- Do all Mercarbs have the unloader?
- No. Many Mercarbs have this function built into the torque and deflection of the choke.
Do All Mercarbs Have a Fast Idle Cam?
1. Standard Mercarb (Electric or Manual Choke)
These models do have a fast idle cam. It is located on the side of the carburetor and is linked to the choke shaft.
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How it works: When the choke is closed, the cam rotates into a position that pushes against the throttle lever (or an idle speed screw), physically holding the throttle plates open slightly to increase the RPM during warmup.
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The "Smooth" vs. "Stepped" Cam: You’ll often find that marine Mercarbs use a smooth cam rather than the "stepped" cams typical of automotive applications (like on a Rochester Quadrajet). This allows for a more linear reduction in idle speed as the choke opens, though some versions do have distinct steps.
2. Mercarb TKS (Turn Key Start)
The TKS models do not have a fast idle cam. These were introduced in the early-to-mid 2000s to simplify starting and eliminate the need for a choke plate.
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Why they lack a cam: TKS carburetors use a thermal wax actuator (TKS solenoid) and an enrichment circuit.
Instead of physically moving the throttle plates with a cam, the system opens an internal passage that allows extra fuel and air to bypass the throttle. -
Identification: If you see a carburetor with no choke plate at the top and a solenoid with two wires plugged into the side of the fuel bowl, it is a TKS model and will not have a fast idle cam or any associated linkage.
Summary Table
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.
