yamaha 115 white plugs

My motor has been bogging and vibrating when i have a load or when powering up to get on plane. I had motor serviced and injectors cleaned. Recently took a look at the plugs and two of the plugs are white in color the other two look more normal, black. the tow that are white come from the same ignition coil. How can i check the coil to see if it ok other then just replacing it? Could this just be a symptom of some other problem?

[robert graham]

White plugs can indicate a lean mixture or a water leak on those cylinders. Better check it out. Spark plugs are a very good way to "read" the general tune of the motor. Plugs should be tan colored, not white or oily carbon black. Of course if you've been *****ing/idling for an hour or so you might expect plugs to be a bit blacker than if running on plane. Good Luck!