TCM Idle Mixture Setting

Cpt_Kirk

Cpt_Kirk

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I read that, as a general rule of thumb, that you should see an approximately 200rpm rise and then a quick fall when bringing the mixture to idle-cutoff from 1,000rpm in a piston pounder. The mechanic says the TCM book quotes a 30-40rpm rise should be seen and that the mixture is set too rich. I think the mixture is set too lean because I'm not seeing the proper FF on takeoff without the boost pump. I'd prefer it to be on the rich side of things.

Is every engine very specific on this rpm rise or is it a pretty general setting to make that particular engine happy?
 
The manufacturer's instructions trump rules of thumb and other old wive's tales.

I'm confused as to why the idle mixture setting has any bearing on takeoff power settings. It's only for IDLE. At any non-idle power settings, the big red knob controls the mixture.

The usual sign of a too-rich idle mixture is the engine dying at idle settings when very hot (like one you slow down on roll out after a long flight).
 
My engine is carb'd so take this for what it's worth.

I've worked hard to get takeoff and rich cruise fuel flows to where they need to be for temperature control and even temperature distributions. The primary thing I'm concerned with is ability to demonstrate at least 150* temp difference between full rich cruise and peak EGT without stumbling. A little more spread is even better for me to assure I have adequate fuel for winter ops in sub-zero temps. Takeoff flows are what they are once that's satisfied. I get no recognizable rise at idle cutoff.

O-520 assumed to produce 275hp, takeoff fuel flow is 24 +/- and full rich cruise is just shy of 20.

Are you using a good flow instrument or a Cessna instrument? Cessna's is notoriously inaccurate.
 
Cpt_Kirk said:
I read that, as a general rule of thumb, that you should see an approximately 200rpm rise and then a quick fall when bringing the mixture to idle-cutoff from 1,000rpm in a piston pounder. The mechanic says the TCM book quotes a 30-40rpm rise should be seen and that the mixture is set too rich. I think the mixture is set too lean because I'm not seeing the proper FF on takeoff without the boost pump. I'd prefer it to be on the rich side of things.

Is every engine very specific on this rpm rise or is it a pretty general setting to make that particular engine happy?
Click to expand...

What kind of engine ?

We spent some time chasing our tail on this on our TSIO520-UB. If the unmetered fuel pressure is sufficient to give us full takeoff fuel-flow, we end up with a somewhat rich mixture at idle. If we turn the unmetered pressure down to where it allows for the right mixture at idle, we can't get full takeoff fuel-flow.

I can fix the rich mixture during taxi with the red know. To fix the insufficient fuel flow on takeoff, I need to turn on the boost pump and monkey around with the mixture at the most inopportune time.
 
flyingron said:
The usual sign of a too-rich idle mixture is the engine dying at idle settings when very hot (like one you slow down on roll out after a long flight).
Click to expand...
The engine would die... if I left the boost pump off. Bringing it to LOW would keep everything running smoothly.

Stewartb said:
Are you using a good flow instrument or a Cessna instrument? Cessna's is notoriously inaccurate.
Click to expand...
'99 Bonanza instruments and a JPI 700.

weilke said:
What kind of engine ?
Click to expand...

TCM IO-550.
 
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Add an FS450 and calibrate it per instructions. Find out what your actual flows are.

Are your takeoff and climb out temps scary?
 
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Stewartb said:
Add an FS450 and calibrate it per instructions. Find out what your actual flows are.

Are your takeoff and climb out temps scary?
Click to expand...
The JPI shows what the FS450 does... just without the fancy turbine wheel.

This is the coolest engine I've ever flown. 350* in the climb at max power, 320*-330* in cruise.
 
Cpt_Kirk said:
The engine would die... if I left the boost pump off. Bringing it to LOW would keep everything running smoothly.

'99 Bonanza instruments and a JPI 700.



TCM IO-550.
Click to expand...
a hot engine dying at idle is a CMI classic hot engine, vapor lock, issue. The POH/AFM does address this and calls out use of the low boost.

Sounds normal to me.....

IMHO.....fuel flow at max power is more important than idle. I'd verify that your fuel flow is 29-31 gph at max power....and deal with idle using the mixture knob.
 
