I searched ebay for a 42 mikuni hsr carb and there were lots of them on there. I know they wont come with rev limiter, cable, air cleaner, etc.. however, they were 250 brand new. would these carbs work on boat motor? would the rest of accessories still cost over 500 more?

thanks

Yes & yes

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Intake adapter is the largest cost. You will also need the float needle and seat for this application.

This is what a BPS kit looks like: