Okay, so my buddy has a 1987 Scarab Excel with a 330 hp 454/ Alpha.
Long story short, last year he had the engine replaced with a jasper, boat ran great but intermittently throws steam through the exhaust.
The manifolds and risers were replaced less than 20 hours ago, impeller also, and the boat shoots plenty of water out.
it doesn't run hot either.
What could it possibly be?
The old motor never did this.
The mechanic that replaced the engine has some theory's, but nothing solid.
We are changing the water circulater pump this weekend and firing it to see if that improves anything.
Thanks for the help!

would it steam randomly or after a hard run? would it steam only when it was cold out?

It steams randomly, cold, hot, there's really no constant, it will even stop steaming randomly.

raw watercooled? have you changed the thermostat recently?

Raw water cooled, thermostat changed 2x since engine was replaced...

Does the engine miss or change Rpm when this occurs?
Have you examined the thermostat housing for proper flow of water? Say when the thermostat is closed. Depending on what type you have the spring loaded check balls could be sticking. Some housings have 4 outlets and some 2. If there are 2 of course they will connect to the manifold lower fitting. If 4 I think the outside ones with the check balls go to the elbow, the other 2 enter the manifold.
The manifold-riser gasket, is it full open flow or 4 open , 2 restricted small holes?

The engine runs excellent through the steam. I don't think the temp even rises, but I don't recall 100% as it was last season. We eyeballed the thermostat housing, but I'm not sure how to check for proper flow. He has, the check balls, and they seem to be functioning 100% as well. The manifold riser gasket is the other thing we will be checking/changing, which gasket is better/worse or right/wrong?
Thanks for all the replies!

We replaced the check balls and bolt etc, also changed the water circulating pump, which was in terrible condition. Nothing has helped the temp was getting close to 180, but stayed there, a new symptom occurred, it seems the manifolds are getting very hot and when we shut it down you can hear water gargling, any ideas???
we are breaking down the risers this evening to see about the gaskets.
thanks

Just a thought, when you changed the impellor in the leg did you check the water tube from the leg up to the engine? Could be partially blocked and causing intermitent slowing of water flow?

This is my internet guess, blockage there.