I have a 1990 model 100 HP evinrude on the back on my Stratos.  It has ran fine, but recently it has decided that it doesn't want to idle.  Is this telling me that it is time for it's annual service?  Spark Plugs?  Hoping someone has ran into this issue before!  Thanks in advance!

Annual service couldn't hurt.

I would start with the basics, do a proper linc and sinc. For this you will need a manual, understand everything they a talking about and don't leave out anything or try to take any short cuts.

The next step take you a little deeper, remove the carbs and do a good cleaning on them, then do be sure to do a proper sinc when reinstalling them.

The next step is to check the recirculation valves. These are oneway check valves in the intake that keep fuel puddles from buiding and dumping large droplets into the cylinder. It absolutely will not idle smooth if any of them are bad.

If none of this makes since to you, take it to a dealer.

First thing I'd check is your fuel.

Is it fresh and have you been using an additive like stabile marine or sea foam.  If the gas gets stale an/or contaminated with water it won't idle properly from my experience.  That's where I'd start.