surging after carb rebuild

[jonny rotten]

Fist run of the season with last years gas after a full carb rebuild
Brand new fuel pump and water separator
The boat ran perfect for about 20 minutes. Then on the return trip to the ramp it was surging what seems like the exact timing with the fuel pump pumping.
Consistent with no hesitation. UP DOWN UP DOWN for the entire 20 minute ride back

It seems like it only happens in mid to low range RPM's
It idles fine, doesn't hesitate or stall and starts right up
Never got further than mid range because I didn't want to push it

I thought it could be the inline screen at the carb but it's new with the rebuild and looks spotless. I was expecting it to have dried white ethanol crud on the screen blocking the gas flow but that's not the case.

Had a full tank of gas when I left so I can't see low fuel being an issue. And if it was old gas I would think it would run crappy all the time and like I mentioned it ran flawless for around 20 minutes at all RPM ranges and on muffs

Any ideas?
I REALLY don't want to rip the carb off for a third time in 2 weeks
1990 4.3 Mercruiser

 

[Scott Danforth]

Carnac says....... you have a 1962 renault motor and a 1 barrel carb, or a 1996 7.4 liter and a 4 barrel carb or a 2013 4.3 and a 2-barrel....

you get the point..... what year, what motor, what induction...... etc.

could be dirt in the carb
could be a leak in the line between the pump and the tank sucking air
could be a hole in the pickup tube sucking air
could be a clogged vent
could be a stuck anti-siphon valve
could be fuel pump going south

 

[jonny rotten]

Actually could it be a clogged gas tank vent line?
The boat sat for 5 hours before starting for the return trip and it starting surging immediately upon giving it gas. I would think it would have had plenty of time to depressurize if the vent was the issue.

 

[jonny rotten]

Carnac says....... you have a 1962 renault motor and a 1 barrel carb, or a 1996 7.4 liter and a 4 barrel carb or a 2013 4.3 and a 2-barrel....

you get the point..... what year, what motor, what induction...... etc.

could be dirt in the carb
could be a leak in the line between the pump and the tank sucking air
could be a hole in the pickup tube sucking air
could be a clogged vent
could be a stuck anti-siphon valve
could be fuel pump going south

I mentioned it was a 1990 4.3 Mercruiser in the OP. Is a 2 bbl Rochester

 

[Scott Danforth]

I would measure fuel pressure - if the fuel pressure is 4-6 psi at all times the motor is running, look at the carb or the fuel.

 

[2550SX]

Years ago I rebuilt a rochester on my 1986 silverado and once i got it adjusted it was running fine and then one day not too long after I did the rebuild I was driving home from work it started surging on the highway. I recall trying a few different things but what ultimately fixed it was me taking it off and cleaning out all the fuel passages with some gumout and compressed air with the regulator on my air line turned pretty far down. I recall thinking that wasnt going to solve it while I was doing it because I never did see any debris come out of any of the holes but it ran like it should have after that. There had to be some gunk lodged in one of the circuits or jets somewhere. I am not saying that will fix your problem I just wanted to share some personal experience.

 

[jonny rotten]

I would measure fuel pressure - if the fuel pressure is 4-6 psi at all times the motor is running, look at the carb or the fuel.

It's a brand new fuel pump. Would the test pickup an air leak in the line dropping the pressure?

Could you give me some insight how to do the test. I've never done a fuel pressure test.
Where does it attach?
Do I need an adaptor?
Will an auto tester work? I would think it's the same

It's a mechanical fuel pump if that makes a difference

 

[poconojoe]

This might or might not apply to your situation...

Seems I remember someone having a problem with the gasket thickness on a mechanical fuel pump.
The pump's lever was not making proper contact.

Again, I don't know if that's the answer. Hopefully someone with more knowledge will comment.