Russell,
You may recall I had a fuel problem just prior to OSH this year
(see below). I did some testing, inspection and cleaning before the next
flight and got the opinions of several experts.
My conclusions are warm fuel and higher ambient temps increase
the likelihood of Fuel Vaporization in the system. Higher density
altitude is also a significant factor. I managed to get away with not
using the Low Boost Pump at some high cruising altitudes for a couple of hundred
hours.
Reasons to use Low Boost:
On Takeoff, if the engine driven pump fails you are covered.
Climbing out, fuel pressure will be stable and allow accurate
leaning.
At cruise, density altitude will not have a detrimental effect
on fuel pressure.
At Airventure, I spoke with several Lancair pilots, Continental
and a Cirrus Test Pilot with over 8 years experience, all seemed to think this procedure
is a good idea. If it means buying a Boost Pump sooner rather than later,
I’m OK with that.
Steve Colwell Legacy IO-550
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 Placerville, CA.
I did not mess with the mixture on descent 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 20 seconds, no leaks.
Steve Colwell 916 505-6099
Legacy IO-550N mostly stock, no ram air or injector mods., 225 hours tt.
I'm a little
confused over your use (or non-use) of the Electric Boost Pump at altitude?
Try turning it on LO at 5,000 feet, and HI at 10,000. That's SOP for the
turbos. The boost pump is your friend! On hot days, with hot fuel,
you may need the boost pump for an hour or more at altitude.
Sounds to me like simple vaporization.
Best...
John Deakin
Advanced Pilot Seminars