Re: Differences between 1988 125hp and 1990 120hp
Yes, the 88 will work. In fact, there is not much difference between the two engines at all.
Merc did not change to a Mercury lower unit until after about 1992 so the lower units on both engines will swap. In fact, as stated above, the 1.93 lower unit will give you added performance over the 1.73 ratio on the 88 125.
L drive was a problematic drive system that used what appeared to be an I/O lower unit with the engine mounted inside the boat. It had specialized parts and some people, thinking it was an I/O ran them without oil, destroying the engine.
A B and C refers to the configuration of the lower unit with one being a single exhaust lower unit with one type water pump, another being a dual exhaust with a Mercury type water pump, and yet the third having two anodes bolted to the outside of the casting just above the anti-ventilation plate. For all practical purposes any one will bolt right up to the 1988 125 BUT as I remember all three are two piece drive shafts so you MUST have the top portion and coupler in order to use them. Drive shaft to crankshaft splines are the same.
Personally, although it is supposed to give better performance, I don't really like the dual exhaust lower. BUT as I said, that is personal preference.
The 120 is a 2.80 stroke engine while the 125 is a 2.875 stroke. You can not swap the crank from one into the other as port timing is different--oh, the engine will run, but not produce full rated power. Pistons, rods, head, and carbs will swap right in
As long as you keep the block complete, all other equuipment should bolt right on. You only need to cautious not to mix ignition parts--use a complete system.
