IO360 fuel distribution instability

[gear1]

Some fuel distribution issues have been bothering me lately. After matching the injector nozzles, I have been operating lean-of-peak for a number of years with no problems. However, lately I have been experiencing some strange fuel distribution issues. The engine is an AeroSport Power IO 360M1 with Hartzell constant speed prop, Electroair ignition on the right magneto pad supplying spark to the top plugs, and a P-Mag on the left magneto pad supplying the lower plugs. A typical FF/EGT/CHT spread is:

Altitude: 12000
OAT: 58 degrees F
Fuel Flow: 6.7 GPH
CHT #1: 352? EGT #1 -47? (from peak)
CHT #2: 338? EGT #2 -54?
CHT #3: 372? EGT #3 -34?
CHT #4: 373? EGT #4 -32?

At times when initially setting the mixture for lean of peak, the distribution will split, with the #4 cylinder going much richer:

Altitude: 12000
OAT: 58 degrees
Fuel Flow 6.7
CHT #1: 375? EGT #1 -25?
CHT #2: 365? EGT #2 -55
CHT #3: 365? EGT #3 -45?
CHT #4: 385? EGT #4 -15?

The CHT and EGT seem to be in correspondence: EG- the highest CHT cylinder has the richest mixture as shown by the smallest drop from peak. The distribution seems random, and sometimes changes during the flight by itself. Sometimes going back to rich of peak, then resetting LOP fixes the distribution.

Does anyone have any idea what is causing this? Is it a case of dirty injectors or a problem with the flow divider? What might be the solution?

Thanks to all for your thoughts.

[dynonsupport]

The first thing you need to look at is your GAMI spread (fuel flow of the first cyl to peak vs the last). This will tell you how balanced your injectors are.

If they are balanced, then you could have ignition issues, EGT probe issues, or other things.

If they aren't balanced well, then you look at the fuel system.

--Ian Jordan
Dynon Avionics

[MarkW]

Hey Craig,
As Ian says the GAMI spread is more important than these other numbers.

I also suggest using https://www.savvyanalysis.com/home for a free service to chart your results. Very informative website.

[jdeas]

I'd consider checking the fuel divider and lines . If you have some trash or an obstruction in #3 it could manifest itself this way. This is a balancing act so fuel diverted from 3 could make 1 and 4 richer.

[David-aviator]

These number vagaries could simply be the result of a little chaos in the Lycoming world.

I'd try starting over from full rich and see if it is different. I bet it will be. These engines are not all that predicable when looking at numbers that closely.

There was a time when that was not possible or of consequence.

If the engine is running smooth and all else is normal, press on.

[Kevin Horton]

Are these two conditions at the same throttle setting? On my aircraft, with IO-360-A1B6, I find that the cylinders reach peak EGT almost simultaneously, if I?m at full throttle. But, if I?ve pulled the throttle back a bit, I get a much larger difference between cylinders. I?ve never understood it, but I usually cruise at full throttle, so I haven?t worried about it much.

[gear1]

Thanks everyone for your thoughts. For the record 1) all readings have been at WOT, 2) the gami spread is .2 Gph and 3) when the imbalance occurs, the problem can usually be corrected by going full rich, and then going back to Lean of Peak.

[RV10inOz]

EGT imblance is not the point, but my observation is that if your spread is 0.2GPH typically, then this is good. But what you may be suffering from is an intake leak upsetting the F/A ratio's and some leaks for some reason are random in the amount of leakage from one time to another

Also I think your LOP ops should be yielding lower CHT and I figure you are running too much advance.

YMMV

[JonJay]

Although unlikely in a known good system, a contaminant could be "floating" around in the fuel divider. A buddy of mine had this happen. Symptoms where much more severe, a complete loss of one cylinder, from time to time, then a different cylinder, and so on. The piece of debris was large enough to cut off the fuel flow to the injector line, then it would move around after shut down. Sometime show up, sometime not. While it seems obvious, it was very difficult to troubleshoot.

I think this is a long shot but if you run out of things to check....

[Guy Prevost]

Talk to Don Rivera at Airflow Performance.

I'm pretty sure it's the overall low fuel flow. At flows that low, the flow divider is playing a bigger part than the nozzles. Because I live at high altitude and routinely fly LOP at high altitude, Don had me switch all of my injectors nozzles to smaller orifices. This raises the fuel pressure at the divider and brings it back out of the equation.

I wouldn't do full power takeoffs on cold days at sea level because of this change, but I've never had the opportunity to try anyway. Landing at DA less than 6000' feels really strange to me.