New Exhaust Gas Valve Pattern: Cylinders 3 and 4 only...

[Pixleyad]

1989 M20J 
IO-360-A3B6

Noticed an odd pattern after boroscoping cylinders during routine maintenance. Only changes to engine management is setting taxi lean after start and going full rich before takeoff vs. leaving full rich on the ground. Interested in everyone's thoughts. 

 

[carusoam]

great pics pixley!

Are you sure you have before and after correct?

Looks like you have some excess deposits on the after...

the before is holy cow clean...

All show excellent pizza patterns indicating even cooling related to good valve rotation...

One challenge... how many hours were on each case/situation...  I have no idea how long deposits stay on the valves and how long it takes to refresh the really clean image if you go back to the old way of running...

PP thoughts only, not a mechanic...

Best regards,

-a-

[ArtVandelay]

Don’t know if it matters, but both before the valves are fully closed and both after they look partially open.

[Pixleyad]

13 hours ago, carusoam said:

great pics pixley!

Are you sure you have before and after correct?

Looks like you have some excess deposits on the after...

the before is holy cow clean...

All show excellent pizza patterns indicating even cooling related to good valve rotation...

One challenge... how many hours were on each case/situation...  I have no idea how long deposits stay on the valves and how long it takes to refresh the really clean image if you go back to the old way of running...

PP thoughts only, not a mechanic...

Best regards,

-a-

I do have the before and after correct. I checked back and forth several times!

I heat distribution looks good and not concerned, just an odd pattern to be found in both rear cylinders when not present in front cylinders...

100 hours between inspections - 1050 SMOH vs. 1150 SMOH 

 

Only other thought I have is that I've been running long cross countries (4 hours plus) at relatively higher altitudes 10K+ and need to keep mix a bit more rich than normal to prevent roughness at the higher altitudes. 

Edited December 18, 2020 by Pixleyad

[Vance Harral]

Your "before" photo on cylinder #4 looks like an intake valve, while the "after" photo looks like an exhaust valve.  I've never seen an exhaust valve look as smooth and clean as your before #4, and I've never seen any kind of deposits on an intake valve like after #4.  My WAG is you're looking at the two different valves in the cylinder.