History:
My IVP with a TSIO 550 would have slight to severe hesitation after
climbing to altitude, leaning for cruise and switching tanks.  About 50%
of the time it was a severe pucker factor, often loosing partial
pressurization.  Most of my flights are to the flight levels. (Usually
220 to 250)  It only happens on the first fuel tank switch and is worse
in hot weather than cold.  Mark and Orin at Lancair have been working
with me on this trying various things.

Improvement:
Climb to cruise altitude, bring back RPM and manifold to cruise, change
tanks, wait for 30 seconds and then lean.  This eliminated most of the
"burps" although I could see the fuel flow drop occasionally without the
loss of power.

Solution:
I use solution hesitantly.  I have now had 17 flights to flight levels
without a single burp or decrease in fuel flow when changing tanks.  I
still switch tanks before I lean.  What is different?  I relocated the
fuel boost pump from the side wall to the floorboard on the right side
forward of the left foot.  This lowers the fuel pump to a point where it
does not have to suck as it is below the top of a full tank.  

Someone is going to ask why I don't try leaning and then switch.  Bottom
line: I am chicken.

Theory:  
As we climb to the flight levels the pressure drops in the fuel tanks up
to the selector valve to about ½ of an atmosphere.  The fuel in the tank
cools as we climb.  The fuel in the line in the cabin is still at some
elevated temperature (let's say 70 degrees F).  The vaporization point
of Av gas changes with temperature and pressure.  (I tired finding a
table for this and was unsuccessful.  Av gas has to have a vapor point
between 5.5 and 7.0 psi.)  We are near or at the point of vaporization
of the warm fuel inside the fuselage.  We switch tanks which formerly
sucked and now is pumping.  The vapor pressure doesn't drop any further
and  the fuel doesn't vaporize.  The engine doesn't burp.  Side benefit:
The upholstery lasts longer.  I don't have to explain to the passengers
that it is normal for the engine to die when I change tanks. (No, I
really don't say that, but I do tell them that it may happen.  And what
the procedure is if we don't get restart.  A passenger did turn on the
O2 and don the O2 mask once before restart occurred.)

Disclaimer:
I have only taken off from 80+ degree days in 4 cases.  I am not an
engineer and all the theory could be bunk.

Promise:
I will report back if this does not work.

Thank you Mark and Orin for all your help solving this.

Carl Cadwell, N25CL, IVP
1 year, 172 hours and in annual now.