1990 Merc 100hp Weak Spark

Hi all...

I've tried searching the forums for similar problems to the one i'm experiencing, but I can't really find anything. I have a 1990 100hp mercury, and i'm having trouble getting it to start, due to weak spark.

When I turn it over without any plugs in, I am able to get spark out of 3 of 4 cylinders. The top and bottom cylinders have a decent spark, the 2nd has a very weak spark, and the 3rd gets nothing. I've switched the green wires coming into the coils on 1 and 2, and the spark follows, so I know all the coils are fine.

Now when I put all the plugs in, i get nothing...no spark off of any wire. I'm not sure if it's because the motor is spinning much slower or what. It doesn't seem like it's cranking slowly, and i've tried 2 different batteries (both were completely charged).

Could I have a bad stator? Or could it be a bad power pack? I tried disconnecting the rectifier and the kill switch, and that made no difference.

I'm going to try to pick up an in-line spark tester tomorrow and see exactly what kind of gap I can jump.

Any suggestions are much appreciated

Re: 1990 Merc 100hp Weak Spark

I think you have a bad switch box.

Re: 1990 Merc 100hp Weak Spark

oldgradywhite said:

I think you have a bad switch box.

Click to expand...

is there any way I can test that so I know for sure? Could it be the stator or the trigger??

I don't want to spend money on the parts unless I know for sure that part is bad

Re: 1990 Merc 100hp Weak Spark

Don't start changing parts and hope for the best test the system, parts are to expencive and not returnable. Here's the CDI test for the system.

Re: 1990 Merc 100hp Weak Spark

So while inspecting this motor this weekend I noticed a chunk of the flywheel is broken and loose against the stator. I'm assuming this is what's causing my problems...Am I correct in assuming the stator is probably damaged and will need to be replaced along with the flywheel. Can you repair a piece of flywheel or does it need to be replaced? Average price that I found was around $300 plus another $150 for the stator