My first vote goes for a leaky fuel bowl. This could be the drain valve o-rings beginning to leak some. The fuel sits on the top of the engine and just bakes off. I had the same issue but mine leaked enough so that if I parked nose high it would run off the back of the engine and drip on the ground. Did a complete fuel bowl rebuild and solved the problem. Complete rebuild kit of o-rings from DieselOrings.com was only about $20.
If you can get up and reach or look behind the fuel bowl you probably will find fuel.
Hope this helps. PM me if you need more details.
Terry
