This is very interesting..I have been having this problem over 3 years now thinking my engines were loading up when the exhaust was in the open/loud mode. When they were in the quiet mode closed they idled fine. EMI thunder short ss risers. with merc silent choice. I learned today that its loading up with water/mist reversion not fuel. The reversion is not from my cam size which is very mild,114 lba, 224/236 [email protected] its from too much water in the short EMI risers. The excess water also is cutting back performance. I was told to either install a 3/4 tee fitting entering the ss tail pipe and them dump it out the transom or find another 3/4 port and dump it from there.
Has anyone else had this situation. Is 3/4 " too big and will there be enough water so the riser pipes don;t overheat?.

I had a set of EMI reverting because the gasket between the risers was bad. The exhaust was leaking out causing a vacuum to suck the water back.

Bob Mandra helped me figure it out.

We had dry exhausts on my friend's ZZ572 originally.Obviously no problem with reversion with the water dumped at the tips.He swapped the Stellings out because of the 100+ dB levels, his marina didn't agreed with,lol.We replaced the dry exhaust with EMI with s/s short risers .We had reversion problem right away.
we decided to have a custom , taller risser made.Fixed the reversion problem pronto.Every extra inch could make a big difference.

Here's the tall riser setup.

I called Eddie marine and they said they can extend my shorty's to a full length pipe. The only thing is I like my silent choice..I mentioned too much water in the risers to them and they agreed that could cause the reversion also. When I run the boat on the trailer with a garden hose it runs fine in the open loud mode..A garden hose has less volume than when in the water. I know these merc pumps pump 38 gallons a minute at 3000 rpms.. Thats alot of water volume pressure in the exhaust. Also I was told that reducing the water volume in the exhaust pipe will give more power and less restriction, equals more HP, better fuel ecomony etc. I guess i will first start with a 3/4 or 5/8 hose off the top port of the manifold.

You might find it easier to put a water pressure relief valve on.
I run the adjustable ones and they work great.

all this sounds great but for sure check the riser Gaskets ....they do leak ...otf

I higly doubt that will give any added hp or anything else other than more exhaust noise.

No water in riser gaskets, dry type

My friends boats, Fountains, Spectre's, Cigarettes all have water dumps on the transoms relieving excess water from the exhaust. Prochargers dump water out also..There will be plenty of water left over for the exhaust hose cooling. I also ordered a set of Shotgun III's.. Drew marine said to dump the excess water and also the shotguns will stop any reversion I have.

only issues i have seen with the emi's is that if your engine is not up to temperture the maniflds will sweat internally because their is way too much cold water running through them..make sure your enfine is up to temp's first..if you see your risers sweating on the outside you can be dam sure their sweating on the inside..