1978 MERCURY HP115 INLINE 6

[mwbos]

WHAT IS THE PURPOSE OF THE MERCURY SWITCH THAT ATTACHES TO THE SWITCH BOX?<br /><br />ENGINE SEEMS TO HAVE A BAD MISS AT IDLE. WHEN I PULLED OFF THE #1 PLUG WIRE THE ENGINE SPEEDED UP INSTEAD OF MISSING MORE ? GOT ANY IDEAS ON THIS MISS ? <br /><br />SPARK AT PLUG SEEMS WEAK? SHOULD I CHANGE THE PLUG OR DO YOU THINK I NEED A SWITCH BOX ?<br /><br />ENGINE HAS NO POWER IN THE WATER OVER 2200 RPM? IS THIS CARB PROBLEM OR RELATED TO ABOVE IGNITION PROBLEM?

 

[sho305]

Re: 1978 MERCURY HP115 INLINE 6

The switch is to kill the motor if you tilt it up too high, like out of the water.<br /><br />I would go directly to your ignition, the distributor cap first except I think you may not have one on that. It may be shorting spark to that cylinder from others. Get a $5 spark tester and check the spark, it should jump 7/16" or so.<br /><br />Do the usual new plugs, get some dielectric grease and put on all plug boots to water proof them. Read your plugs and see what ones are different. Check that coil/wire/pickup for problems. Check for poor connections first. <br /><br />If you have two CDI boxes they are known to fail. You can switch them to test, normally one goes bad and those three cylinders get no/poor spark. <br /><br />Some of them have two alternator windings; a high and low and if your high is bad it will run poor over 3000 rpm or so I think it switches at. You will have more than 2 wires to your alternator under the flywheel if you have that type.