I bought this motor knowing it had no fire, I've tested and replaced the trigger and the stater but still not fire. The coils test good. Switchbox?

Could be switchbox, ignition switch, landyard switch or main wire harness.

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I have a 1986 90hp Merc inline 6....I also had eninge trouble this spring. No SPark at any cylinder, just would turn over and over and over.....I replaced the stator, trigger and rectifier. Recently unhooked the blk/yellow wires from each of the two switch boxes and it started! Try doing that on your 70hp!!! Now though I have replaced the key switch but the problem still exist. Will start with blk/yellow wires unhooked from the two switch boxes but will not start with them hooked up.
I am thinking it is in the throttle box now.....Any Ideas anyone.

the wiring digram I have shows the black -yellow going to the kill switch (other side of kill is black) and also goes to your key switch tab M (black is going to other M tab. On the 8 pin connector to the motor pin 4 is black and pin 6 is black-yellow. I would disconnect the connector and measure the resistance with the key turned on and kill in run you should have high resistance(black-yellow not grounding), pull the kill and you should get low resistance (black-yellow connect to black and ground) my thoughts hope they help