1988 Merc Black Max 150 V6, strange

Dennis4b

Seaman
Joined
Nov 10, 2001
Messages
51
Hey!<br /><br />Just posting this as I am pondering what is going on with a freshly-put-into-the-water 20'er with a "new" (for us :) 1988 Mercury Black Max, 150 HP.<br /><br />After being properly winterized last year, it started immediately when we installed it this spring, ran as fine as they do while running on mufs.<br /><br />Since it was dyno tested to be well over 150 HP at the prop, decided to try the 21" SS one we got with it. <br /><br />It maxed out at 33 knots, which works out (theoretically) to 3570 RPM. (RPM signal disappears above 2000 RPM so the gauge is no help). Way below the 5600 it likes, also ran rough and smokey, but the fuel is a bit old and (diluted) shomewhat premixed.<br /><br />Tried it then with a 19" aluminium prop, and it maxed out (tried different directions, i.e. for currents and winds etc) at 29 knots, which works out to 3470 RPM (very roughly calculated). Boat weight and distribution was the same between tests.<br /><br />There can of course be lots of things wrong, and tomorrow will be a full day dedicated to finding out, but right now I am just wondering about these measurements. Why did a 19" get less performance? <br /><br />If it runs on 4 or so cilinders, would that max out the RPM before the load does? I doubt it can turn a 21" to 33 knots on less than 6 cilinders.<br /><br />Can (stale, slightly oiled) fuel make so much of a difference? Wouldn't then the different props at least get different readings?<br /><br />I know this engine can run a lot smoother, quieter and less smokey than it does now, just wondering if these prop measurements ring any bells with the more knowledgable crowd on this forum :) <br /><br />Best regards,<br />Dennis
 

clanton

Rear Admiral
Joined
Jul 9, 2001
Messages
4,876
Re: 1988 Merc Black Max 150 V6, strange

I would repair the tach and engine, then play with the props
 
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