Hey guys, I have a 78 Sleekcraft Aristocrat with a lightly modified 350 SBC. When I first put her in, I let her warm up, and then when I do WOT, I turn about 4300-4500 RPMs. If I drive this way for a while, my RPMS seem to drop a little bit, to 4200 RPMs. If I back off and slow down a bit, then WOT again, it won't go over 4200. If I turn the boat off and let it sit a while, then try again, it will be back at 4300-4500 for a short while.

Any idea what is causing this?

Here's my boat's build thread for reference:

http://www.offshoreonly.com/forums/c...ld-thread.html

Heat soak? Nice looking boat, btw.

Maybe a tired coil?

Did you check your fuel/water separater? Could be clogging at WOT, then when it sits for a while it will settle. Just a thought. Nice boat!

Just a guess . Sounds like automotive engine tolerances used in a marine engine resulting in expanded pistons in a cold non expanded block among other things . Again just a guess from the gut .

Is this a new problem ? IE: Been running fine and now recently this ?

Anyhow, you are running too much of a prop. Even the small dinky cammed 260hp Merc 5.7 liked being 4600-4800. Ideally, closer to 4800rpm.

So, if not a new problem, to make you think,
Are you running a vacuum secondary carb ?
Reason for asking, is between the too low WOT rpm and a strong vac secondary spring, the secondaries may be closing a touch. When you shut off engine, the vac secondaries will totally close, even if binding a touch.

I one boat I have a spot in rpm and throttle where the smallest movement in the throttle is close to a 6mph difference. Small light boat. I need to convert that one to mech secondary so operator can have total control over the throttle openings.

Can u see the rotating assmbly through the hole in the side of the block?

My guess too...

Throttle cable stretch

good lookin boat, nice lines on it.