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Idle Mixture Adjustment?

Mark Dickens

Well Known Member
Patron
I am helping a friend get his plane ready for first flight and we had to cancel today because the engine won't run reliably or smoothly at idle.

Details: Titan IO-360, Precision Silverhawk injector, EFII ignition.

Symptoms: Engine runs fine at higher RPMs, but dies when pulling throttle to idle with light backfiring and rough running just before quitting

Question, I figure the culprit is a misadjusted idle mixture. Should we go richer or leaner? I am thinking leaner. What ye think? Any other adjustments suggested?
 
Idle

I would get someone familiar with the adjustment to look at it before a first flight, it is no time to be dealing with a cranky engine.
 
Lean. Its backfiring because there's unburnt fuel in the exhaust. You want a 50 rpm rise like a carb when you pull the mixture.
 
Backfiring during deceleration is typically due to an overly lean idle mixture. It is technically called an after fire, not a backfire and is caused by the lean condition creating intermittent misfires that place an unburned fuel air mix in the exhaust. The next combustion event that properly fires, ignites the mixture in the exhaust pipe. You have to be REALLY rich to get afterfires from a rich mixture.

The Bendix or Marvel manual should give a recommended starting position in number of turns out from fully seated.

Larry
 
Last edited:
Heads or tails?��
The best part is that either answer could solve the problem.
 
Backfiring during deceleration is typically due to an overly lean idle mixture. It is technically called an after fire, not a backfire and is caused by the lean condition creating intermittent misfires that place an unburned fuel air mix in the exhaust. The next combustion event that properly fires, ignites the mixture in the exhaust pipe. You have to be REALLY rich to get afterfires from a rich mixture.

The Bendix or Marvel manual should give a recommended starting position in number of turns out from fully seated.

Larry
I failed to note that this is on the ground. We never got the idle sorted out so the the flight didn't happen. The next thing will be checking everything out per the manual.
 
Too Lean likely

Thanks for the responses. This is my airplane. I sure wanted to go flying...

I am idling too low (~600), but also I do not get any rpm rise when leaning. The engine will idle smoothly that low, but only if the throttle is retarded slowly.

Per the Silverhawk manual I should be idling 700-750 and that is the first place to start. Also per the manual, ?an immediate decrease in rpm toward cut-off without a preceding increase indicates too lean of a mixture?.

It appears to me the first fix is to (1) set throttle idle stop to 700-750, (2) enrichen the mixture until I am getting (ideally) 10-50 rpm rise.

The issue became apparent following some high power run-ups with a couple of throttle jam accel/decel. The engine acceleration had no issues. The rapid decels to throttle idle would cause engine to die.
 
I failed to note that this is on the ground. We never got the idle sorted out so the the flight didn't happen. The next thing will be checking everything out per the manual.

I was referring to RPM decelleration, not airspeed. When you pull the throttle from High power to idle, that is decelleration (prop is decellerating) even on the ground.

Larry
 
Thanks for the responses. This is my airplane. I sure wanted to go flying...

I am idling too low (~600), but also I do not get any rpm rise when leaning. The engine will idle smoothly that low, but only if the throttle is retarded slowly.

Per the Silverhawk manual I should be idling 700-750 and that is the first place to start. Also per the manual, ?an immediate decrease in rpm toward cut-off without a preceding increase indicates too lean of a mixture?.

It appears to me the first fix is to (1) set throttle idle stop to 700-750, (2) enrichen the mixture until I am getting (ideally) 10-50 rpm rise.

The issue became apparent following some high power run-ups with a couple of throttle jam accel/decel. The engine acceleration had no issues. The rapid decels to throttle idle would cause engine to die.

You may find that richening up the mixture will bring your RPMs up near 700 if you are as lean as I think you are.
 
You can download a manual for the silver hawk and it will cover all the details of how to set the idle mixture.
 
I idle smoothly at 600rpm with the Silverhawk and standard mags. 750 would be too high for me and I am CS.
Just a data point for you.
 
Heads or tails?��
The best part is that either answer could solve the problem.

I'm in the camp that your mixture is already too lean. Fatten it up (enrichen) and make sure you get an increase in RPM as you move the mixture to idle from full rich.

-Marc
 
Update... I enrichened the mixture which helped a little, but I believe the issue I was having had more to do with heat. The engine stumble and die issue I was experiencing was occurring after ground ops where the engine was really warm. I think there was some fuel vaporization causing an even leaner mixture. I flew the airplane for first flight today with no related issues after a proper (but brief) run-up. The engine idles nicely and seems happy at ~630-650 rpm.
 
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