Maybe gas in my engine and knocking Bravo 3

"I have a Bravo 3, twin prop I

"I have a Bravo 3, twin prop I/O on my 2001 rinker 270, the engine starts first time every time, it sounds great when idle, it gets up on plane and it runs about 40-45 easy and I can cruise at about 25, my problem is after about an hour cruise and on my way back the engine starts to knock very loud, not a lot just now and then. Then it starts to lose power and knock some more, to the point where I can only come back at about 5-10km. I had it fully tuned, changed plugs, wires, and everything. It still does the same. The next day it is like nothing was wrong. When I get back to the marine I ask people to listen to it and nothing it runs amazing. I think I might have water in the gas tank could that be the problem, also temp and pressure is perfect when this is going on, please help. thanks"

What fuel octane are you using

What fuel octane are you using?
What oil weight and brand are you using?
How long has it been since the oil was changed?
Does the oil pressure drop a little or remain the same?
Do you run the blower all the time?
Is the sound more of a hard ping like shaking a tambourine or a knocking rod?

It may be carbon buildup on the piston tops. Run a can of SeaFoam thru the engine per directions; not in the gas or oil. It will smoke like a forest fire.

"What fuel octane are you usin

"What fuel octane are you using? I fill at my marine only one type
What oil weight and brand are you using? Not really sure
How long has it been since the oil was changed? I have it changed at the start of every season
Does the oil pressure drop a little or remain the same? the same, doesn't change
Do you run the blower all the time? no only at low speeds, start and coming in
Is the sound more of a hard ping like shaking a tambourine or a knocking rod? it sounds more like a bang.

all the other times it runs great. If I run it for more than an hour straight that is when it does this. I am thinking that I might have water in my tank}"

"If water was the problem it w

"If water was the problem it would not wait an hour to knock. Make sure the WCP (water circulating pump) on the front of the engine doesn't rock from side to side. A WCP bearing wearing out can knock as water infiltrates the grease seal. If it starts the noise again, stop the engine and remove the belt(s) and run the engine a minute to see if it stops. If it does then the WCP or alternator is failing or the idler pulley if it has a serpentine belt."

"[b]"I think I might have

""I think I might have water in the gas tank could that be the problem,"

Ayuh,.... Sounds like a Plugged tank vent to Me...."