Checkout_my_Six said:
a hot engine dying at idle is a CMI classic hot engine, vapor lock, issue. The POH/AFM does address this and calls out use of the low boost.

Sounds normal to me.....

IMHO.....fuel flow at max power is more important than idle. I'd verify that your fuel flow is 29-31 gph at max power....and deal with idle using the mixture knob.
Click to expand...

And that fuel flow gauge should be checked. The OEM gauges are pressure gauges that are calibrated in GPH. The system makes assumptions based on pressure at the fuel manifold, and if the gauge is tired or the fuel injectors are worn it could read low.
 
Cpt_Kirk said:
I read that, as a general rule of thumb, that you should see an approximately 200rpm rise and then a quick fall when bringing the mixture to idle-cutoff from 1,000rpm in a piston pounder. The mechanic says the TCM book quotes a 30-40rpm rise should be seen and that the mixture is set too rich. I think the mixture is set too lean because I'm not seeing the proper FF on takeoff without the boost pump. I'd prefer it to be on the rich side of things.

Is every engine very specific on this rpm rise or is it a pretty general setting to make that particular engine happy?
Click to expand...

200 rpm rise is way high, indicates a too-rich idle mixture. Don't confuse idle mixture with full power mixture, they are controlled differently. The idle mixture can be too rich and you still will not see correct fuel flow on takeoff if you have a fuel delivery issue.

So sayeth the brand-new A&P :lol:
 
IO-XXX engines the proper rich/lean mixture proper settings are in the fuel control manuals, do not confuse the carb'ed engine with fuel injection types.

The 0-200/300 with the MA-3SPA 10-4894 carb, the proper idle mixture setting should be 10-50 RPM Rise after pulling the mixture all the way out.
All other engine carb configurations see the MM for the aircraft/engine.
 
Cpt_Kirk said:
I read that, as a general rule of thumb, that you should see an approximately 200rpm rise and then a quick fall when bringing the mixture to idle-cutoff from 1,000rpm in a piston pounder. The mechanic says the TCM book quotes a 30-40rpm rise should be seen and that the mixture is set too rich. I think the mixture is set too lean because I'm not seeing the proper FF on takeoff without the boost pump. I'd prefer it to be on the rich side of things.

Is every engine very specific on this rpm rise or is it a pretty general setting to make that particular engine happy?
Click to expand...

200RPM rise is not correct. Has the fuel system been calibrated recently? Suppose to happen at each annual. Call Continental support - they will discuss this in detail with you. The rise is indeed smaller than 200, and right before the engine dies.
 
GLMS_NC said:
200RPM rise is not correct. Has the fuel system been calibrated recently? Suppose to happen at each annual. Call Continental support - they will discuss this in detail with you. The rise is indeed smaller than 200, and right before the engine dies.
Click to expand...

I gather that you didn't read post 12.
 
As has been pointed out "rule of thumb" is 50, not 200. You got your thumbs mixed up somewhere.
 
Silvaire said:
As has been pointed out "rule of thumb" is 50, not 200. You got your thumbs mixed up somewhere.
Click to expand...
Guess so. The post was more curiosity than anything. The real issue I have with this engine is the inability to run LoP without roughness (with GAMIjectors).
 
Which has nothing to do with "TCM Idle Mixture Setting"

just saying...
 
weilke said:
What kind of engine ?

We spent some time chasing our tail on this on our TSIO520-UB. If the unmetered fuel pressure is sufficient to give us full takeoff fuel-flow, we end up with a somewhat rich mixture at idle. If we turn the unmetered pressure down to where it allows for the right mixture at idle, we can't get full takeoff fuel-flow.

I can fix the rich mixture during taxi with the red know. To fix the insufficient fuel flow on takeoff, I need to turn on the boost pump and monkey around with the mixture at the most inopportune time.
Click to expand...

There are charts from tcm that tell you what the metered and unmetered pressures should be. If your mechanic has the proper test equipment and you follow tcm's specs it should be right. Have never seen one that couldn't be dialed in with one exception. But that one was an outlier due to the mechanic (not me) failing to put oil in the engine before doing the fuel trim. The runs were all different right up until the point the engine seized. Go figure
 
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