Re: What sound should my engine be making on start up?
Yup, Screw in to increase idle. Check for TOO MUCH slack in your throttle cable, but that won't affect the Idle if the idle screw is set right.
I'm thinking more like an air leak in the intake somewhere. A cheap and easy way to check is to get a mustard or catsup bottle with a squeeze top. fill it with water and drizzle it on both sides of the intake fitting. If the engine dies (or slow's down,) a little quicker than it did before, then ya got a air leak. A drop or two of water won't hurt the engine, and it is an almost free air leak test. Just don't dump all the catsup down the drain or your lady will hit you with the empty bottle. Trust me on this, I still have nightmares.
I used marine oil for a while. Then I replaced my engine. It's not exactly the same as the regular stuff. It IS meant for water cooled engines. But use what you feel better with. I'm no expert on the subject.