Hello Everyone,

I am working on a Mercruiser 454 magnum that I recently put a rebuilt carburetor on. The boat will start, but won't idle unless I am giving it a little throttle. Supposedly the carburetor was tuned already by the company I got it from.

I read that the IAC valve can be what is causing this, however I cannot seem to locate it on my engine.

Does anyone know if this engine even has one?

Carburetors don’t have idle air control valves

There is no such thing as a carburetor that is 100% drop-on and go, regardless if it was "tuned by the Company I bought it from".

Let's pretend that all of the fixed jetting inside of the carb is exactly what your particular motor needs. You will still need to adjust the idle stop screw, as well as the idle mixture screws to suit your motor. The reason is that "your motor" in one boat may be tilted upwards toward the front at idle (a normal performance sterndrive usually sits nose-high at idle) while the same motor in a houseboat may sit perfectly level or in a V-drive houseboat it may actually tilt a little the opposite way. Additionally, one boat may have exhaust down the outdrive thru the prop hub while another has thru hull exhausts above the water line which creates two totally different backpressure values with the boat stationary at idle.
All of the things mentioned will present a different idle scenario to the carburetor.

And once again, we assumed at the beginning of my reply that the carb jetting is correct for your boat which it may or may not be.

And if you bought a Holley carb (which your motor did not come equipped with), it will need the float levels adjusted "on the boat in the water sitting still with 1/2 tank of fuel".

Since you were not aware of carb idle adjustments, etc, I recommend that you find a guy experienced with carb adjustments and pay him to
1) verify that the carb is installed properly with the correct base gasket
2) make certain that the fuel line and vent hoses are attached and routed correctly
3) make sure the choke linkage/thermal spring/ electric actuator is hooked up properly
4) make certain that (if it's a Holley) your float levels are set properly
5) make sure that your idle stop screw and idle mix screws are baselined correctly (boat sitting in the water - not on a hose - not backed into the water on a trailer)
6) make sure the throttle cable is attached and adjusted properly, so that the cable fully closes the throttle against the stop screw at idle,, and also properly opens the carburetor fully when the throttle is in the max position.
7) there I'll need to be some additional tinkering with idle mix screws once the motor is up to operating temp and you start shifting in and out of gear. Sometimes the mix screws need to be opened an additional 1/4 to 1/2 a turn from where you adjust them "by ear".
8) make sure the boat carburetes smoothly and properly from idle up to cruise speed, and also at wide open throttle.
........ unless you feel qualified to do this yourself.

I close this with a question.
Why did you replace the carburetor in the first place?

MC