Mariah 351 surges at low speed

[joe Schnur]

My dealer is working on the boat and is having trouble with the above problem .

Runs great at high speed. At low speed and idle it surges.

Work done wires , plugs, coil , ballast resistor, carb rebuilt . What else could cause this any help appreciated

[Thor]

I’m having the same issue. Could be fuel pump issue, carb issue or a gasket leak on the intake side. I plan on spending time diagnosing this week. Please post if you find the issue first.

[joe Schnur]

New fuel pump too

[Thor]

I found these items relating to carb diagnostic.
18733
18734
18736
18735

[NorCalPR]

Double check the fuel screws. Should be about 1.25 turns back from fully seated.

If too rich, it'll run rough at idle. I had the same problem with my boat till I readjusted the screws and idle

Sent from my SM-G920V using Tapatalk

[CJD]

Surging is always caused by a lean mixture, period.

First would be to make sure you have no vacuum leaks, including the carb base, manifold, or hose connections. The fact you have an issue even off idle tells me there is very likely a leak somewhere.

Once you have that addressed, then remember that boats are always under load. you can set a car's carb for max idle speed, which puts it on the lean side of the curve. Cars idle with zero load. For a boat, you have to favor the rich side of the curve, because as soon as you put it in gear at idle it is under load. Therefore on a boat you must accept a bit of idle roughness when not in gear...and it will smooth out once in gear.

[joe Schnur]

Yes well horror story dealer rebuilt carb did a crappy job said primaries were flooding over not the case the primary bowl is leaking because the needle valve is sticking trying to figure out why.

Leaks gas through booster at idle causing the surging not all the time and when it sticks closed obviously no start

[CJD]

Yes well horror story dealer rebuilt carb did a crappy job said primaries were flooding over not the case the primary bowl is leaking because the needle valve is sticking trying to figure out why.

Leaks gas through booster at idle causing the surging not all the time and when it sticks closed obviously no start

Well, it sounds like you know what to do. It's time to rebuild the carb properly!