Engine no power under load

[bones774]

Hi, I just launched boat and after replacing the fuel pump (New Carter) i am still having problems under load. Boat starts fine, runs good for 1st ten minutes then after that the boat will not get up to full power. When i reduce throttle then accelerate I can feel a little increase in RPM for a second then it just goes back to sluggish power. I think it's some type of fuel starvation. (Checked vent tube, its ok). ANy on water tests or diagnoses i would appreciate.
1988 350 Merc V8
thanks

 

[alldodge]

Have you looked at the carb or checked fuel pressure?

 

[bones774]

I don't have fuel pressure gauge, however this is a brand new fuel pump, If it's the fuel pump that would mean this one and the one removed were both defective,(same symptoms both pumps) it's possible but how likely?
What should i look at on the carb?
thanks

 

[alldodge]

New pump doesn't mean it is pumping like it should. If the antisiphon valve is sticking then it will be unable to suck fuel very well out of the tank. If pickup tank tube has an issue then same problem. If the inside of a fuel line has collapsed, same issue.

If the carb is clogged up internally it will not supply enough fuel so it will bog.

 

[bones774]

How can i check some of the issues you state?
Funny because my son when it happened to him he disconnected the vent line at the tank and it worked fine, but that would be after the anti siphon valve?

 

[alldodge]

Funny because my son when it happened to him he disconnected the vent line at the tank and it worked fine,

What do you mean it worked fine after the vent line was disconnected?

but that would be after the anti siphon valve?

The antisiphon valve is connected directly to the tank. Its a barbed fitting which the rubber fuel line is connected to that goes to the fuel filter. It has a spring and check ball inside which can get clogged up and stick


To test, remove the valve and see if you can blow thru it fairly easy.

Otherwise, crank the motor with carb fuel line disconnected and see how fast it fills a container with gas

 

[Scott Danforth]

crack your fuel fill cap. if you hear hissing when you do, your vent is plugged.

 

[bones774]

ok, im gonna go thru those steps and see.
In the meantime ive been fiddling with the carb, the larger back(transom side) throttle plates connected to the choke dont seem to open, they are not frozen, i can open them with fingers, but seem to stay closed at WT.
That anti siphon vale if its where i think it is , is a tough part to get at, i have a boat structural stringer going right across it and leaves very little access, looks like id have to remove fuel tank brackets and slide tank backwards, with 50gals of gas ???

 

[bones774]

My son disconnected the vent line from the top of fuel tank and it seemed to run fine.

 

[Scott Danforth]

Your choke needs to open. It needs to be properly adjusted

 

[bones774]

Your choke needs to open. It needs to be properly adjusted

Can i do it myself? That rear plate should be opening at sometime during operation? Would that cause the symptoms described? thanks

I see there is a sticky about the adjustment.

 

[Scott Danforth]

Can i do it myself? That rear plate should be opening at sometime during operation? Would that cause the symptoms described? thanks

Anyone with a modicum of knowledge and the shop manual or a bit of knowledge on carburetors can set the choke

 

[nola mike]

Anyone with a modicum of knowledge and the shop manual or a bit of knowledge on carburetors can set the choke

Do we know what carb we're talking about? Is op talking about the secondaries?

 

[alldodge]

Scott I think he's talking about a 4BBL carb and the rear barrels are not opening.

 

[alldodge]

My son disconnected the vent line from the top of fuel tank and it seemed to run fine.

Then your tank vent is clogged.
Normally if the tank vent is clogged it will run fine for a while, can be an hour or 2 then start bogging down. If the vent was disconnected and it runs good, and when reconnected it starts bogging almost right away there could be other issues

Which is it?

 

[bones774]

Scott I think he's talking about a 4BBL carb and the rear barrels are not opening.

yes that is correct, i

 

[bones774]

Then your tank vent is clogged.
Normally if the tank vent is clogged it will run fine for a while, can be an hour or 2 then start bogging down. If the vent was disconnected and it runs good, and when reconnected it starts bogging almost right away there could be other issues

Which is it?

he disconnected and ran fine and after we reconnected it started bogging, i blew thru lie with pressure hose and its clear, He also removed gas cap separately but engine was still bogging

 

[alldodge]

Something is clogged up but don't know where

 

[bones774]

Something is clogged up but don't know where

yup, i know it;s small but frustrating. Just wondering about that throttle plate too

 

[alldodge]

The carb uses vacuum to open the secondaries. If the motor is not getting full power then the vacuum is weak. A weak vacuum will not open the secondaries