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I used to think it was a schlock engine---until I got one. It came in a couple of versions:1. An earlier model, small bore (3.3125) with either small WB carbs or large TC carbs. and 2. A big bore (3.375 version with large TC carbs. I have it from a good source that the stroke was 2.875 but have never measured it myself.
I have both the small bore/big carb version and the big bore big carb version. I have not yet run or water tested the small bore but I can tell you that the big bore version with dual exhaust will make your 125 look sick. For example: On my 21 foot cuddy cabin a 125 topped out at 33 MPH. With the 150 the same boat tops out at 40, even with the significantly increased weight.
Although the 150 uses the same mounting bolts, mounts to the same
midleg, and uses the same spline pattern on the drive shaft. It is far and away a better design. That massive plate on the exhaust side hides tuned exhaust runners.
The only problems I have heard about them are that the top semi-keystone ring tends to break. Indeed, Both engines I bought (cheap) had two pistons needing replacement. I have also heard but never experienced that they MAY POSSIBLY break the crankshaft.
Since it is a 5 cylinder in line, water needs to be pumped high and it needs a LOT of cooling water. The water pump is the same as for the 3 and 4 cylinder engines so you should pay particular attention to the impeller and change it on a regular basis as scheduled maintenance. An old impeller will cause the engine to overheat