| [FONT=Verdana, Arial]The 115/125 HP Mercury "Classic Carbureted Two-Stroke" outboard is a 113 cubic-inch displacement in-line 4-cylinder engine.[/FONT] [FONT=Verdana, Arial]It uses the same bore and stroke as the 90-HP, 3-cylinder engine. It weighs about 45-lbs more than the 90-HP, has a taller cowling, and is otherwise quite similar. There are two significant differences, however.[/FONT] [FONT=Verdana, Arial]The 115/125 engine is designed to run only on two cylinders at lower speeds, and shifts to four cylinder operation when the crankshaft speed gets above about 1800 RPM. The engine will be running on just two cylinders at idle or trolling speeds, so it may not be quite as smooth as one would anticipate. I really don't know precisely how it idles; I don't have one.[/FONT] [FONT=Verdana, Arial]The low speed operation on only two cylinders probably saves gasoline if you are doing a lot of fishing or trolling.[/FONT] [FONT=Verdana, Arial]The shift from 2-cylinder to 4-cylinder operation is accomplished in a rather crafty manner, and does not rely on anything exotic. The two cylinders that are cut off are fed with carburetors that do not have idle jets. At low speeds the venturi effect does not have enough force to pull fuel from the high-speed jet, so those cylinders do not get fuel. Once the engine spins up to higher speeds these cylinders come alive and start getting fuel. The engine shifts from 2-cylinder to 4-cylinder operation. I believe that when this happens you could say it really "kicks in."[/FONT] [FONT=Verdana, Arial]The other difference from the 90-HP 3-cylinder engine is that the 115/125 HP engine has a lower maximum speed. Mercury recommends holding the wide open throttle (WOT) speed to 5250 RPM maximum. This may be in part to the inherent in-line 4-cylinder design. My understanding of engine design is limited, but I believe that there are certain problems in vibration with any in-line 4-cylinder engine. To counteract these vibrations some engines employ balancing weights on the crankshaft. I do not know for certain if the 115/125 has or does not have these weights, but inasmuch as they are not mentioned and the engine has a rather low maximum speed, my inference is that these models to not have balancing weights on the crankshaft. Or, if they do, it is still advised not to run the engine over 5250. You could conclude that perhaps there are balancing weights and they are the reason not to rev the engine too high![/FONT] [FONT=Verdana, Arial]As for the general reputation of this engine, I will add this anecdotal data:[/FONT] [FONT=Verdana, Arial]My local Mercury dealership is owned (in partnership) by a fellow who began in the marine business as a mechanic. I mention this so that when later I tell you that "my dealer said..." you understand that this dealer did not get into the marine engine business after his coin-operated laundry business failed, or his bowling center burned down. This dealership is owned by a guy who is also an experienced outboard mechanic and has all the Mercury "master mechanic ratings."[/FONT] [FONT=Verdana, Arial]I have been talking with him lately about buying some engines, and I have been leaning toward buying a pair of the 90-HP Mercury "classics." He keeps trying to talk me into the 115-HP versions. He says they are very good engines and would be a real "rocket ship" on my boat. And his current master mechanic out in the shop agrees. He likes them, too.[/FONT] |
