When your Mercarb marine carburetor is getting too much fuel there is a list of items you can check. Gas fumes, rough idle, dies after idling for a few minutes and black smoke are all indicative of too much fuel being pumped into the engine. We will assume the electrical system is correct.
- You replaced the float – the black nitrophyl can be a problem, so be sure it gets replaced.
- After rebuilding gunk from the gas tank and fuel lines can rush into the carburetor getting into the needle & seat – remove the top and flush the needle out with silicon spray lubricant.
- The power valve in the bottom of the bowl must have a good gasket, or fuel can leak by the valve. Check for missing gasket, or damage.
- Too much pressure on the needle when adjusting the float may have damaged the viton tip, thus it won’t seal.
- The float is adjusted to the correct spec. Measure from the top to the toe of the float (toe sticks out the furthest).
- Fuel pump had too much pressure. Check the pressure. This is always suspect if you recently replaced the pump.
- The check ball in the main discharge isn’t sealing. Remove the carburetor, fill the bowl with mineral spirits, take the T and the spring out. Hold the check ball down with a brass drift punch (gently), push the accelerator pump down. You should feel some pressure. Reseal by tapping the drift punch a couple of times. Not very hard, or it will get stuck.
- Perhaps you used teflon tape on the fuel inlet fitting. Teflon tape could be in the needle & seat. (never use teflon tape).
- On some marine engines the fuel pump will have a return line to the carburetor, or into the spark arrestor. This is so that if the fuel pump ruptures, the fuel will go into the carburetor and not the bilge. The line is generally made of clear plastic, so look for gas going through the line. A very small amount is OK.