My 3 cyl. 59 ci Merc has a mysterious knock/rattle at mid throttle. The smart money says it's a clogged jet. I'm not too keen on running it too much until the carbs come off and get a good cleaning.

Here's a suggestion that will help a LOT- change whatever fuel lines and filters BEFORE you clean your carbs. Get all of that right, because when you start knocking that stuff around, you're bound to get some of that in your jets.