Yanmar surges briefly when throttle reduced

[Persy]

I have an 01 Hunter 420 powered with a Yanmar 4JH3E. Engine runs fine with 1 hopefully minor bug that has only recently developed.

When I reduce throttle to idle, the engine slows down immediately to idle, then very briefly 'surges' for a 1/2 second or so, then back to idle.

Engine has about 1,100 hours. I have never had valves or injectors adjusted. Scheduled maintenance suggests I do this at 600 hours, but my mechanic said I shouldn't unless there was a problem.

Ideas?

 

[KD3PC]

could be air/foam in the fuel...
the return line to the tank may be clogged
else injector pump or governor will need looking at..

likeliest is the governor linkage is grimy...