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Mercarb Marine Flooding

  • Engine is running rough due to ignition problems – we will not address the various causes.
  • Choke valve isn’t opening completely when the engine is at operating temperature. 
  • Idle mixture incorrectly adjusted.
  • Needle & seat is damaged – putting too much pressure on the needle can damage the viton tip.
  • Inspect the seat closely to be sure the gasket is there and not cracked.
  • Check the seat gasket surface area to make sure old gasket material is present.
  • Check closely for cracks around the seat.
  • Float level , or float drop is off.
  • Float is bad. Mercarbs have Nitrophyl floats and over time they will absorb fuel. Did you replace it when rebuilding?
  • Fuel pump is putting out too much pressure. Check the pressure. Consult your engine manual for proper pressure. New fuel pumps are especially suspect.
  • Check the site tube (clear plastic tube) that runs from the fuel pump to the carburetor for kinks, or collapsing.
  • Make sure power piston is moving up and down freely. Any sticking may cause too much gas.
  • Be sure the gasket under the power valve is there, not duplicated, or cracked.
  • Look down the carburetor while idling. Is there gas dribbling out of the venturi? If so then perhaps the main discharge check ball is leaking and fuel is being siphoned.
  • Follow the Idle mixture hole in the bore up to the top of the venturi. Blow through both (idle & off idle port) and see if air is getting through the venturi. Might be plugged.

been watching this several times and rebuilt mine. however I am running into fuel flooding my intake…so I am getting a no start (marine… mercruiser 3.7 165. I have taken my carb apart several times to check things… notice the power piston refuses to stay in its place. I can peen it in place…but just wondering what is causing fuel to just flow into my intake. accel pump works great. the float bowl is only half full each check…float is in spec. (using the spring needle and seat setup) 

Staking the power piston is the correct thing to do. Just make sure it move up and down freely.

Did you replace the float? Also be sure to run thin wire through all of the small passages. A plugged vent would cause the gas to siphon off at idle. Also test your main discharge. If it isn’t correct, that would also siphon fuel. Check ball, then spring, then T. You can also test the check ball to make sure it seals. Hold the check ball down with a brass drift punch, put some liquid in the pump well and press down on the pump. There should be a little pressure if the check ball is sealing.I have a 1990 Mercuriser 4.3 with a two barrel carburetor. I am getting fuel all the way up to the filter on the carburetor I pull the fuel line off and turn the motor over and the fuel pump is working .But can get fuel into the carburetor would this possibly be that the float is stuck up ? 

Could be a stuck needle & seat, but your test of the pump isn’t good enough. You need to use a fuel pressure tester. There is also a volume test you can perform, but I don’t know what the measurement should be. Fuel pump pressure is around 7 lbs. Refer to your motors manual to be sure.

Another good possibility is the carburetor is dirty and needs to be rebuilt. Boats sit around quite a bit and that is very hard on carburetors. Ethanol seperates from the gas and then attacks rubber. Corrosion also builds up.

We recommend Ethanol Defense to protect the rubber parts.

Updated on 12/12/2020

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