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115 merc detonation problem

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greghost

New member
I just bought a Mercury 1996 115 2+2 serial # OG466864 It seems to have a problem with detonation when backing off the throttle It has the correct plugs in it and the magnets on the flywheel are in good shape also the keyway.The timing is set at 23 degrees BTDC at wide open throttle per my service manual .Originally it was at 30 degrees AWOT.When 30 degrees it was detonating when you advanced throttle.We have replaced trigger and CDM's .The stator checked out fine with the ohm meter. Incase your wondering what 2+2 is,it runs on two cylinders up to 2200 then the other two cylinders kick in.There is no idle circut on the bottom two carbs.Also we disconnected the rev limiter as told by a dealer friend.any ideas would be appreciated thanks in advance for the reply's.
 
So do you back off the throttle gradually?----Abruply ?---If the latter you might be hearing normal propeller ratcheting.
 
My experience with detonation is that it occurs upon acceleration when there is glowing carbon in the cylinders, or timing advanced too far and it explodes before time to fire the plugs, aggravated by low octane fuel.

I agree that on fast deceleration, or maybe not all that fast, you could be hearing the overrun clutch in the LU clicking indicating that the engine isn't turning as fast as the water pressure is turning the prop.
 
Texasmark said:
My experience with detonation is that it occurs upon acceleration when there is glowing carbon in the cylinders, or timing advanced too far and it explodes before time to fire the plugs, aggravated by low octane fuel.

I agree that on fast deceleration, or maybe not all that fast, you could be hearing the overrun clutch in the LU clicking indicating that the engine isn't turning as fast as the water pressure is turning the prop.
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Seems that you might be confusing preignition with detonation.
 
boscoe said:
Seems that you might be confusing preignition with detonation.
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Maybe I am after you mention it.......but, "detonation definition" from my Apple computer's dictionary: " the premature combustion of fuel in an internal combustion engine, causing knocking." My comment: "...............and it explodes before time to fire the plugs." Looks pretty close.
 
Texasmark said:
Maybe I am after you mention it.......but, "detonation definition" from my Apple computer's dictionary: " the premature combustion of fuel in an internal combustion engine, causing knocking." My comment: "...............and it explodes before time to fire the plugs." Looks pretty close.
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Lets apply cause and effect: Pre-ignition causes Detonation.
 
Mercury put this in the manual so you can go "oh my God", notice that they mention prop pitch.
preignition.jpg
 
greghost said:
I just bought a Mercury 1996 115 2+2 serial # OG466864 It seems to have a problem with detonation when backing off the throttle It has the correct plugs in it and the magnets on the flywheel are in good shape also the keyway.The timing is set at 23 degrees BTDC at wide open throttle per my service manual .Originally it was at 30 degrees AWOT.When 30 degrees it was detonating when you advanced throttle.We have replaced trigger and CDM's .The stator checked out fine with the ohm meter. Incase your wondering what 2+2 is,it runs on two cylinders up to 2200 then the other two cylinders kick in.There is no idle circut on the bottom two carbs.Also we disconnected the rev limiter as told by a dealer friend.any ideas would be appreciated thanks in advance for the reply's.
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What does the engine sound like when its doing this? I had twin 2+2 1995 115's on my Aquasport 22-2, one of them had an annoying rattle on deceleration not caused by clutch/prop overun. it always did it at 3200-3500 rpm, above and below that rpm, no noise. The engines ran perfect to this day. Another annoyance was a deceleration shake on the starboard engine when transitioning from 4 cylinders to 2 in that 1800-2000 range. Please record this noise if you can.
 
When you back off the throttle the cylinder pressures go way down.----Not sure how there can be detonation under those conditions.-----Original poster should come back and say if he backs off the throttle suddenly.----Also should put motor in forward in the driveway and turn prop clockwise.
 
Detonation occurs after the spark plug has lit the air/fuel mixture.

Preignition occurs before the spark plug has a chance to light the fire.

Preignition can lead up to detonation and detonation can lead up to preignition.

They are the red headed ugly step sisters of catastrophic engine damage.
 
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