Calling old Merc Mechanics Need help with a 1974 Merc 150

[Hydrostreamviper]

So I am having troubles with my middle carb leaking. All 3 carbs have been rebuilt with float levels set correctly. As soon as I prime the bulb on the fuel line, fuel pours out of the throat of the middle carb. The primer bulb never gets hard. The motor will start up and continue to dump fuel out of the middle carb until it gets to higher rpm. I have removed the carb and pressure tested it on the bench with the same fuel tank in the boat. The primer bulb gets hard like it should and the carb doesn't leak a drop of fuel. What could be causing this issue?

[Capt.Insane-o]

new floats?

[Hydrostreamviper]

Yes all 3 are new

[Capt.Insane-o]

make sure that float is not sticking on the pin or in the bore.

[Hydrostreamviper]

They move freely, It just doesn't make sense to me why it leaks when all 3 carbs are hooked up, but doesn't leak when it is pressurized by itself with the same source of fuel.

[Capt.Insane-o]

Which float bowl cover does it have, the one with the filter on top or the non filter? If it has the filter cover swap it with one of the others, and/or swap a float out too.

[David Borg]

Check fuel pump diaphragm

[Hydrostreamviper]

It has a filter on top. And how would the fuel pump be affecting it?

[largecar91]

I never seen a diaphragm pump, pump too much pressure but too much fuel pressure will blow past the needle and seat. Are the needles and seats new?

[Hydrostreamviper]

Needles and seats are new and float heights are set. I can't see the fuel pump being the issue. Has anyone ever seen a hole in a fuel line cause a carb to leak? Usually a bad fuel line will suck air, which seems possible due to the soft primer bulb. But could air in the fuel line overcome the needle and allow excess fuel to flow in?

[faztbullet]

Did you put the sealing washers under the needles???

[Bumblebeeman]

You must have some part of the assembly leaking fuel that doesn't come out when you do the bench testing. Look carefully at the parts that remain with the motor when you remove for bench testing.

[flabum1017]

which cylinder does the fuel pump get it's pulse from? if it's the middle cylinder, then the fuel pump diaphragm would be suspect as said before

[Hydrostreamviper]

Yes the washers under the seat are new

[Hydrostreamviper]

It doesn't leak from anywhere in the system but the one carb, thats what is throwing me off