Bill

Sounds like after it warms and things grow, it sucks air form some place or another.

I would look at the reed block manifold, make sure there are gaskets both sides... Verify the surface is true with a straight edge.

Is this the new light-weight version engine?

Tighten all the bolts on the engine.

Look for obvious leaks at the cases and at the cylinder head to case areas. Should be a film of oil and or dirt if leaking.

As Helidude mentioned you may want to check the needle settings, I suspect this is not the problem though...