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Ask Dominick Your Own Question
Dominick, Marine Mechanic
Category: Boat
Satisfied Customers: 1009
Experience:  25 Years Experience as Mechanic, 20 Yrs Boat Yard Owner/Mechanic, Factory Certified
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I have a 1988 Evinrude 120, recently lost 10mph with same weight

Customer Question

I have a 1988 Evinrude 120, recently lost 10mph with same weight load, sounds like I am 2-300 rpm short. Same fuel, same everything. Compression hasn't budged, plugs look perfect. Ideas?
Submitted: 2 years ago.
Category: Boat
Expert:  Jeff G. replied 2 years ago.
Couple things. You could have weak fire on one cylinder.First check the spark plugs with an ohm meter. Check from the center electrode to the metal end where the spark plug wire connects. It should be the same on all 4.Keep in mind that with hotter weather and warmer water the engine will lose HP. This can easily transfer to a loss of rpm and speed. This is one of those things we have to put up with in the summer.Are you using a tachometer? What is your full throttle rpm? What kind of boat?
Customer: replied 2 years ago.
No tach, going by sound. It is a Hewes River Runner with an outboard jet. Problems started when it was 60° during a cold spell, water temp about the same. Will check Ohm meter later today
Expert:  Jeff G. replied 2 years ago.
You really need a tach, I can't tell of a 200 rpm loss at wide open.If you can tell a difference it may be lot more that that.Do you have access to a timing light? If not you can borrow one from most auto parts chain stores.Put the timing light on each cylinder in turn and watch the flashes, are they even and bright from each cylinder? Do this at idle and wide open. Keep cords and body parts away from the flywheel!