Dan, Did you scarf the inner tube? I see it is mounted straight in on the exhaust tube. Bill
Cut the end at an angle? Yes, but let's consider
why.
There's a tendency to think the slash cut end somehow generates low pressure. It does, but not much more, if any more, than any other shape which avoids capturing dynamic pressure.
Just for fun, I drilled a 5/8" hole in the wall of my big leaf blower's outlet tube, then made up a 5/8" tube with a pressure measurement connection on one end. The blower stream is not the same velocity, temperature, or density as an exhaust stream, but no matter. This is simply a comparison of pressure tap end shapes in a flow.
Note the pressure for the 1/2' insertion of the square end, as compared to the other two ends. It doesn't make much difference what sort of end you put on your pressure tap, as long as it is not faced into the airstream. If you reverse the airflow so that the moving air impacts the open end of the slash-cut or tipped tubes, you will capture some dynamic pressure, i.e. convert dynamic to static like a pitot tube. Instead of a vacuum you would probably measure a positive pressure, not at all what we want here.
The real reason to slash cut the tube is to simply avoid dynamic pressure. The negative pressure we wish to tap doesn't stem from airflow, but rather from the positive and negative wave pressures due to cylinder events and pipe tuning.
Take a look at Figure 1 in the CAFE report titled "Aircraft Exhaust Systems IV linked fro this page:
http://cafefoundation.org/v2/research_reports.php
Note there are three points in 720 degrees of crank rotation in which a single cylinder's exhaust pipe is subject to significant negative pressure....about 10" Hg in this graph. This is the real source of negative pressure for our evacuator. The reed valve blocks the positive and we keep the negative.
BTW, my install looks like the one on the right, above