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

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?

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.

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

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.

drainge said:

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.

Click to expand...

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.

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.

drainge said:

I'm using a venier. The knob position was dramatically different for the same mixture settings. .

Click to expand...

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

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.

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?

Deuelly said:

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?

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It would be quite unusual for an FI engine to have a primer setup.

Larry

Right, no primer.

Just a note -- this engine (and red cube) have less than 300 hours since new. If the cube died it sure did it in a hurry.

How can I test the functionality of the cube?

I'm going to pull the cowl today and examine the linkage and mixture cable for any issues. If I don't find anything there I'll look at the fuel injection system.

drainge said:

Right, no primer.

Just a note -- this engine (and red cube) have less than 300 hours since new. If the cube died it sure did it in a hurry.

How can I test the functionality of the cube?

I'm going to pull the cowl today and examine the linkage and mixture cable for any issues. If I don't find anything there I'll look at the fuel injection system.

Click to expand...

If done on the ground, pull a line downstream of the cube and put some type of valve on the end to control the flow. Nothing fancy. Adjust the valve to get a reasonable flow rate (around 10-15 GPH) with the boost pump on. Then, with instruments on, hit the boost pump and a stop watch at the same time and find the number of seconds to fill a one gallon container. Then do some math and compare the actual flow to what your EMS was displaying as the flow.

If all of your investigation points to the cube, you can test it in the air. Playing around with the mixture around peak EGT and look for anamolies. Sounds like you have a good idea of what it should present. They usually don't fail a little. It is usually pretty significant and 300 hours is not uncommon for a failure.

You can put a coffee filter in your funnel and put the gas back in your tank when done.

From all that you have posted, my money is not on the red cube as the problem.