Visual checks of your outboard are often helpful
Here's a question I received: Wayne,
I have a '80 Mercury 40hp 2 cylinder that has spent most of the year in or going back to the shop. It's been great up to now and the mechanics have always done a honorable job in the past..
Early spring I had a full prep service done..
Fresh lube, plugs, carb rebuilt, new wiring harness, new fuel fiters and lines, etc..
Since then a problem started with intermittent firing problems..
It would run great for about 20 minutes at first, then loose power and miss fire badly..
Let it cool off and the same routine would happen.. The iginition coil was replaced..
Then the switch box was replaced..
Hit the lake ,, starts perfectly, runs smooth.. But has no power, wont plane.. Does par at 1/4 throttle..
After that lack of power shows greatly.. What effect would both spark plugs being backed out show..
They were no more than 75% screwed in .. Not even close to hand tightened.. How would the engine react to this??? Loss of compression / loss of power ?? Still firing to run smoothly ??
Many times your boat problems are obivious!
Here's My Reply:
Sounds like your mechanic was having a bad day. I would tighten the plugs and make a close visual inspection for anything else possibly left loose such as: Ground wires, switch box nuts and mounting screws (there should be a ground wire going to the block from one of those mounting screws).
Check to be sure carbs were tightened properly and that a mounting gasket was installed. Check those new fuel lines for kinks or extreme bends that might restrict fuel flow and check that all hose clamps (should be wire tys) are tight.
If none of that helps, send me serial number of motor and I'll tell you some more areas to check.
One of your ignition components is a stator located under the flywheel. The stator has a low-speed coil and a high-speed coil with different outputs.
This may be your problem and it can be checked at switch-box terminals.
Engine missing, surging and failure to start or run can be caused by
loose or corroded electrical connections.
Check all terminals and plug-in connectors for tight clean contact.
Also check all wiring for shorts to ground.
Weak, inconsistent spark, or spark from only one or two ignition
coils; suspect sensor coils or ignition coils.
Weak, inconsistent spark from all four ignition coils; suspect charge
coils. No spark at all; suspect power pack.
You'll find a couple of ohm-meter checks on this page.
Important points that help isolate a problem when seeking repair service or advice!
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It would help if you knew the compression from each cylinder and a description of spark plugs from each.
What condition and color is each plug? One cleanish tan and others wet and black, or one extremely clean etc?
What steps have you tried thus far?
Did the problem start all at once, or over time?
Do you know the repair and maintenance history of the boat?
Knowing this additional info always helps to narrow it down.
And leads to lower repair bills!
Reversed Polarity This will cause some of the strangest boat problems you've ever seen! The changing or reversing of the normal polarity of
a battery, which commonly occurs when battery cables or charging cables
are connected backwards.
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