1991 Mcercury Black Max Bogging and will not plane

I have a 1991 Black Max that is driving me crazy.

Back in April or May it started running bad and I diagnosed a bad stator. I replaced the stator and it ran good, but did not seem to quite plane out as quick as usual and did not quite seem to run as fast as it normally did. This would come and at times it would run perfect..

Sometime during the summer it got where it would no longer plane out. It will crank and idle as it should and will rev up with no stumble, but it just will not plane. It tries to and it will push the boat at a trolling speed but it will either shut off or just run at trolling speed with the throttle pushed to wide open.

I checked the compression and I am getting about 110 on each cylinder. I checked the fire and each cylinder has a good strong fire.

The first problem I found was that the fuel filter had black specs in it which told me the fuel line was breaking apart.

I replaced the filter, and all the hoses. It still ran the same.

I pulled the carbs. I saw no signs of blockage, but I still cleaned them out and replaced the gaskets. It still ran the same.

I then pulled the fuel pump. I saw no signs of a problem, but I rebuilt it anyway. It still ran the same.

So, I am at a loss of where to go next.

Any suggestions???

How are you testing spark?

faztbullet said:

How are you testing spark?

Click to expand...

I tested it by using pliers to hold spark plug near metal while having someone crank the engine.

Every cylinder showed strong spark.

Not a good test...need a tester to see if it will jump a 1/4 air gap

faztbullet said:

Not a good test...need a tester to see if it will jump a 1/4 air gap

Click to expand...

So I got a tester and after going back to this thread I may have gotten the wrong thing.

I got one with the light in it that goes between the coil and plug. Every cylinder lit and seemed to be good. When I would rev the engine the light got brighter.

Sounds like you are saying to get one where you can adjust the spark distance.

So, I guess I need to get an adjustable one and try it?

faztbullet said:

Not a good test...need a tester to see if it will jump a 1/4 air gap

Click to expand...

OK, I picked up one of those adjustable spark testers. I set it for a 1/4" gap distance. Ran through all 6 coils and each one gave a consistent spark cross the gap.

What should I check now?

I it possible a switchbox or a coil could be failing when put under a load?