That is not going to change your rate of fall in the mot critical area (first 18" of riser going down) though.

Here is what says for Merc 'black' motors.
Obviously, not modified one's.

The water should stop reverting when you pull the O2 sensors out. That should effectively kill the vacuum signal causing the reversion. Volvo does this on some of their V8s by putting AIR check valves right near the turn in the elbow. I just wanted to share that before I say the obvious, are you sure its reversion? Not trying to irritate you, just give you a thought.

"Late" timing at idle helps reduce reversion by requiring more throttle opening for the same idle rpm. This decreases intake vacuum and allows more air to be pulled from the intake during overlap instead of the exhaust. Not a cure-all, but it helps.

So the more I research this, the more it seems to be a "black science" with lots of opinions but not a lot of absolute data or facts. SB: If I raise the point at which the exhaust starts downward by 2" and it ends up at the same exhaust tip how can that not increase the rate of fall? I mocked up a 2" spacer out of wood, so I'll calculate the fall- thanks for the chart.
As to the reversion stopping when you open the 02 bung, I've read and was told by a mechanic friend yesterday just the opposite- that any exhaust leak will make reversion much worse. The theory I guess is that by opening the exhaust path you lower the outbound pressure wave and make it easier for the water to creep back up?
I am sure it's reversion- I've pressure tested the manifolds and risers, the eng. is closed cooled, and finally I removed the water dump from one of the risers and ran the eng. for a few seconds and the reversion stopped. It was also suggested to me that I should do a leak-down test to make sure I don't have a damaged ex valve sucking the exhaust path full time. I rebuilt the engines a couple of years ago and I caught the reversion almost immediately so I doubt it but I guess I should eliminate the possibility. Thanks for the input-

If you raise the risers with a solid, non angled spacer, the solid inner and outer pipes that turn down to go towards the transom don't change angle. They can't. Again, they are solid.

So I put a level on top of the riser and measured the vertical drop 18" down: With the 2" spacer between the manifold and the riser base the drop was 5 3/4" (3.8"/ft) and without it's 4.25" (2.8"/ft). Without the spacer the exhaust pipe extension terminates low on the tips. The spacers make the inner exhaust pipe extension line up almost perfectly with the tips on vertical so I'm going to go ahead with that mod. It doesn't explain however why I'm getting reversion with the current set up of 2.8"/ft. of fall. However, I experienced the initial reversion before I had the inner pipes extended. When I reinstalled them after extending the pipes I checked for reversion by removing the o2 bungs, which, depending on who's right should have increased or decreased the reversion pulse. If it increased it, that would explain it. Either way the spacers make everything line up better and increase the fall so it seems like a no-brainer next step.

SB: Well, you make a very good point. I guess I should mention another mod that I didn't bring before because I was trying not to get too bogged down in every detail. As I mentioned, the inner pipe extensions terminate too low on the tips so last week I had the (ignorant) brainstorm to raise them by having a "wedge" made by my machine shop to raise the angle enough to meet the tips. Therefore, I have plates that are 1/4" at the front, 1/2" at the back between the manifolds and risers. This raised the terminal end of the pipes perfectly but obviously flattens out the drop. Weather that's enough to explain the difference in fall I describe above I'm not sure but it must be because I measured both very carefully.
Thanks-

Rule #1 when trying to repair anything: fix known issues first. Then go from there if need be.

BTW, and again, those are minimum angles for Merc black motors, and notice the 8.1's min are even a greater angle.