I have a mysterious oil leak on my '78 DT 400 that I cannot seem to pin point. Based on the color and scent of the oil, I believe it is Yamalube injector oil.
The attached pictures show (what appears to be) oil dripping off of the lower left bolt of the intake manifold then down onto the engine case.
Does this make sense? Is there any possibility that the injector oil is leaking through the carb and somehow winding up here? I would think any weeping oil would wind up in the intake or combustion area?
Any pointers here will be most appreciated.
Thanks,
Scott

Hi Scott, any gasket sealing issues there can let a little of the fuel mixture escape. Gasoline portion evaporates pretty quick leaving the oil.
Check gaskets or do an engine pressure test.
Third method as I’m in the HVAC business is to clean up dry and dust with a powder made for cleaning up oil stains. Any leaks show up as a darker color

Thanks Dave.

I can't find it but years ago I posted my old school method for finding small seepages that seem to have no source... clean off the oil, run the engine and dust the suspect areas with baby powder or flour... source of leak or leaks will become obvious.

Looks like now they have a fancy expensive powder that does the same thing but probably better.

P.S. You can dust over the area without cleaning first... reapply as required and you'll find the source. I think it works better starting clean.

Thank you. I will try it.

So, I haven't ran the bike in a month or so and I check it over this weekend and notice the "leak" is bad enough to actually put residue all over the engine casing and even develop a small puddle on the floor.
Is there some type of check valve on the oil feed line that I should check?
I am at a loss here.
It is definitely injector oil.
Thanks in advance for any insight.
    

A change in atmospheric pressure can lead to oil coming out of the oil tank overflow pipe. Don't ask me to explain, but I just know that it happens on our RD200DX and a pump action soap dispenser!

I'm not sure that this helps with your leak, but best to follow earlier suggestions and find out where it's coming from first.

I will try to determine exactly where it is emanating from, but it sure looks like it is dripping from the lower left bolt on the intake manifold.

You might find that on the other side of the manifold (upper right bolt) is a clamp that secures the oil pump feed line or that the line might be rubbing on the engine there?
 
It may have rubbed a tiny hole into the line...  I had that happen on a DT175.

Or as was said, could be a leak from overflow hose up under tank that's dripping down from above.