I removed the exhaust from both engines and found water in the back 2 cylinders (of the exhaust manifolds) both sides of both engines. The good news is I now know what took out my 02 sensors. 20 years ago or so my wife and I took a ride with a salesman on PQ 290. When he came off plane, he would hit the throttle (maybe 1500 rpm or so) when the wave hit the back of the boat. He said it would keep the water from getting up into the exhaust. Made sense at the time and I have done that ever since. Fast forward to this year and the first time out with the new straight through tips I loose both of my 02 sensors. Don't think much of it as everything is new this year and I guess things like that can happen. The new 4" tips have the rubber reversion tabs in them and NOW I understand what probably happened. The first time I came off plane I of course hit the throttles to keep the water out not thinking that that action also moves the rubber reversion tabs up and out of the way and water got in. With the stock muffler tips installed for the first part of this season I didn't have any problems. These new tips will accept muffler inserts however that defeats the purpose of the new tips. I am thinking I will look around and get some rubber hose and clamp onto these tips for turn downs. Anybody running rubber hose turn down's and if yes, (I feel your pain) where did you get them?

Rather than completely closing the throttles when coming down off plane I stop them at 1500 rpm until the wave catches up. When it dissipates I proceed to lower rpm, that way it doesn't "blow the flaps open."

People who "rev" their engines can cause reversion. You slam the throttle plate shut at 3k plus and the vacuum shoots to the moon. Ittl suck water up that exhaust like a pump.
CMI recommends on shut down bringing engines to 25-3500, turn the key off with the throttle still advanced. It will clear any water out of the exhaust. Its a great prractice to get into ESPECIALLY for people whos exhaust gets buried pulling the boat out of the water.

Never heard of that before, but makes perfect sense. I will try that, hopefully there will not be any dieseling.

Never heard of that before, but makes perfect sense. I will try that, hopefully there will not be any dieseling.

Every off season I have a project or problem which forces me to remove the exhaust and this year is no different. I am replacing the front crank seals and during the disassembly process I knocked the dipstick tube loose and because of this I had to remove the ONE exhaust manifold and I found about a teaspoon of water in the manifold at number 6. I have the pictured exhaust system. The tips (not shown) are 7" straight through angled cut with internal rubber flaps.
I no longer hit the throttles when coming off plane just before the wave hits the back of the boat. In my mind this should allow the internal flaps to shut and prevent reversion. I was pretty confident that no water would be getting up the exhaust especially with the silicone turned down extensions I added.
I am at least heading in the right direction because last year I actually heard the engines "gurgling" and the water reversion took out my 02 sensors.
FYI, the last time I ran the engines this season was on the hose to change the oil before storage. Basically idle. I don't think this type of exhaust is prone to cracking but am thinking I should at least perform a test to find out. I have no idea what's involved in pressure testing this stuff. A picture of the procedure and or tools necessary would be great.
Thanks for your help.

do your downturns have relief holes? we drill 1in dia holes on the inside portion of the 90

So I normally bring the throttles back pretty slowly (not chopping) and really don't know the relationship in time from when I actually bring them all the way back and the wave hitting the back of the boat. Or is this not really what you are referring to? Just the action alone of reducing the throttle can bring water back up the exhaust? So at any given time throughout the day there is some water in the manifolds at the heads and it's important that at the end of the day to run the rpm up and then turn off the keys?

No they don't. But they're going to
Thanks for the tip.

more referring to people to rev their engines at the dock..you know..because its "cool"
do it at the of the day, or while pulling it out of the water. a lot of people drive their boats on the trailer, bury the exhaust while its running and never think twice about it. pull the boat out a few feet until the exhaust is above water, bring the engine up to 2500 or so and kill the ignition with the throttle plates open.