90 hp Merc - Spark comes and goes on whim

[Emjaysee]

Hi. I have a 1979 merc 90 that runs awesome, except that it will just quit for "no" reason (obviously there is a reason! ). When it dies, it doesn't sputter or bog or anything, it literally just dies, and then you can't start it for anything, seems like there's no spark to any cylinders. If you let it sit for awhile it will start again like nothing happened. Conversely, it will run great for several hours and we'll go to lunch or whatever, then when we try to start it, boom, no spark. It's very strange and of course my mechanic can't make it happen in the shop.

Any ideas for this aggravating issue are greatly appreciated. Thanks!

 

[Chris1956]

Re: 90 hp Merc - Spark comes and goes on whim

You should run the CDI Troubleshooting tests on it when you have no spark. If that motor is a distributor model, you may have an intermittent trigger or heat-sensitive switchbox, or you may lose voltage ot the switchbox on the white wire or red wire. If it has the ADI ignition (six coils), you may have an inadvertant ground on the kill circuit (orange wire) or a bad trigger.

 

[Emjaysee]

Re: 90 hp Merc - Spark comes and goes on whim

What is meant by "intermittent trigger or heat-sensitive switchbox", I have no idea what that means. Also, what is meant by "or you may lose voltage ot the switchbox on the white wire or red wire". Please assume I know nothing about this. I do have a meter so I can check things, but I don't normally work on 2-cycle outboards so I'm kind of at a loss as to where to run the meter. There are lots of wires going to the CDI, is that what you're calling the switch box? Thanks.

 

[Emjaysee]

Re: 90 hp Merc - Spark comes and goes on whim

Well, maybe spark isn't the issue, although more time will tell. I noticed in squeezing the fuel bulb that it doesn't stay hard, so I swapped it out with another fuel line and now it's running good. I'll use it this way until it quits again.