The Mercarb marine 2 barrel carburetor is outfitted with an electric choke, which will get hot only with the key on. When the engine is cold and the key is off, the choke valve should be closed. Note: you may have to open the throttle slightly to allow the valve to close. As the thermostat heats up the choke will close. The thermostat is grounded through the carburetor so do not put a gasket behind the thermostat.
Mercarb Marine Fast Idle & Choke Setting.
When the engine is cold and the choke is closed extra fuel is being sent into the carburetor bore. To keep the engine from flooding and dying, the fast idle cam is used to raise the RPM (the throttle is opened slightly). This allows more air to be mixed with the extra fuel. As the engines gets warm, the choke opens and the fast idle cam moves to where the idle screw hits the low point. This is where the idle should be when warmed up.
Set the idle RPM with the engine warmed up and the choke valve opened all the way. This automatically sets the fast idle at the same time.
Choke Pulldown
This is one of 2 types of pulldowns. If your Mercarb doesn’t have a pulldown tab, then it is built in. No adjustment is necessary.
The choke pulldown is used to open the choke valve slightly when opening full throttle. This allows more air into the carburetor as needed.To adjust open the throttle fully. Measure the gap from the choke valve and the carburetor body. Adjust by bending the dog ear.
Do all Mercarbs have an unloader?
No, on many of them this function is built into the torque and deflection of the choke.