We used to break in newly rebuilt motors on the hose for hours before we ran them. we would burn six gallons threw them and water test. Maybe he has a fresh rebuild.
The rabbit ears just have to fit well enough, and the hose has to have enough flow, to keep the water pump circulating water through the water jacket and cool the motor. How would that affect carbon build-up? (Just asking, I'm not a marine mechanic, just know enough to be dangerous!) All the failures I've heard of in 2-cycle OBs have been from overheating (blown gaskets, water pump failure) when the ears didn't fit right or the person ran the motor at high rpms and the water flow wasn't enough.
If it's a 4 cycle OB, I have no clue if there's any difference in how it would be affected.
if there is enough water flow and he's idling then it's the same as being out on the water and idling for a couple hours. Same carbon build up. The problems arise if the hose falls off, there isn't enough flow, or if he's running it higher than idle.
