|
-
POSTING RULES

-
Donate yearly (please).
-
Advertise in here!
-
Today's Posts
|
Insert Pics
|

02-25-2019, 12:53 PM
|
|
|
|
Join Date: Jan 2017
Location: Alameda, CA
Posts: 29
|
|
Mixture seems to adjust itself in flight
Hi all,
I have been experiencing some mixture issues in flight and I could use some help thinking about the problem. I believe the issue has been present for the last ~100 flight hours, but yesterday developed from curiosity to concern.
After leveling off at 2000' (for a short local brunch run), I leaned my IO-375. My typical procedure is to pull power to 62%, then lean for 60%, which gives me LOP burn of ~7.8 gph. After a few minutes I heard the RPMs drop and found that my fuel burn had dropped to 7gph and power was low. I enriched the mixture and continued the flight. A few minutes later I looked down and found that my burn was up over 10gph. Again I adjusted and continued the flight.
On the way home I experienced the same issues, but with wider and more frequent swings in consumption and power. I bee-lined for home and pushed mixture to full rich, thinking that the last thing I wanted was for my engine to lean out in flight. Full rich was burning >20gph! For reference, I burn ~15gph on take off. I started to worry that I was going to fowl the plugs, but by this point I had the runway made, so I left mixture at full rich and landed.
What could be going on here?? Maybe the mixture cable is slipping in its housing? Possibly an issue with the fuel injection?
Thanks,
Dan
__________________
Dan Ainge
RV-7A N375M
2019 =VAF= dues paid
|

02-25-2019, 01:00 PM
|
 |
|
|
Join Date: Jan 2005
Location: Twin Cities, MN
Posts: 1,565
|
|
What kind of mixture control do you have (i.e. vernier, push/pull, etc.)? Is it possible it's slipping in flight and simply needs more friction?
__________________
Brad Benson, Maplewood MN.
RV-6A N164BL, Flying since Nov 2012!
If you're not making mistakes, you're probably not making anything
|

02-25-2019, 01:05 PM
|
|
|
|
Join Date: Jan 2017
Location: Alameda, CA
Posts: 29
|
|
I'm using a venier. The knob position was dramatically different for the same mixture settings. I.E., after I discovered the lean mixture I enriched and the knob was nearer the firewall. It was almost like I was chasing the correct point as it moved through the throw of the knob.
__________________
Dan Ainge
RV-7A N375M
2019 =VAF= dues paid
|

02-25-2019, 01:10 PM
|
|
|
|
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
|
|
I would first confirm that the heim joint connection from the mixture cable to the arm on the FI servo is solid, including the attachment bracket. I would also experiment a bit more to insure that the red cube is measuring correctly. It wouldn't seem that is the issue, given the RPM drops with reduced measured FF. However, it warrants confirming, given the other symptoms. You can play around in the neighborhood of peak EGT to watch for consistent relationship between EGT and FF.
Is the 375 capable of consuming 20 GPH at the altitude/RPM you were flying? It seems pretty high. If not, that might point to the red cube. Once you get too rich, you will see power drop off and possibly rough running. You mention takeoff FF. Is that at 2700 with a CS or lower with an FP? I am guessing the latter.
There are a few rubber parts in the servo that could cause this, but not an expert on their operation.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 02-25-2019 at 01:18 PM.
|

02-25-2019, 01:42 PM
|
|
|
|
Join Date: Jan 2017
Location: Alameda, CA
Posts: 29
|
|
I fly out of Oakland, which is basically at 0'
I have the 3 bladed FP catto prop. My take off is ~2400 RPM. I've never seen fuel flow as high as 20, and honestly I'm not sure whether it's possible to push that much fuel through the engine or not.
I have suspected that the red cube is slightly off for some time. Each time I fill the tank I add ~1-2 gal less than the totalizer would indicate.... buuut 1-2 gal isn't bad and I'd rather be conservative than optimistic with burn rate so I haven't adjusted it. One thing to note is that the cube does not have the recommended length (1" if I remember correctly) of straight tubing leading into the unit. Instead I have a 120deg fitting (this is as-purchased). Fixing that has been on my list for some time.
__________________
Dan Ainge
RV-7A N375M
2019 =VAF= dues paid
|

02-25-2019, 01:45 PM
|
 |
|
|
Join Date: Jan 2005
Location: Twin Cities, MN
Posts: 1,565
|
|
Quote:
Originally Posted by drainge
I'm using a venier. The knob position was dramatically different for the same mixture settings. I.E., after I discovered the lean mixture I enriched and the knob was nearer the firewall. It was almost like I was chasing the correct point as it moved through the throw of the knob.
|
Time to pull the cowl and check the linkage then. I wouldn't fly it until I found the source of the issue. Could be as simple as the mixture arm being loose or one end of the control cable not being secured well. Could also be something more as Larry said - all things to check.
Good luck - really interested to hear what the root cause is/was.
__________________
Brad Benson, Maplewood MN.
RV-6A N164BL, Flying since Nov 2012!
If you're not making mistakes, you're probably not making anything
|

02-25-2019, 01:49 PM
|
 |
|
|
Join Date: Mar 2006
Location: SoCal
Posts: 626
|
|
Induction fod?
Check the FI intake. If any FOD is blocking a portion of the throat you will get wild swings in metering of the fuel regardless of the mixture setting.
I would not fly until that intake path has been checked! Can't think if anything else that could allow a 20gph flow rate outside of a fuel leak.
__________________
JD
----------------------
RV-7 N314SY (KWHP)
IO-360-B1B
CANbus based trim/flaps and electrical
Last edited by jdeas : 02-25-2019 at 02:25 PM.
|

02-25-2019, 02:02 PM
|
|
|
|
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
|
|
Quote:
Originally Posted by drainge
I'm using a venier. The knob position was dramatically different for the same mixture settings. .
|
That points to a linkage problem. definitely get the cowl off and examine closely. That said, you would likely need to be turning 2700 at a low altitude and full rich to get 20 GPH with a well functioning servo. If that wasn't the case, loose linkage can't account for that fuel flow indication.
Also, red cubes typically fail in an instant vs a slow degradation in performance, in my experience, so don't rule it out due to it's past performance. Being consistently off 2 gallons per tank is a calibration issue and not an indication of performance.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 02-25-2019 at 02:20 PM.
|

02-26-2019, 05:20 AM
|
|
|
|
Join Date: May 2011
Location: Roseville
Posts: 386
|
|
I was seeing similar behavior with my AFP system, but to lesser degree. It was nearly 10 years since new and due for inspection and overhaul. After sending it in, problem gone. Several bushings were in need of replacement as was the flow divider diaphragm. That's too many variables to pin on the symptom but the point is that, after time, the components needed attention.
__________________
RV-9A N435KR, Vans calendar March 2018
LIO 320 B1A, Dual LS Plasma III, AFP injection
G3x touch
Roseville, Ca
|

02-26-2019, 06:50 AM
|
|
|
|
Join Date: Sep 2014
Location: Minnesota
Posts: 33
|
|
Primer?
Just a question, and trying to learn a little from the guys that know more than me. Could this somehow be a leaking primer problem? Or is there something indicating that wouldn't be a problem?
|
| Thread Tools |
Search this Thread |
|
|
|
| Display Modes |
Linear Mode
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
All times are GMT -6. The time now is 07:03 AM.
|