There are many things in the engine that could cause that, but as lightning says, sounds a lot like either a slack cable or something loose in the throttle mechanism.
First pop the lid off the engine, & hard port or stbd it so you can see the cable ends.
Push the lever from neutral into Fwd. Only the gear cable should move. As of then, there should be instant movement on the throttle cable. Same thing should happen in reverse.
If you aren't getitng that then threre is somethig loose inside your remotes. Unless youve taken them apart before, post back - there's a couple of useful tips to doing that without loosing half the bits...... Also lots of plastic bushes in there, so when they have all worn a little bit the looseness all adds up through the mechanism.
If, however, the engine ends of the cables move properly, you'll need to follow the linkages through from the end of the cable to the throttle looking for slack. Not sure if your engine has this, but on the old 70s Johnsorudes they had a cam follower which regularly lost it's roller, introducing half a roller's width of slack into the throttle movement......