Fountain Vibration Mystery

[Powerquestboy]

I was hoping somebody had some ideas on this Mystery my buddy is trying to solve. He just bought a 1998 29 Fountain with twin 454's/Bravo 1's. At idle this thing has a vibration. They noticed when you turn the silent choice exhaust on it goes away? They had a Merc dealer look at it and they said it was just the exhaust because its under the swim step??? It also goes away once they get it up to speed. Anyone have any ideas. I've had U joints start to go bad in outdrives that caused a vibration at idle but have no idea why an exhaust would change that.

[riverbound]

I was hoping somebody had some ideas on this Mystery my buddy is trying to solve. He just bought a 1998 29 Fountain with twin 454's/Bravo 1's. At idle this thing has a vibration. They noticed when you turn the silent choice exhaust on it goes away? They had a Merc dealer look at it and they said it was just the exhaust because its under the swim step??? It also goes away once they get it up to speed. Anyone have any ideas. I've had U joints start to go bad in outdrives that caused a vibration at idle but have no idea why an exhaust would change that.
The exhaust under the swim step makes sense. I have seen that a couple times on some of the boats I owned in the past.

[RiverDave]

If he's idling and the tips go under the water line from the boat rocking / waves etc.. The boat will "shudder" for lack of a better word. All boats with transom exhaust do it.
RD

[BajaMike]

It is probably the exhast....I have heard this on brand new boats with Bravo One and transom exhast....kinda of sounds like u-joint noise when idleing....nothing to worry about.

[Powerquestboy]

I dont know much about silent choice since none of my boats have ever had it. But doesnt it redirect the exhaust to exit through the outdrive???? Which would change in turn cause the boat to not "shudder" when the water hits the exhaust? They did say it was pretty choppy when they had it out for the first time, but this is not their first boat so I figure they are use to that vibration when big waves hit under the swim step.

[DEEZ NUTTS]

My Formula with twins and a huge swim platform shutters at idle from exhaust and little waves getting trapped under platform.