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Check the easy stuff first. Is the throttle opening the carb throttle butterfly all the way? Is your choke opening all the way? Is the throttle spark advance properly sync’d to the carb (part of the economizer linkage Fleetwin mentioned)? Try swapping the two spark plugs around and see if it’s still the bottom one that’s wet. I’ve seen bad plugs new out of the box in recent years with Champions. Have you put a tach on the motor to see what RPM it is revving up to? If you don’t have one, buy a Tiny-Tach or equivalent cheaper one online – link below – these hook up super easy with just a wire wrapped around a spark plug wire (no direct connection) for the RPM signal. Note that is could be running on one cylinder – these motors run pretty smooth on one. Try taking one spark plug wire off at a time and see if it makes a difference – you may find a dead cylinder this way (take them off when not running and then start it again or you may get zapped). Prop pitch has been mentioned – did your friend have it on a similar type boat? or a bigger one where he may have run a low-pitch prop? You asked about running it in a barrel. To check out max throttle in a barrel, you need a “test wheel” instead of the prop to put a load on it in the barrel, and keep it from cavitating. If this easy stuff doesn’t fix it, you could have a bad reed. Have you checked compression to know you don’t have a stuck ring? Be sure you aren’t running a short shaft motor on a long shaft transom, or vice-versa. Here is a link to buying a tachometer: