Posted for "Chuck Jensen" <cjensen@dts9000.com>:

 Do you know if your cabin is at a negative or positive pressure to the
 ambient/surrounding air (one way to tell is by opening/closing the
 alternate air source and watching the altimeter)?  Since it happens
 primarily in climb and slow flight, the plane is at a higher angle of
 attach and there is less air flow past the cabin, it may be the cabin
 pressure is slightly negative to the area toward the rear of the plane
 where your exhaust stream travels past the plane.  If the cabin is
 slightly negative, it'll pull a few of those fumes into the cabin.
 
 The dynamics may change at cruise and you have a slight positive
 pressure in the cabin, hence the traces of the exhaust stream no longer
 seep into the cabin.  If you operate your fresh air vents wide open and
 don't get a CO alarm, it may be because you are pressurizing the cabin
 and preventing migration of CO into the cabin and not just diluting and
 flushing away the in-seepage of CO.
 
 That's one advantage of flying a Velocity pusher.  All those problems
 are behind us.
 
 Chuck