When I stripped my 650 to fix the fuel pump issue I noticed these big cracks in the rear manifold.

Do they look dangerous?

http://s274.photobucket.com/albums/jj246/Austentatious_Productions/?action=view&current=manifold.jpg

Mine are exactly the same mate n not had a problem as of yet, they are about £50 to buy

http://www.wemoto.com/bikes/yamaha/xvs_650_drag_star/97-04/picture/carburettor_to_engine_inlet_rubbers/

Thanks mate that's great

Lucky me in my xv125 the manifold is totally metalic. There is some plastic covering over it. But in my other xv125 the manifold is heat resistant rubbery plus plastic thing it does seem to be cracky. cracks dont lead to inside forming vacuum leaks anyway. Of course the leaks will create vacuum issues so the petcock and fuel pump wont operate as they should. Plus the air/gas mixture distributed by carb to each head would leak out. There will be miss fires and the bike wouldnt pick up well. Try to put some gum all over the manifold, a gum that is both heat resistant and wouldnt melt the manifold.

If I were u I would go for considering a metal manifold which would last a life time, u can easily modify a metal tube ( well this needs lathe machines etc, some guy dealing with a metal workshop u know), make it exactly fit the carb and heads. This way it will be more reliable. I would be trying manifolds from other makes of bikes, should be able to get them modified.