How do I tighten the throttle?

[Jeff Weiss]

I am having issues with my throttle. Sometimes I would like to have both hands on the wheel but when I take my hand off the throttle it moves back on its own and the boat will drop off of plane. It is a Mercury panel throttle with the idle throttle enganged by pushing a button on the throttle. There is no adjusment on the throttle itself that I can see. This only happens when the motor is running. If it isn't, the throttle will stay where you put it. I would like to fix this at least until I can install a foot throttle. Any suggestions?

Thanks.