Twin Merc Inboard 3000 RPM WOT

[grizer]

Ok I have a strange one which will require a real GURU to give some new hints. I have read lots of posts about RPM limits under WOT but none of them seem to be my issue. At dock and at idle, WOT will go up to 4500RPM+. Under load (in FWD), both engines are limited to about 3000RPM w WOT. Both engines behave exactly the same. If I try 1 engine at a time under power, no change, meaning each engine still limited to about 3000RPM. We pulled the boat, inspected the shafts, props, bottom, trim tabs and everything seems to check out. Drew some fuel into a glass jar and looks great. Replaced fuel filters and checks fuel lines even cleaned out the fuel vents. Still same issue. For giggles we did a compression test and both engines seem fine. We put 2 different diagnostic tools on and no codes are being thrown. Both engines purr like a kitten at idle and also run strong - just limited to the 3000RMP thing. We looked at the fuel pumps (CoolFuel2) and the input voltage is correct. According to Merc, with WOT we should see about 40psi+ but only get about low 20s. That seems to be wrong of course but the question is "is something causing it to be limited to 20psi or is it truly the pumps"..it.seems strange that both would behave exactly the same. Other than the input voltage to the fuel pumps, what are the other inputs and conditions that it needs to see in order for it to supply the full 40+psi? Furthermore, is their something common to both engines with regards to those other inputs/conditions needed for the fuel pump? Any suggestions welcome please as I am completely STUMPED!

 

[alldodge]

should see about 40psi+ but only get about low 20s.

This is your problem, just need to figure out why

is their something common to both engines with regards to those other inputs/conditions needed for the fuel pump?

Is there are common input fuel supply?
If so this will probably be the issue. Look at the antisiphon valve which is a barbed fitting screwed into the tank and the rubber fuel line is connected to it

 

[alldodge]

If there are separate fuel supplies, then is there only one tank or two?

 

[grizer]

2 separate tanks, vents, fills and supply lines 1 running to each engine. when you say "look" at the antisiphon valve, how do I check it and what am I looking for?

 

[alldodge]

Both doing the same thing is the puzzler
The antisiphon valve is a barbed fitting on the tank which has the rubber fuel line attached to it. Its a spring loaded check ball inside which closes off fuel from siphoning out if the is a leak

 

[hogbare35]

What about your props. They can limit rpms. Just a thought.

 

[harringtondav]

Is this a new issue, or have the RPMS always been at 3000? If it ran at higher rpm, follow the pros above. If not, it sounds like you have a big boat, and maybe loading and prop pitch are too high.

 

[Scott Danforth]

you should be seeing 43psi or more

try running the motors off a small 6 gallon tank

what was the compression numbers?

reving in neutral does nothing since the motor is not under load and only needs about 1% of the fuel that the motor needs under load

is this a new problem or has it always been there

when was the last time the drives were serviced?

how are the props, shafts, etc?