0-320 leaning at altitude

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UngaWunga

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UngaWunga
Flight back home yesterday I was playing with the throttle and mixture settings to get max rpms at 7500'. Per the book, full throttle at that altitude (leaned appropriately) should be a certain % of power and should give a specific rpm. I don't have the POH with me right now. Outside temp was 0deg C at altitude.

What I ended up doing was just leaving it at full throttle and leaning until engine stumbled a bit and then click it in a notch of mixture. I let things settle, and then pushed in the mixture another notch for max rpms. Figured all was good per the book settings. Engine was around 2450rpms.

But when I was going to begin my decent, I left the mixture alone and backed the throttle off. I noticed the rpms rise with a bit of the throttle out to a little over 2500. The further I backed off the throttle, the rpms began to decrease.

This tells me something is wrong in the carburetor. Like the butterfly is partially over vertical at full throttle or something. I'll have to try a test on the ground to see if backing off the throttle during runup affects rpms, but I don't recall that it does. Or am I just mistaken in my understanding on how the engine mixture setting is working in an airplane carburetor? I know how motorcycle and car carburetors work from years of racing, but I'm not sure if that completely carries over to aircraft stuff.

Are there other tests I could do to make sure things are correct?
 
I don't know anything about anything, but some carbs have enrichment circuits at WOT. It is possible that, by backing off the throttle, you leaned the mixture a bit more.
 
I was wondering about enrichment circuits, but shouldn't I have been able to lean out to get max rpms at altitude using full throttle and mixture alone?
 
If you leave throttle the same and pull mixture (leaning it), then RPMs first increase, then drop.

If you leave mixture the same and close throttle, and the RPMs increase, that tells me that you were leaner at wide open throttle.

The mixture to throttle-position relationship is not always the same over the RPM range of any given internal combustion engine (there are many variables).

Open for correction here (my usual disclaimer, I'm a carbureted car dude).
 
455 Bravo Uniform said:
If you leave throttle the same and pull mixture (leaning it), then RPMs first increase, then drop.
Click to expand...

yes

If you leave mixture the same and close throttle, and the RPMs increase, that tells me that you were leaner at wide open throttle.
Click to expand...

I did try multiple mixture settings trying for max rpms.

The mixture to throttle-position relationship is not always the same over the RPM range of any given internal combustion engine (there are many variables).
Click to expand...

Yes, depending on air density/temp.
Guess I should go flying again to try out specific settings. Another reason why I asking is that a mechanic that I no longer trust did rebuild the airbox and may have messed up the carburetor settings. Most of my flying is local and lower, so I'm not completely familiar with higher altitude leaning. Yes, 7500 isn't that high, but I lean the mixture no matter what the altitude is for either max rpms or fuel savings, depending on where I'm going.
 
GeorgeC said:
I don't know anything about anything, but some carbs have enrichment circuits at WOT. It is possible that, by backing off the throttle, you leaned the mixture a bit more.
Click to expand...

That carb doesnt have any enrichment circuits. What it does probably have is a poor fuel nozzle that fails to atomize the fuel properly. Closing the throttle a bit puts the butterfly in the way where the fuel stream hits it, breaking it up better and allowing a more complete burn in the cylinder, delivering more power. I have seen it on some 172s, and even in the climb you might pick up a few RPM by pulling the throttle back a little.

The MS carbs are terrible things. The ancient Stromberg on my A-65 does a far better job.
 
Could be you are lean of peak and reducing the throttle just a bit you are going back toward peak increasing power. It is possible the butterfly is a bit over center but that should not make a difference in power. Sometimes the butterfly being backed off just a bit will result in better atomization of the fuel and better power. I would try the throttle reduction being sure you are rich of peak. You can assure that by running on one mag and lean to the threshold of roughness then go back to both mags. Now you are at about 75 rich of peak assuming the mag timing is correct. Charlie Melot
 
not enough info given to make an intelligent answer.
 
charlie said:
Could be you are lean of peak and reducing the throttle just a bit you are going back toward peak increasing power. It is possible the butterfly is a bit over center but that should not make a difference in power. Sometimes the butterfly being backed off just a bit will result in better atomization of the fuel and better power. I would try the throttle reduction being sure you are rich of peak. You can assure that by running on one mag and lean to the threshold of roughness then go back to both mags. Now you are at about 75 rich of peak assuming the mag timing is correct. Charlie Melot
Click to expand...
he could also allowed the nose to drop. not noticing that he blamed it on power.
 
at that altitude and those conditions you are roughly operating at 65-70% HP, the optimum altitude for leaning....and you can do no wrong leaning at that altitude.

You should of leaned, using full throttle position, for best "SPEED" or power....and the way to do that is to either use indicated AS or your GPS to determine your best power/speed. If what you wanted is best economy then lean for Carson Speed (same full throttle position)..... (1.32 times the best-L/D speed).

also, a little trick...use a touch of carb heat to help with fuel atomization....then perform the leaning with mixture knob.

UngaWunga said:
Flight back home yesterday I was playing with the throttle and mixture settings to get max rpms at 7500'. Per the book, full throttle at that altitude (leaned appropriately) should be a certain % of power and should give a specific rpm. I don't have the POH with me right now. Outside temp was 0deg C at altitude.

What I ended up doing was just leaving it at full throttle and leaning until engine stumbled a bit and then click it in a notch of mixture. I let things settle, and then pushed in the mixture another notch for max rpms. Figured all was good per the book settings. Engine was around 2450rpms.

But when I was going to begin my decent, I left the mixture alone and backed the throttle off. I noticed the rpms rise with a bit of the throttle out to a little over 2500. The further I backed off the throttle, the rpms began to decrease.

This tells me something is wrong in the carburetor. Like the butterfly is partially over vertical at full throttle or something. I'll have to try a test on the ground to see if backing off the throttle during runup affects rpms, but I don't recall that it does. Or am I just mistaken in my understanding on how the engine mixture setting is working in an airplane carburetor? I know how motorcycle and car carburetors work from years of racing, but I'm not sure if that completely carries over to aircraft stuff.

Are there other tests I could do to make sure things are correct?
Click to expand...
 
charlie said:
Could be you are lean of peak and reducing the throttle just a bit you are going back toward peak increasing power. It is possible the butterfly is a bit over center but that should not make a difference in power. Sometimes the butterfly being backed off just a bit will result in better atomization of the fuel and better power. I would try the throttle reduction being sure you are rich of peak. You can assure that by running on one mag and lean to the threshold of roughness then go back to both mags. Now you are at about 75 rich of peak assuming the mag timing is correct. Charlie Melot
Click to expand...

Thanks, I'll give that a shot. I'm basing everything on the assumption that at 7500' full power won't be at full rich.
 
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