posted 11-04-2001 08:08 AM ET (US)
I have an older Mercury engine (1976), and the confidence stream output appears to be taken from the water cooling jacket after the water has passed through some of the powerhead. I haven't actually traced the route, but I make this assumption because:--the temperature of the water warms as the engine warms up;
--the water output takes a few seconds to appear after the engine starts and runs.
I also own a Yamaha engine (1987), and the confidence stream seems to be taken right off the pump output. I make this assumption because:
--the temperature of the water is always cold or lake temperature;
--the water output begins as soon as the engine cranks and often precedes the engine start.
Also, in the older Mercury the amount of flow in the stream varies with engine speed, and at low idle speeds it can be a weak stream, especially when running on muffs and not in the water.
In many of the newer Mercury engines, the stream seems to be much higher pressure and has little change in output with engine speed.
When I first saw my engine side-by-side with a newer Merc, I concluded my water pump must be going bad (since the stream output was so much lower) and I replaced the impeller. The new impeller did not change a thing. I asked my trusted Mercury mechanic about this and he cited a difference in the water pump design. The way my engine worked was typical of the older designs; the newer models used a different style of pump.
Mercury has a line of "classic" engines, which I interpret to mean "engineered long ago and proven to work" (as well as "tooling is still good so let's get some more engines out of this"). Perhaps the model you have is a "classic" Mercury 2-stroke engine. If so, the behavior of its water pump may be more like what I have described above.
In any case, I would be worried if the stream output stopped completely at slow speeds. That does not sound right.