Yamaha 115 Hole-shot Power (or lack thereof)

[Volabama]

I have a 1987 16' Starcraft with a Yamaha 115. It's been running great all spring. Then last week, it lost its hole-shot power. While idling, if I really push the throttle, it is very sluggish. But if I ease it down, within a few seconds, it builds up power and will plane out and cruise all day at around 45-50 mph.

It just started acting like this. It starts fine, idles fine. Any suggestions on where to start? I already replaced the plugs (was going to do this anyway). Could this be a fuel filter or carb problem? Other?

Thanks.