I have a speedboat with a Yamaha 90 outboard. It misfires at about half throttle. Any ideas why?

Question

I have a speedboat with a Yamaha 90 outboard. It misfires at about half throttle. Any ideas why?

Answer 1

CDI box is bad or is getting that way

Answer 2

bad plugs

Answer 3

A cylinder's dropping out.

If you mean it misfires *only* at half throttle, but runs smoothly at all other throttle settings, inspect the wiring coming off the stator & trigger, look for chafing, worn, missing or burned insulation -- could just be a bare wire or two touching something they shouldn't just at that setting.

If you mean misfires at 1/2 throttle all the way up to full throttle, do a spark test.   Not a "spark plug" test -- use an inline tester to determine whether the ignition is delivering enough power to generate spark under heavy load.  

Set the gap on the tester to 7/16", clip it to the engine block, attach the plug leads (one at a time), and look for strong bright blue spark while somebody turns the engine over with the key.  If it jumps a 7/16" gap outside the cylinder is a good indication that everything is okay.

If you get a bad cylinder, coil, powerpack (aka switchbox) & trigger can be tested with a multimeter, but you'd need a manual to determine the specs of your particular engine.  

Other possibility is, misadjusted float or sticking float valve in one of the carbs.