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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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1983 Mercury 115 HP outboard stalls/dies after revving up to

Customer Question

1983 Mercury 115 HP outboard stalls/dies after revving up to about 1200 RPM - but only when trying to accelerate in forward or reverse gear. Motor usually starts OK and idles fine and will rev up to higher RPM when in throttle-only mode and no load.
Motor has good, even compression, new stator, new trigger, new rectifier, rebuilt carbs, new prop, and new battery. The boat is equipped with a built-in aluminum gas tank and it appears to be functioning properly since I've tried testing with a separate portable vented gas tank and get the same results.
So far three separate mechanics have worked on this motor and have removed and replaced many parts without solving the problem. What else can be checked to diagnose the root cause problem?
Submitted: 2 years ago.
Category: Boat
Expert:  Jeff G. replied 2 years ago.
The synchronization of the engine is important. That will definitely affect the acceleration.Set your idle mixture initially at two turns out from lightly seated.Remove the air jets in the top of the float bowls on the carbs, leave them out.What is the timing set at?Run the engine on the hose to 1500 rpm, never higher. Stop engine abruptly and immediately pull the spark plugs out and with a magnifying glass check for any small droplets of water or milkiness.
Customer: replied 2 years ago.
The carbs have been synchronized and the timing set to spec by the last mechanic who worked on the motor. The motor accelerates as it should as long as it's in the throttle only mode and not in gear. As soon as you shift into gear, the motor will no longer accelerate much above idling RPM and will stall out as soon as you try to apply half to full throttle.
I don't believe that water in the gas is the cause either because the boat tank has been drained and filled with fresh gas and stabilizer. Independently the motor was tested with a separate portable gas tank. The results were always the same - stalling under load conditions.
The only other root cause that I can think of is binding in the lower unit or the wrong propeller pitch, The propeller shaft seems to turn freely. Tomorrow I am going to install a 13.25 X 17 pitch propeller instead of the 21 pitch propeller that's on the motor now.
Could a 115 hp motor be that sensitive to propeller pitch to cause this type of problem?
Expert:  Jeff G. replied 2 years ago.
It's not the prop. Probably not the lower unit.Did you check for water on the pistons as I outlined? The inside exhaust plate are notorious for leaking into the cylinders.Why was both the stator and trigger replaced?The trigger and stator have a yellow sleeve on 1/2 of the wires. Are the yellow sleeves both on the same switchbox?Did you check spark on all 6 cylinders?Did you remove the air jet as I outlined and readjusted the idle mixture screw?