It could be a number of things but it doesn't sound expensive, just a tune-
up and maybe some ignition parts like condensor and or plugs or coils. The
coils aren't likely because it runs smoothly and not all three are going to
go at once. Solving yourself is a hassle but doable. First thing I'd check
is that the ignition advance lever is connected and adjusted properly - it
is a little bar that advances the spark when the throttle lever moves. Try
adjusting it one way and then the other to see what happens.

Are you sure that throttle is opening all the way? Sometimes the cable gets
a kink in it that prevents full movement - actually a few other parts can
prevent cable or throttle movement - check for something blocking full
travel. Could even be in the throttle box. Disconnect the cable from the
throttle bar and see if it moves farther.

Get a can of carb cleaner and run it through the carbs with engine running
- don't over do it so engine stalls. Modern gas has so many additives that
if there is old gas in the tanks or cabs they may be gummed up and you have
to take them apart and clean the whole mess - this is a lower probabilty
thing so check ignition and try cleaner first. If it was gummed up carbs it
shouldn't start at all and/or run rough.

The 17' pitch prop is a good pulling prop so it shouldn't be causing enfine
to bog down. If the hub was spun you would get lots of rpm and no go.