Mercruiser 260 Bogging after 45min

[sgs01]

I have a riddle for everyone here. My boat (1982 Sea Ray Vanguard 310EC with Mercruiser 260 inboard engines; starboard engine replaced 3 years ago) has a problem with the starboard engine losing power and bogging after cruising for 45 minutes at around 26-2700rpm. I'm on plane at that point. It bogs then catches again, bogs then catches again, but never comes back up to that RPM.

When that happens, if I run at around 1500rpm for a few minutes then throttle back up, it's usually fine for a few minutes then it happens again. Other than that, the engine runs fine.

I'm stumped here! Any ideas?

 

[scoflaw]

Water in your fuel.

 

[sgs01]

Ordinarily I would agree with you, but when I dumped the water separators and let it sit for an hour or so, I saw no separation at all. That's why I'm stumped.

 

[Lazy Daze]

Another possibilty is a blockage in the fuel vent line. Could be a low spot in the line (accumulating fuel), a blocked tank fitting or even a blocked vent on the hull side. Take a look at things. Try running with the fuel cap off - that will tell you if the problem is with the vent system, or not.

 

[sgs01]

I already tried the trick with opening the fuel tank when it happens. Still no joy. Air flies through that vent when I fuel up, so I don't think it's that.

 

[scoflaw]

Bogging and catching makes it sound like fuel. Could be heat soak of the coils, maybe. What ignition system ?

 

[sgs01]

I think it's Thunderbolt IV. I know it's electronic.

 

[scoflaw]

That's a reliable system, keep looking on the fuel side