This may be old news to some...
If you do any riding in deep muddy water, you may want to remove your pull/recoil starter housing and clean it out. Here's a picture I just took after removing the recoil starter housing... Muddy recoil

 

I spend lots of time in mud & water & don't have that problem, make sure your pullhandle is seated properly in the cup & it shouldn't leak.

 

clean it out. pull the cord out some and clean it. goop 100% grade silicon inside the boot and seat it back down. thats it, fool proof.