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Mercruiser 140 Miss Fire

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nic_surry

New member
"I have a Mercruiser 140 3.0 4

"I have a Mercruiser 140 3.0 4 cylinder which developed a missfire initialy at high revs but within 1 hour of driving progressed to mid revs,I have changed plugs,points,condensor,rotor arm,dist cap,plug leads,coil,stripped carb ane replaced all gaskets,fuel supply OK,compressions all ok and still misfires.removed cylinder head no obvious gasket failiures or cracks/corrosion on cylinder head or manifolds.any sugestions appreciated"
 
"Do you have a voltmeter? If s

"Do you have a voltmeter? If so, what is it showing when the misfire occurs? I had this problem on a 470 and it was a bad voltage regulator - running v was up to 16, causing too high secondary voltage in the coil so the sparks were firing adjacent cylinders. A new belt-driven alternator with built-in vr solved the problem."
 
"Thanks for that I have 3 week

"Thanks for that I have 3 weeks ago had the alternator overhauled because it was charging at 16 volts but the engine ran OK,it was charging at 12 volts when re fitted butI've not looked at the charge rate with this recent missfire but will do so when I have the engine up and running again,I'm sure it is something unusuall but I'm a bit stumpped as to what"
 
did you check the riser for le

did you check the riser for leakage into #4 cyl. that plug should be pure white is water is getting into the cylinder
 
"I have heard of this before a

"I have heard of this before and have checked riser and elbow no signs of leakage,fortunately the boat had predomenantly been used in fresh water so no major corrosion on these parts,combustion chamber carbon colour is slightly different on no 2 cylinder 1,3 and 4 all the same,I did discover a slight blow buy on the exhaust manifold which might explain this.Thanks for the feed back"
 
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