On climb out to 14,500’ yesterday I was having trouble leaning to 1300 degrees.   The mixture knob was extra sensitive so I favored the rich side.   I had just topped off with 40 gal. from an above ground tank that was probably around 80 to 85 degrees.   Low boost was used from takeoff to about 1500’ agl.,  RPM was reduced to 2500 and leaning began.   Normally, maintaining 1300 degrees on the hottest cyl. in the climb (per The Advanced Pilot Course) is easy.   (On the previous flight to 15,500’ for 2.5 hours I noticed it did not lean normally but I was able to go LOP and it held the fuel flow @ 9.8 gph.  No fuel had been added and temp on the ground was about 70 degrees.)

 

When we reached 14,500’, we accelerated in level off, reduced RPM to 2300 and started to ease the mixture knob back to go Lean of Peak.  At 16.5 gal. per hr., slowly turning the Vernier about 1/8” as normal, fuel flow jumped to 9 gph. with the engine faltering.  I tried this twice before concluding “this ain’t right”.   I wish I had thought to use Low Boost or to check Fuel Pressure, but I didn’t.  We were over the Sierras and headed for the nearest airport but maintained altitude and finally circled down over home base.   I did not mess with the mixture on decent thinking a running engine was the best option. 

 

The control cable, bracket, arm and movement on the mixture are OK.  Testing for leaks, I had the mixture in idle cut off, throttle open full.   With Low Boost for about 10 seconds, no leaks.  On High Boost for about 10 seconds, I got about a pint of fuel out the sniffle valve.  Hmmmmm, it’s not supposed to do that. 

 

Sounds like the Metering Unit, what do you think?

 

Steve Colwell   Legacy IO-550N mostly stock, no ram air or injector mods., 225 hours tt.