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Post Annual Rich Mixture


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Annuals drive me a little crazy when a well working machine can come out of an annual with squawks.

Post-annual, when on takeoff and I firewall the mixture, I find that the mixture is too rich (feels like it floods a little bit). When flying from near sea level, I feel like that I have to lean like I'm flying from the mountains in the summertime.

Before I do a lot of digging or discussions, I'm trying to figure this out in my minds eye - is there anything post annual that could cause the mixture to become too rich (the mechanic said that he didn't adjust anything on the control knob). M20E with an injected 360.

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This idea may only apply to Continental engines... (thanks Rich!)

Part of the annual may be to set the FF to book settings...

If you have A FF indicator, what is it reading at FT on take-off?

Some engines are common, others not so much.  If the mechanic sets it up without looking into the detail you might get a bad set-up.

The LB Mooney has three variations of IO550. The FF needs to match the hp of the engine at FT.

PP understanding of what the mechanic does, not a mechanic myself.

You may want to have a discussion with the mechanic that set up the FF..?

Best regards,

-a-

Edited by carusoam
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The RSA does not have a FF adjustment like the Contintal system. 

The mechanic is correct that the only adjustment that would affect the WOT FF would be the cable adjustment. The only other thing would be the idle mixture. All other adjustments are not field adjustable.

 

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Somebody posted a guideline for max FF at SL the other day.  

It was 90% of total hp.  For my engine 310hp the guideline setting is 27.9.  Actual FF setting for my engine from the STC is 27.2 gph...

The official FAA documentation for your plane/engine will have a FF number to check against...

For a 200hp engine using the unofficial guideline... 18 gph

Rich, Is this guideline any good for the IO360?

Best regard,

-a-

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Thanks for the info, my recollection is that FF is around 17 gph at TO with WOT.  I'll double check.

This is a stretch, but could an air filter change impact the mixture and be observed at a "too rich" mixture - in other words (at the extreme) if I completely block the air intake, would the show itself as a too-rich mixture?  

And now to back off from that extreme view, if a new air filter lets in (say) 70% of the previous air filter, could that manifest itself to the PIC as a too-rich mixture?

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8 hours ago, carusoam said:

You may want to have a discussion with the mechanic that set up the FF..?

 

I did chat with him about this.  He said that he didn't touch FF or mixture adjustment.  He followed up with "that is strange, last week I had a pilot tell me that his mixture was too lean after his annual - I don't know what that would have happened either".

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8 hours ago, N201MKTurbo said:

The mechanic is correct that the only adjustment that would affect the WOT FF would be the cable adjustment. The only other thing would be the idle mixture. All other adjustments are not field adjustable.

1

I think that there is a mixture adjustment near the bottom of the engine where one can turn an adjustment ring to change (I think) the mixutre - I'll have to take a picture of the ring/location (hopefully) later today - will post it.

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MP should give a direct hint to airflow available to the engine.  If something were restricting flow, the MP will be lower than typical.

Keep in mind the atmospheric pressure can make this shift about an inch.

The other thing to consider that gets checked/changed at annual is timing of the mags. The IO360 has two possible standards of 20 or 25°BTDC.  25° is more spry at the cost of higher CHTs.

Pp thoughts only.  A big fan of having brief discussions with people working on my machinery.

Best regards,

-a-

Edited by carusoam
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1 hour ago, 211º said:

I think that there is a mixture adjustment near the bottom of the engine where one can turn an adjustment ring to change (I think) the mixutre - I'll have to take a picture of the ring/location (hopefully) later today - will post it.

That is just for idle mixture.  To check idle mixture, you should see a 50 RPM rise when you smoothly shut down using ICO.  Less than 50 RPM is too lean.  More than 50 RPM is too rich.   But it's for idle mixture ONLY.  Zero effect on the main metering.

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One annual, mine came out running poorly like it was flooding. It turned out that the shop set my timing to 20 instead of 25 as it had been in the past. Verify your timing.. The optional SB 1325a to retard timing to 20 sucks

 

 

Sent from my iPhone using Tapatalk

 

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1 hour ago, Cyril Gibb said:

That is just for idle mixture.  To check idle mixture, you should see a 50 RPM rise when you smoothly shut down using ICO.  Less than 50 RPM is too lean.  More than 50 RPM is too rich.   But it's for idle mixture ONLY.  Zero effect on the main metering.

