16hp Vanguard - Runs Rough @ Full Throttle When Hot?

Hi all, I've got a Cub with a 16hp Vanguard. It seems to run fine when cool, but after mowing for a while it starts to run a bit rough. I can hear little burbles and coughs and when I throttle down, they become more apparent.

I'm hoping it's a carb problem, but my head tells me it may be a valve issue.

BTW, there's about 700 hours on the engine.

Any ideas?