We've got a Formula 330 ss with the 496 Mag's. The port side idles at 650 rpm and shifts easily. On the starboard side it's idling close to 800 rpm which makes it a little difficult to shift. If I move the throttle forward just a small amount the idle will drop to 650 and everything shifts great. I already switched the throttle position sensor from my port engine to starboard and put in a new IAC. What could cause the idle to climb 150 rpm at fully closed throttle?

Sounds like a throttle linkage problem. If you move throttle lever fwd and you drop rpm something is wrong. This action should open the throttle plate and increase rpm. I would unhook throttle linkage off tb and verify manual operation. If you have access to a scantool you can monitor tps and throttle # when working your throttle lever.

Marine motors don't have a return spring, and I'm guessing you need to adjust the cable. There is a barrel nut which holds the outer jacket. With throttle in neutral, remove nut and rotate the barrel so it gives slight pressure holding the butterfly closed

If not throttle related could be iac or vacuum leak.

It is best to check actual throttle position with a Rinda or other scanner. That way you can set your cable position adjustments specifically according to what the Throttle Position Sensor Percentage is stating, at both end of the swing. This way you can match the motors. You really can't do this by sight alone. In a nutshell, with the throttles pulled all the way back, you want to ensure the TPS reads 0% open. And with the throttles pushed all the way open, you want to ensure the TPS reads 100% open. You want to be certain they are trued up like this. You do this with the engine off and the ignition turned on.

And yes, you could very well have an IAC problem if syncing your throttle blades doesn't correct the problem.

throttle blade position not closing enough so related parts with all that end including throttle cable and or its adjustment

IAC

IAC muffler missing ?

TPS and its parameters

dirty flame arrestor

SCAN the engine to see if your tachs are reading correctly - I would check this 4 sure

fault codes ?

idle running parameters -- ie : coolant temp sensor and reading correctly also possible stuck t stat open ? MAP readings ?

Vacuum leak on any of the intake side of things

A very good friend of mine is a retired Merc master tech in your area. If you want me to ask about hiring him I can or see if he is interested working on your app. I could ask. He is very honest and super smart with Mercruiser marine engines.

He is not some hack lets see if I can fix it and prob at one time the best Marine tech Minnesota had for Inboards and I/O - OEM stock apps. He will not work on pieced together apps. OEM stock apps only.