Thank you all for your help.
As mentioned, I'm not much of a mechanic
so I ended up calling a Yanmar dealer.
Problem is now solved
and my engine runs great.
Here's the technical explanation I asked them to provide me with:
We found that your Injection Pump
may have been removed, or not.
Possibly the previous attempt to
remove it was aborted due to the very
The Injection Pump repair company
said the pump was never dissasembled
for reapir. They found the Internal Fuel
Control Lever in the pump was dragging
and not moving freely. This combined with the
engine temperature which eventually
reaches approximately 180 degrees +/-
resulted in the injector lever sticking
inside the pump and not returning to the
cold start position until the engine and
injection pump cooled down. At which
time the lever moved to the cold start
This may have been a problem from when