mercury 115 inline 6 - Major engine problems?

I have a mercury 115 inline 6, 1975'ish engine. I have replaced the plugs, leads, distributer cap and rotor arm. It does not have points but i still dont seem to be getting a spark. Checked all wiring to be secure with good contact. I was thinking that it may be the switch box or the coil. Whats the likelyhood or is there something simple im forgetting.....(yes it had fresh fuel in it)

Where should i go from here?

Many thanks

Stan

 

Sounds like the Thunderbolt IV system and it consists of four basic parts, coil, amplifier, sensor and sensor wheel.

The sensor wheel is mounted under the rotor inside the cap, the sensor is mounted next to it on the distributor body (sort of a magnetic switch) The amplifier provides ignition advance and the coil the jolting stuff. If the battery is hooked up backwards the amp will die in about a minute. If a regular coil (standard ignition) is used then the amp will overheat and not give much spark.

The amp, wheel and sensor are reasonable tough parts and don't fail often. The coil and shift cut out switch can be trouble spots.

I'm betting it's the shift cutout switch that's causing the problem. This switch kills the ignition during shifting to release load from the gears for easier shifting. This switch usually gets screwed up when a shift cable is replaced by folks unaware of this little guy hiding in the back of the boat.

The switch is normally open (off position) and grounds out the ignition when shifting. This switch is mounted on the transom and is controlled by the shift cable. There will be two wires attached, a gray and a black. If this switch is not adjusted correctly it will kill the juice to the ignition, if it's not operating correctly it may short the ignition. Pull the gray wire off the switch and this will prevent the switch from grounding the ignition. Making sure the gray wire doesn't touch any ground, check for spark with a plug attached to a plug wire and grounded to the block, by cranking the engine.

I'll bet you have spark. Adjust or replace and adjust the switch.

If no spark still, the coil is suspect, have it tested or test it by holding the lead from the distributor tower (the big fat one) to within a 1/4" of a good ground and crank. If you didn't use insulated pliers you've have known right away if the coil was good. Besides the spark, you'll get a hell of a shot of juice. If you did use insulated pliers then a blue spark should jump the gap between the ground and the wire coming out of the coil.

 

Also try disconnecting the gray wire that goes to the tach. If the gage is shorted it will kill the ignition.