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1983 Black Max 150 Switch Box to Coil Wire Issue

S

second cast

New member
I recently purchased a boat with the 1983 150 V6 motor. It had an issue where it would not idle. Open the throttle a little and it will start and run, but simply will not idle. If you could keep it running to get it into gear and get the throttle back open it will stay running, but it is very sluggish getting onto plane. Once it planes out it will run, but clearly issues. So as soon as I began to investigate I noticed the wires from the swithbox to the coils for number 3 and 4 cylinders were shot, to the point of crumbling apart. I believe number three was completely severed before I even touched it. I haven't checked to see if there was any continuity through number 4 or not. So as a start to my repair I've ordered replacement harnesses from the switchbox to the coils for both banks as each bank has a bad wire. My question is before I install them, is there something that is causing these to wires to go bad. The wires for the other four cylinders are all fine it's only these two that are bad. I don't want to put the new wiring on only to waste them. Any help with this would be greatly appreciated.
 
The motor is 30 years old so "some" dry rot is to be expected if the motor was sitting any length of time.

Dry rot of wires can be from one of a number of causes. Temperature fluctuations (so sitting outside through winter/summer cycles) can cause it, if the hood was left off and the sunlight got to it (sunlight will "eat" plastic after prolonged exposure), salt air or other air pollution can also have an effect.

Why the other wires survived and these didn't - can only guess. Perhaps the other wires had enough of a fluid film on them to give some extra protection (when I store a motor for the winter I give all the wiring a shot of "fogging oil"). While this may not have been done on your motor, under the hood can sometimes be oily just from leaky 2 stroke exhaust - those two sets of (leads) may have just been the pair that "least benefited" from the oily environment and rotted quicker than the others (which are probably on the way out, just to a lesser extent).

The above is just a "big maybe".

In any event, I do not think that you have some underlying electrical or mechanical issue that caused them to rot - it was simply a result of the environment that the motor was stored in with a corresponding lack of proper storage preparation and maintenance while stored.
 
Thank you so much Graham! This was my first instance as well. I've seen wires do this on trolling motors and what not with no real reason why one deteriorates faster than another. It was after I forked out the money for new wire sets for both banks that I started thinking about making sure there would have been any other ignition component that could have caused that, and not wanting to ruin the new wire sets.

I do have a couple more questions if any Old Merc experts could help me out. First I see two different NGK plugs listed for this motor. I'm going to change them out when I put the new wiring on. Should I use the BUHW or the BU8H for this motor I see places refer to both? My other question at least in this moment is what is the recommended fuel grade for the motor?
 
Both BUHW and BU8H are surface gap plugs. They work best at "sustained" higher rpm running (so not for a trolling motor etc - although it will work, a gapped plug is better if you keep it well throttled down most of the time).

The BUHW is a "colder" plug than the BU8H. The (general) advice from NGK is - if you want the motor to run cooler, use a cooler plug (and vice versa).

When it comes to outboards, unless you run very early in the spring or very late in the fall, getting to operating temperature "quickly" is not an issue, so I would personally use the cooler plug (BUHW).

As far as fuel - 86/87 octane "regular unleaded" is all you need. Some will go out of their way to try and find gas "without" ethanol in it. Merc says you can run up to 10% ethanol (I ran E-15 for three seasons and had no issues), however, if the gas does contain ethanol you absolutely should add fuel stabilizer to the tank.

The single biggest issue with ethanol is that it attracts/bonds with moisture and can separate out (as water, called phase separation) in the tank - causes no end of issues.

Ethanol is also really hard on older gas lines (that were made before alcohol rated gas lines went into wide use). So if you are going to run ethanol gas you might want to spend a couple bucks at the local auto parts place and get a couple feet of fuel line to replace what you have under the hood. I (personally) don't know what rubber compounds they were using in the gas lines back in 83 (if they are original), and for 3 or 4 bucks worth of line, its not worth chancing it.
 
Thank you once again. I will use the motor year around, but based on my past experience I wouldn't anticipate any issues getting the engine up to temp. I won't ever be out below mid 40 temps. That said I will have period of inactivity no doubt and won't use a lot of fuel just fishing in the cooler months so I certainly want to use a reliable stabilizer in the fuel. I purchased some Stabil Marine. Will this cover me or is there a better product that I should go with? I really appreciated the advice on the fuel lines. I'm going to go ahead and change it out while I am replacing the coil wires and plugs. Being able to use an ethanol fuel will save me a lot of towing around to get fuel without ethanol.
 
Yes, Marine Stabil is fine (just a little expensive) - just about any stabilizer is fine.

Some will tell you that you should avoid ethanol at all costs - I have just never had any negative experience running it. I run my motors a lot, have been running E10 for as long as it has been around and even ran E15 for three seasons since there was a station in my area that was selling it on a test basis. Likewise, the E15 caused no issues with my motors (1986 Evinrude/140, 1986 Merc/9.9, 1987 Merc/9.9, 1987 Evinrude/2.5 and 1990 MercMariner/15).
 
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