The cam that came with these engines is a Crane 134241 going by the specs on my dyno sheet. There is 1 thing that is different. The valves intake and exhaust are longer than the stock height. I found this out when i did the heads 2 years ago. Maybe that is creating a higher lift..I don't know, that's too technical for me.The exhaust is fairly new and Eddie Marine just redid all 4 risers.. They used to be the short style and now they are the long style. As i said in previous posts ..It only reverts when the exhaust is in the open free flowing mode. It does not happen when the Gil mufflers are engaged. When the select sound mufflers are engaged it is creating back pressure in the exhaust system which in turn is stopping the reversion. I can tell this by the sound of the engines. Much smoother running even when revving from idle. BUT if i rev from idle while opened its sloppy and hesitates like it was loaded up Maybe the 2 merc resonators will be enough back pressure to stop this when in the open mode. We will soon see

To help with reverison, Make sure your idle rpm, in gear is at 750rpm or higher.

Thanks i will check the idle in gear..I also just received the merc resonators. I will put them in for a try also

Back in the 2001 era Volvo Penta with 496 had problems with water reversion because Volvo in 1999 did away with all exhaust water shutters / exhaust flapper valves. Volvo on stock apps installed check valves in the side of the risers that help a lot and for thru hull and silent choice exhaust they installed an exhaust crossover upgrade kit in which Corsa helped make part of the kit for Volvo. This crossover help a lot with water revesion.

The Volvo kit part # is 3862055 and the Volvo Service Bulletin about it in 2001 was 25-2-7 describing the problem and fix.. I can not pull any of this up to post here. maybe someone can find the info and post it. I will check with Corsa and see if they have any info about it It has worked out very well for Volvo installing the crossover kit as they do it on all their apps with thru and silent choice exhaust.

The crossover and or the check valves in the risers help equalize exhaust pulsations in which help against water reversion. It helps a lot but not 100 % plus there is other factors that comes into play with water reversion. Also too high of idle on a hot day at engine shut down can make the engine diesel / run on in which causes water version to. Just saying. You have to find that happy medium with idle rpms to help against water reversion and prevent engine run on at shut down - I say help not cure it 100 %.

My guess is that the reversion is caused by high vacuum at idle, caused by the 34 degree advance.
My engine is a 509 with 500 efi cam and intake, CMI headers and silent choice. To cure the reversion problem i have set the advance at 1 degree at idle, which gives 9 Inch hg vacuum at idle

I raised the idles to 650-700 in gear and it seemed to be better. I had the exhaust on the open position for 5 minutes idling and no load up. I'm going to install the resonators anyway and see what happens.

Interesting. High vacuum. :party-smiley-004:

1 degree advance at idle??? off of TDC how did it run?...doesn't make any sense....My timing is locked can;t be changed. There is no reversion with the mufflers in the closed position..That's because in the closed position i am adding back pressure and the back pressure is stopping the pulsations of the exhaust. Nothing to do with timing or vacuum. BTW. My vacuum is 14 with my cam. stock should be around 20 or more. 9 vacuum is low. There's your problem. You must have a large overlap cam, and silent choice means your dumping the water before the y pipe and its creeping back up the pipe. I bet it doesn't revert in the silent mode and if it still does your cam is wrong. What is the LSA and the duration @ .050

It runs OK but it is weak just of idle due to the low vacuum, It does not matter if it is weak at idle since i never accelerate hard from idle anyway.
I know the reversion is caused by the cam and the silent choice, but it is the cam and exhaust i have so the easiest and cheapest was to tamper with the ignition curve.
I bought the cam second hand and where told that it is a 500 efi cam 230*/236* @ .050 and 114* LSA.
Your vacuum is not high. I have been told that 525 efi has 14-15 at idle.

I just installed the Mercruiser Resonators today. Started up on garden hose. Open or closed position of the exhaust...very smooth revs, no hesitation. Resonators took out the low end bark sound from the exhaust. Hopefully not to quiet while cruising at 3200. we shall see. Test ride tomorrow.