1974 7,5 mercury

[Mason Sowell]

Im back! Finally had the chance to take my mercury to the water,well it
Was a river a low one at that, ran sweet going down, but coming back up the river it dropped to one cylinder again, after replacing the coils. When I got back home I checked for fire on the number two cylinder and there was none, the same coil position that went bad before, I tried checking the coil leads from cdi/switch box with my blue point multimeter in which was of no use being that the meter only goes to 700 volt dc and 1 kv on ac either ac or dc both gave eratic readings, on both coils before I replace the stator is there a true way to test the stator voltage or pulse? About ready to fabricate me a coupling for main shaft and set a 20 horse v-twin four stroke in the place of this merc power head! Talk about a money pit! Thanks!

 

[Rmoore45]

Re: 1974 7,5 mercury

700 volt Dc meter is good, if you can set it in the 200v range that would be better. I can check the positive lead voltage on my 9.8 so I would think that you could do it on a 7.5. Take off the Positive wire on the coil, connect the positive lead on the multimeter to it, and connect the negative lead to engine ground, then pull the rope. It should jump up to 140-190 volts for a split second and then drop down to 0. Are you using a digital readout meter?

 

[Mason Sowell]

Re: 1974 7,5 mercury

Yes sir, it's digital. Perhaps I should check it without motor running, LoL" did in a hurry, wife was standing there telling me it was time to eat, as I was checking the spark, always convenient when there's someone else to blame,. Got her engine head back yesterday, for her car, doing a rebuild on it.looks like thats gonna take priority over my merc project. Plus the cold weather, and the limited daylight, thanks Rmoore, I'll try it with out it running.

 

[Mason Sowell]

Re: 1974 7,5 mercury

Checked voltage, to coil 168 give or take, swaped coil. Positions same thing no spark, went ahead and replaced stator, no help! Gonna replace trigger advance coil, or what ever it's called when I get home, guess the next item will be the switch box/cdi, speaking of is there anyone out there that has a spare switch box for a1974 7.5 mercury? Thanks

 

[Rmoore45]

Re: 1974 7,5 mercury

Whoa Whoa whoa, hold your horses on replacing parts. I still have a few tricks up my sleeve for getting spark for these engines. Follow these steps and if you still don't have spark, THEN think about replacing parts.

Put everything back together like you're going to start the engine, leave the cowling off and take the spark plugs out and connect them to a good ground (the middle cover plate bolt works great for a ground). Next disconnect the 2 negative wires from the coils, pull the little rubber covers down the wire to expose the ring terminal on the wire. Ok here's where it gets weird, pull the starter rope a couple times. Grab one of the negative wires that connect to the coil and touch it to the negative on the coil. You should get a spark at the negative terminal on the coil and the spark plug should spark at the same time. Do this to both coils a couple times, then reattach the wires to the coils and check for spark. You "should" get spark.

I had the same problem with my '78 merc 9.8 (the 7.5s big brother, same block and everything) except it only did it when it was cold. Then it finally lost spark and I ended up buying a used power pack off of eBay only to have the same problem, after doing all of the tests to see if all the components were good and switching out stators, coils, and triggers, I finally did this trick and got spark.

The only explanation I can come up with is that the electronic trigger inside the power pack sometimes gets "stuck" between firing the coils. So when you take off the negative wires on the coils, the coils build up some juice and shock the electronic trigger back to life. The first time I did this and got spark I was so happy because I knew that I could get the engine running again without having to buy even more parts.

It's late, I hope I explained well enough. If you're not quite sure just ask some more questions before buying parts.

 

[Texasmark]

Re: 1974 7,5 mercury

Whoa Whoa whoa, hold your horses on replacing parts. I still have a few tricks up my sleeve for getting spark for these engines. Follow these steps and if you still don't have spark, THEN think about replacing parts.

Put everything back together like you're going to start the engine, leave the cowling off and take the spark plugs out and connect them to a good ground (the middle cover plate bolt works great for a ground). Next disconnect the 2 negative wires from the coils, pull the little rubber covers down the wire to expose the ring terminal on the wire. Ok here's where it gets weird, pull the starter rope a couple times. Grab one of the negative wires that connect to the coil and touch it to the negative on the coil. You should get a spark at the negative terminal on the coil and the spark plug should spark at the same time. Do this to both coils a couple times, then reattach the wires to the coils and check for spark. You "should" get spark.

I had the same problem with my '78 merc 9.8 (the 7.5s big brother, same block and everything) except it only did it when it was cold. Then it finally lost spark and I ended up buying a used power pack off of eBay only to have the same problem, after doing all of the tests to see if all the components were good and switching out stators, coils, and triggers, I finally did this trick and got spark.

The only explanation I can come up with is that the electronic trigger inside the power pack sometimes gets "stuck" between firing the coils. So when you take off the negative wires on the coils, the coils build up some juice and shock the electronic trigger back to life. The first time I did this and got spark I was so happy because I knew that I could get the engine running again without having to buy even more parts.

It's late, I hope I explained well enough. If you're not quite sure just ask some more questions before buying parts.

If I understood your wiring correctly, with a magneto driving an open circuit the voltage will jump up until it is high enough to jump the gap. This allows the magneto to develop more "energy" so that when the spark jumps it's hotter. I don't know about solid state devices (probably the opposite would be true), but capacitor problems could be cured/improved with this process.
I don't know what type of capacitor is used for the storage cap in the CDM. I remember back in the days of TV troubleshooting, you could smack a leaky electrolytic in the power supply with another charged capacitor and it would blow out the leak and restore performance. The same process could probably work with paper/plastic which is probably what is used since the rise time of the 40kv pulse is faster than most electrolytics can operate.

Interesting that you discovered what you did.

My 2c,

Mark