Engine does ot rev till runs for a while.

[johnrmclean]

So I have an 83 2 stroke 50 HP Mariner. Every time I take it out it will not go to full throttle until its been running at open throttle for a while , say 10 minutes. It runs at maybe 1/4 throttle till all the sudden it will surge up to open throttle. After that its ok for the rest of the day even if I stop for a while . Any suggestions whats going on?

Thanks