Engine needs throttle to start

One of my engines will only start if the throttle is advanced about 50%. The other engine starts just by hitting the iginition switch. It does not matter if the engine is warm, or cold. It just wants some extra throttle to get going. Once idling, it is fine.
Techs have looked it over, and confirmed the IAC is good. Is there anything else they should look at?
It is an 03 280 with 5.0 Mercruisers.

Swap IAC’s between motors. It’s easy to do and will confirm/debunk their diagnosis.

I think they tried that, but I will next time aboard.
Thanks