoodard]

There are plenty of other folks on here who are more knowledgeable, but I?ll kick off the discussion by suggesting that you make sure you?re getting full travel on the mixture control arm when you move the mixture cable from stop to stop.

I just helped the engine shop reinstall my IO-320 in my RV-3 and the mixture control in particular needed to be adjusted just perfectly in order to allow full travel in both directions.

To be clear... you?re checking for full travel on the mixture control arm on your fuel servo, not necessarily full travel on the red knob.

I?ll leave any servo adjustment instructions to the experts! :-)

[RV10inOz]

And there is a potential that with the engine running, you might be getting some flex in the cable mounting bracket and the ICO stop is not being met firmly.

[Lizard Lips]

I strongly suspect I will find the mixture control arm is not getting full throw to the idle cutoff position. I'll know tomorrow when I remove the cowling and have a look. Idle mixture could be off by a click or two also.
The 100+ degree heat has caused me to wait until early morning to undertake this task.

Thanks for your comments.

Joe
Longview, Texas

[mahlon_r]

Idle mixture setting has no effect on ICO. When the mixture is moved to ICO, a plate in the servo shuts the fuel off by closing the port that allows fuel out of the servo. At the same time the piston in the flow divider closes. So the servo shuts of the fuel and the flow divider backs it up. Adjusting the mixture will have no effect on that function. If the mixture is hitting the stops then the issue is a score on the plates inside the servo that are allowing fuel out of the servo when in ICO position or the flow divider isn't closing all the way. To check this, simply put the mixture in ICO position, disconnect the hose that goes from the servo to the flow divider and turn the boost pump on. No more then a drip, drip, drip should come out of that hose. If there is any kind of flow the problem is internal to the servo. If the flow divider is the issue the symptom will be that the engine doesn't cutoff cleanly and runs on when the mixture is moved to ICO, but eventually stops as the fuel in the lines is exhausted.
Good Luck,
Mahlon

[Lizard Lips]

Originally Posted by mahlon_r

Idle mixture setting has no effect on ICO. When the mixture is moved to ICO, a plate in the servo shuts the fuel off by closing the port that allows fuel out of the servo. At the same time the piston in the flow divider closes. So the servo shuts of the fuel and the flow divider backs it up. Adjusting the mixture will have no effect on that function. If the mixture is hitting the stops then the issue is a score on the plates inside the servo that are allowing fuel out of the servo when in ICO position or the flow divider isn't closing all the way. To check this, simply put the mixture in ICO position, disconnect the hose that goes from the servo to the flow divider and turn the boost pump on. No more then a drip, drip, drip should come out of that hose. If there is any kind of flow the problem is internal to the servo. If the flow divider is the issue the symptom will be that the engine doesn't cutoff cleanly and runs on when the mixture is moved to ICO, but eventually stops as the fuel in the lines is exhausted.
Good Luck,
Mahlon

That's great advice Mahlon. I will most certainly use your procedure to determine if I have reached ICO. Thanks for the response.
Joe
Longview, Texas