exhaust manifold leak on ym1401

[MarkStewart]

Hi. I'm having a problem with my yanmar 1401. There seems to be fuel leaking from the exhaust manifold. I checked the head gasket and it doesn't seem to be leaking. Also it doesn't seem to be using any oil and the anti-freeze level is the same as far as I can see. The anti-freeze also doesn't appear to be contaminated. When it runs it doesn't smoke excessivly, it puffs grey smoke when I hit the throttle or when it is under a load. Does this seem like a major problem? Will I need to rebuild engine? Thank you
See this link for pics. leaking exhaust manifold