What will cause two of my cylinders to stop firing on my 75hp merc. outboard?

Question

What will cause two of my cylinders to stop firing on my 75hp merc. outboard?

Answer 1

Could be a coil. Those motors have 2 coils, each running 2 cylinders. I think the one that was firing your two ,now dead cylinders, just died.

Answer 2

bad plugs or distributor.

Answer 3

Do a compression test to make sure that the engine is mechanically sound. Once you know the mechanical health of the motor, you can look at the electrical health. Are the cylinders dead all the time or only at low rpms? If you overheated the engine, you also might have made the cast block porous and water might be getting into the cylinders.

When you pull the plugs for the compression test, you should look at the plugs see if they are in good shape. You also may want to find a test spark plug and put it on one of the two cylinders to see if the CD spark units are firing. The problem could be mechanical, or electrical  You will have to test it out.

Answer 4

First thing is to check your switch box for voltage coming in from stator and trigger. If you have both then check the output side of the same switch box. go from there. If no output but you have input your looking at a bad switchbox.  If no input on stator then start looking at stator wires and make sure there is none grounded by doing an ohms test. If you have stator volts and no trigger volts then do the same ohms test. If you have input and output start going to your coils and test both of them but it is very very rare for 2 coils to go out. I am not saying its impossible but it is rare. Remember when you are checking voltage you need a DVA meter or DVA adapter. well worth the money you pay for one. especially if you can diagnose the problem yourself instead of paying a tech the 95 dollars or so an hour shop fee. Hope this helps.