I have a pr. of 540's built from base Merc 502's w/ Scat stroker kits. They are otherwise built out the same as 500 efi's w/ the same Crane cam, Edelbrok Alum. heads, 500 efi intake and injection system, gil exhaust, (TRS Drives). When I built them a few years ago I bought new Gil manifolds and a used set of Gil "shorty" risers that I had extended with inner and out pipes so they were dry out to the transom and added O2 bungs. They worked with no reversion for years until they started springing leaks in the outer pipes- I guess galvanic corrosion form the previous owner not flushing in salt water so I bought 2 sets of almost-new current style BB Bravo wet exhaust risers. One engine reverted so badly the oil was saturated with water and it would hardly run. After eliminating any other possible source of a raw water leak (pressure tested the manifolds and oil cooler @ 60 lbs and the blocks are closed cooled) I had 16" of inner pipe added to keep the exhaust dry to within 3" of the tips. It still reverts big time- if I remove the O2 bungs when it's running it spits huge amounts of water out every time it fires. To make sure it was reversion I removed the water intake hose off the riser and the exhaust coming out of the bung dried out within seconds. Here's my question: I thought that mixing the water with the exhaust as far away from the engine was key but apparently not. These riser are a couple of inches shorter at their highest point than the previous set so would raising the height of the riser above the manifold with a custom machined 2" block help?

What is the rpm at idle?

Around 750 (efi)

Correction: It USED to be around 750. Now it's sucking so much water it barely runs at all.

I'm new to this, but it sounds like you had the solution before. You need the same or more riser height plus run.

Dry exhaust

I hear you. But am amazed that it's a tiny bit of GRAVITY that keeps reversion from happening. It makes sense that getting the water as far as possible form the suction that happens from intake/exhaust overlap would be critical but to have it caused by at most 2" less riser height blows my mind.... I'd appreciate anybody else's input who has any experience or knowledge about this-

I hear what your saying also and sounds like you eliminated all other possibilities. So that leaves you with what did you recently change? Go back to dry or if you have the room find some of the tall anti reversion tails but you may need to make some mods cause the only ones I've seen are for bravo app. One of the two options will solve your problem. I doubt you'd wanna change cams???

Riser height can / wll influence it big time. This is why all manufacturers have min guidelines for rate of fall for the exhaust.

Did you know when you add cubic inches, you pull more air. This means thru the exhaust during overlap too. Makes sense right ?

Thanks for the input. I want to avoid dry because it's obnoxiously loud & I boat on a well patrolled lake. I'm thinking about having 2" spacers made out of billet aluminum to go on top of the manifolds. That would make these risers higher than the ones that were on it for 3 years without a problem. I've also still got 4" between the end of the inner pipe I added to the risers and the exhaust tips. Thinking I might add 3 more inches of pipe too.