Engine races uncontrolled at full throttle

"My friend has a 1997 80'

"My friend has a 1997 80' Summerset houseboat w/ twin 5.7L Mercrusers (2 bbl. carbs). On occasions the starboard engine unpredictably will race at full throttle. He shuts the engine down and then restarts it and it runs fine. This boat has a helm topside but no one has been up there when this has happened and he says it is pretty scary when it does happen. We tried to get it to do it yesterday to no avail (naturally, not when you're looking). He says the throttle will not respond. Any ideas what could cause this?
Thanks in advance."

Look for loose throttle linkag

Look for loose throttle linkage and/or connections on the engine; compare w/the twin.

it's revving up in neutral

it's revving up in neutral? or when travelling under way?

I just asked him that question

I just asked him that question and he told me that it seems to do it when it is just starting however one time he started it at a marina and almost immediately engaged the prop and it took off full rpm on that starboard engine.

Thanks for asking.

I suspect a big flybridge boat

I suspect a big flybridge boat like that might have some sort of aftermarket "fly by wire" system with throttle actuators down in the engine bay... not going to be able to help much with it. Would have to see what's on it. Any new electronics installed on the boat around the time this symptom first began?

"Nothing new installed and the

"Nothing new installed and the linkage is mechanical (big solid cable). I wonder if there is any way that the throttle link can become decoupled with the butterfly valves in the throttle. If not, it almost has to have something to do with the throttle topside. That might explain why he has no control at the throttle below. It's sure a mystery."