4.3 MPI Cold Engine Start Issue

How old are your IAC mufflers? If they are Old pull one out and try starting and running without it. Will not hurt anything just makes loud hissing. Also, you mentioned you had spare IAC. Was it one you bought or did it come with boat?
IAC mufflers are a bit dirty, but not overly so. Just tried starting an engine today with the flame arrestor off and the muffler out. Still wanted to stall within 5 seconds of firing up. Bumped the throttle slightly which kept it going and pulled right back to idle. Ran like a champ as always once it passed that hump, but without the little muffler - man is that intake obnoxious. I'll get new mufflers for the spring, but they're not causing issues at this point and I'll be coming out of the water in a couple of weeks.

The spare IAC came with the boat. It was one of many spare parts, but the old IAC looked perfectly clean inside so I think it was fine too.
 
It's base engine stuff. Mechanical first, IAC control is easy after this.
I'm going to tackle this in the spring since it's the end of the season now. Start off with new plugs/wires and see where that takes me.
Thanks everyone for the suggestions!
 

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