1970 Merc 75 no spark

[bph9742]

I have a merc 75 (7.5hp) that has no spark when the breaker point wires are connected to the coils, but when i remove the white wire that goes to the top cylinder coil i have spark on the bottom coil , i then move that brown wire to the top coil and it has spark. i can not generate a spark at either coil with the white wire which is the front breaker point.


both breaker points are brand new set at .020 with new insulators and wires. i checked the white wire for short to ground but everything is working normal.
it opens the circuit and closes the ground normal.

i need some feed back

thanks

 

[bob1340]

Re: 1970 Merc 75 no spark

I have a merc 75 (7.5hp) that has no spark when the breaker point wires are connected to the coils, but when i remove the white wire that goes to the top cylinder coil i have spark on the bottom coil , i then move that brown wire to the top coil and it has spark. i can not generate a spark at either coil with the white wire which is the front breaker point.


both breaker points are brand new set at .020 with new insulators and wires. i checked the white wire for short to ground but everything is working normal.
it opens the circuit and closes the ground normal.

i need some feed back

thanks

Not totally sure what the issue is, but the kill wire going to the button/switch needs to be grounded to run and open to shut down.
I found that was confusing as my kill button was bad when I got my motor and it was not grounding the wire. The button/switch is a normally closed (NC) switch. Is yours the Thunderbolt? Mine is a '70 model as well.

 

[bktheking]

Re: 1970 Merc 75 no spark

All the wires do from the points is serve as a "trigger" for the coils. The green wire feeds power from the stator to both coils and the salmon wire is a wire that is normally grounded to the switch. As a test to isolate - pull the wire off the switch and attach it to ground. Then remove both wires going from the points (on the coil side). Pull the motor over and take a ground wire and tap each coil individually with a spark tester in place to see if the grounding triggers the spark. If you do get spark on "both" coils then the issue is under the flywheel. I tested a 71 just a week ago with a dva, got 500V with the green wire connected, test your if in doubt. You already replaced the insulation blocks, that plus new coils was the issue with the one I was working on. I think the issue is with the point going to the white wire, if you say you can get spark with the brown wire and not the white it's either the wire, the point or the insulator- what else could it be. If it were a ground issue with the switch you wouldn't get anything.

PS- don't shut this motor off on muffs or in the barrel by the switch, only by choke.

 

[bob1340]

Re: 1970 Merc 75 no spark

PS- don't shut this motor off on muffs or in the barrel by the switch, only by choke.

Why is that? I'd guess muffs can force water up the exhaust, but what difference does it make how you shut it off? I've shut mine off in a barrel many times.

 

[bktheking]

Re: 1970 Merc 75 no spark

Read the warning:


http://www.cdielectronics.com/downl...guide/Mercury 1 and 2 Cylinder - 336-3996.pdf

It "can" happen, not saying that it will every time.

 

[bob1340]

Re: 1970 Merc 75 no spark

Read the warning:


http://www.cdielectronics.com/downl...guide/Mercury 1 and 2 Cylinder - 336-3996.pdf

It "can" happen, not saying that it will every time.

Learn something everyday! Thanks. I'd presume this is not as "true" in a barrel of water? But I'll use the choke.

 

[bktheking]

Re: 1970 Merc 75 no spark

Hard to say but i'm not taking chances on a $300 part, ya know???

 

[bob1340]

Re: 1970 Merc 75 no spark

Hard to say but i'm not taking chances on a $300 part, ya know???

Thanks for the link. I bookmarked it.

 

[malibu66]

Re: 1970 Merc 75 no spark

I have a merc 75 (7.5hp) that has no spark when the breaker point wires are connected to the coils, but when i remove the white wire that goes to the top cylinder coil i have spark on the bottom coil , i then move that brown wire to the top coil and it has spark. i can not generate a spark at either coil with the white wire which is the front breaker point.


both breaker points are brand new set at .020 with new insulators and wires. i checked the white wire for short to ground but everything is working normal.
it opens the circuit and closes the ground normal.

i need some feed back

thanks

I had this exact problem. It turns out the white insulator that hold the points had a small pin hole burned through in the groove area. It was pretty hard to see. I just cleaned the grove out and chamfered the hole with an Xacto knife, and filled the hole with JB weld and that side came right back to life.