OK, before you do, IF it's a Merc based Mariner it might
be one of the ones with a slightly un-conventional choke..... which wen set up correctly don't follow the "normal" rules of starting.
Traditional choke does what you expect - blocks the air flow so the fuel - air mix gets higher.
The Merc "enrichener" chokes basically open a small fuel valve & add more fuel. Thing is if you lift the fist idle lever, you are letting more air in, so the Air- fuel ratio goes back to "normal-ish".
It sounds like if you are cranking with both, you will be pouring fuel in, but at too lean a mix to fire when cold. Needless to say when it finally accumuilates enough fuel in the analsd of the engine to actually fire, there's way too much fuel, hence the smoke.
So, When on initial start, don't lift the fiast idle lever - until it fires.
My cold start sequence goes like this:
"Choke" on - no fast ilde - turn key
When it fires - let go the choke switch and try to get the fast idle lever up.
- IF I can get the fast idle lever up quick enough, it keeps going.
- If not, second start is without "choke" and the fast idle lever up. - The fuel lying around in the engine from the first attempt then does what it's supposed to with the extra air.
Having said that, if it's a Yam based one all that I've just typed is irrelevant! Try it before you take it back.