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I will definitely disconnect the throttle cable as first test.Disconnect the throttle cable before doing anything else- if it settles down, make sure the cable hasn't been damaged or may be restricted from moving in its whole range.
Don't default to messing with the idle- you have three carbs and they need to be synchronized. If someone else messed with them, find someone else and make sure they have a manometer, so the carbs can be set correctly.
If it ran well last year, what makes you think anything needs to be readjusted?
As long as it has more than one carb, my recommendation about synchronization stands. However, the throttle cable is still my first choice- look for info about how far the throttle plates should be opened at idle- if one or both are open too far, it will idle too fast.I will definitely disconnect the throttle cable as first test.
It has always idled fast, I just haven't been using it for several years, putting it back in service this year.
This model is 3 cylinder, but 2 carbs on a manifold, just an update.
Thanks
This YouTube is about motorcycles but it all the info apples to outboards...
more of a 4 strk thing. never seen it done on 2 strk.You use the Uni-Syn by moving it from throut to throut checking the red plastic float level. The Youtube video is for you to understand the concepts of carb syncronization.
All multi carb engines need to have the carbs sycronized, especial the Merc where one of the cylinder is sharing both carbs.
There is no mystery here, it is just fluid dynamics. His two carb are probably just fine since they are coupled together between each other, but it never hurts to check.
The interconnection could of been damage when the carbs were being removed for cleaned...