The engine is running too lean for its warm-up temperature. The cold
start system is fine, but not the warm up. It's OK at 80 degrees C. so
don't adjust the fuel injection.
I don't know the specific engine so the following ideas are more theory
than actual "fix it" knowledge.
If this engine has a warm air duct as part of its air intake, perhaps a
temperature controlled duct from a small "stove" on the exhaust
manifold, I'd check its operation.
But, if not, then its warm up regulator is the place to look.