 

Interesting - I was seeing about 120RPM increase on a slow leaning shutdown and am now (after adjustment) seeinig about 70RPM increase.  Thanks for the info and comment.

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1 hour ago, Browncbr1 said:

One annual, mine came out running poorly like it was flooding. It turned out that the shop set my timing to 20 instead of 25 as it had been in the past. Verify your timing.. The optional SB 1325a to retard timing to 20 sucks

1

That sounds very plausible - I'll ask my mechanic... but unfortunatley, it wasn't written up in my log book (as I've seen in the past in other annuals and other airplanes).

I'll let you know and will hopefully bring this to closure soon.

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Found the issue and learned a few things on the way.

The cut-to-the-chase answer is that a piece of the airplane got sucked into and blocked the air intake.  Blocking this intake caused the mixture to become relatively rich (fuel flow stayed the same but airflow decreased causing the relative change in the mixture).

How it was found...

  1. All spark plugs were removed, inspected, cleaned, and gapped - they looked ok and this crossed them off of the issue list and permitted us to...
  2. Check the fuel flow for each cylinder by doing a run up at 2000 and 2600 to see if one of the cylinders was running hot or colder than the others (different from before), but the temps all looked relatively good, so we did...
  3. A mag check at 2000 and 2600 which caused the engine to stumble and stutter almost to the point of stopping - seemed like both mags were bad, but then just for the heck of it,...
  4. Leaning the mixture and doing the same runup at 2000 and 2600 made for a pretty standard drop on both the left and right mag (and took them off of the issues list) which lead us to think more about relative mixture and "not too much gas", but "not enough air".
  5. After shutting down, we talked about the filter, other things that would reduce the airflow, and then also looked into the bypass gate.  There we found a piece of material long enough to span the intake baffle/hose and to slow down the air intake. If you look closely, you can see where it came from (lower left-hand corner of the photo).

Gratitude to MS for a place to think it through and to the tower that notified me yesterday that I was trailing smoke.

Bypass Gate.jpg

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There are 4 ram air type tubes just about where your FOD stopped.  I beleive they measure air flow/pressure and adjust fuel accordingly.  I suspect that one or more were blocked and thus enriched the mixture.....plus the reduction in air intake that you mentioned.  We are prone to similar issues from bugs, birds or other FOD when flying with ram air open.  I've always worried about that.  It's  good to know the symptoms from your story.  Thanks for sharing.

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3 minutes ago, Yetti said:

Looks like some runway FOD. Looks like it cut your intake boot on the way in.  That thing is a PITA to replace.  work slowly and carefully.

I think they fair the inlet with bondo.  Is that what broke loose or the rubber seal?

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Aluminum except for some filler.  I only ask that because mine had big chunks of "bondo" flake off in big chunks from that same spot.  I was just lucky it didn't happen in flight with the door open.  I still am not sure who put it there, but am now thinking it came from the factory that way.  I chipped it all away at the time and there are some sheet metal seems that they were fairing over.  The plane has since been painted and I can't recall what it looks like now. 

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12 hours ago, takair said:

There are 4 ram air type tubes just about where your FOD stopped.  I beleive they measure air flow/pressure and adjust fuel accordingly.  I suspect that one or more were blocked and thus enriched the mixture.....plus the reduction in air intake that you mentioned.  We are prone to similar issues from bugs, birds or other FOD when flying with ram air open.  I've always worried about that.  It's  good to know the symptoms from your story.  Thanks for sharing.

 

Exactly - that is my understanding of the ram air tubes too and I agree that their blockage added complexity to the mixture situation. For the 3/4-inch MP increase at altitude, I'm leaning away from its use - not enough to gain really to leave the air inlet unprotected.

12 hours ago, takair said:

I think they fair the inlet with bondo.  Is that what broke loose or the rubber seal?

It may have been bondo.  If felt "plastic-like"  And that is what broke away - look at the lower left portion of the photo and you can see a grayish area where the piece came from.  When I took it out of the inlet, it was a perfect match to the missing area.

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