Weak spark on Merc 160?

[thunderroad]

69 Merc 160. I did a complete tuneup in May. Points, condenser, plugs, cap, rotor, wires, fuel filter, set dwell and timing, compression even, all around 130-135. The boat has run like a champ all summer. This Saturday it seemed a little doggy. Sunday was worse yet, to the point of actually running rough out of the hole. So tonight I did some investigating. No. 1 cyl had very little effect. Plug looked wet so I put in one of the old ones I took out in May. Engine was instantly back to running smooth and felt strong. But during the testing I couldn't help but notice how weak the spark looked when I did the old screwdriver-in-the-plug-wire thing and when I hooked on the new plug and watched it fire. Even when it would nail me as I took off plug wires, I didn't even flinch. So my question...would a weak coil cause this, or do coils get "weak"? Maybe they just either work or don't. I've had the boat three years and the coil is the one that was on it when I got it. Connections on the coil are clean and tight. What about those "high performance" coils? Any value in them or is this just the nature of the beast? I'm not comparing it to spark like an electronic ignition. I'm still around a lot of engines with breaker points and this one just seems like it has a weak spark. Or should I just leave well enough alone and be thankful that at the moment I'm worrying about a weak spark instead of something major?
thanks

 

[Don S]

Re: Weak spark on Merc 160?

Look at the points. Are they all pitted? Did you check the resistance across the points (power off of course). Yes it could be a coil, or coil wire, or even the wrong wiring from the ign. to the coil when running. You should have a resistor wire or a ballast resistor to cut the voltage down going to the coil. Keeps the points from burning up too fast.

 

[thunderroad]

Re: Weak spark on Merc 160?

The points didn't seem to be too pitted, but I'm gonna look at them again tomorrow in some better light. How exactly do I check the resistance across the points? What should the reading be?
There are a couple of ballast resistors mounted back to back near the rear of the head. I looked at a wiring diagram last year and it seemed like everything was hooked up as it should be. I posted a question on here about them getting extremely hot and was told this was normal. Like I say, the boat has started and ran fine all summer so maybe I should just move along.
Also....just curious...I've been a Ford guy most of my life, but I love the way this GM engine runs. But as a Ford guy, I've always used Autolite or Motorcraft plugs. What brand should I put in this engine?

thanks

 

[wire2]

Re: Weak spark on Merc 160?

If points are pitted and you don't have a new set handy, the old ones can be cleaned up with emery cloth folded double, (abrasive to outside, of course). Finish with waterproof paper, 600 or 1000 to get the shiny surface back.
Do you have .018" gap when open?
Reading across them should be 0Ω closed.

I'd stick with whatever plugs worked well in the past. My 5.7 has always had NGK's.

 

[thunderroad]

Re: Weak spark on Merc 160?

I started at .018 but to get the dwell right, I ended up about .016. I'll check the face of the points again this evening.
Any other opinions about the so-called high performance coils? If you read the ads they say the spark is hot enough that you can run a wider gap on the plugs to help the power. I'm not trying to re-invent the thing, but I'm game to try anything that makes an improvement.

 

[Fishermark]

Re: Weak spark on Merc 160?

You do not need a high performance coil. As I understand it, your coil will put out enough energy to jump a spark across the gap. Period. If it has 50,000 volts or a million, doesn't matter, it only uses as much as is needed. (That's my layman's understanding - some explain with some fancy terms, but it means the same thing! )

Now if your coil is bad, or going bad, then a replacement coil will certainly help. But a "normal" coil is fine.

 

[wire2]

Re: Weak spark on Merc 160?

There used to be aftermarket capacitor discharge ignition modules available that used the points as a signal input, with very little current through them. That made the points last until the phenolic cam lifter wore out. The signal was electronically amplified to 150 volts. The coil didn't heat up because the discharge to the coil was very short duration.

Fishermark is correct, the gap distance between the spark plug electrodes determines the voltage output of the coil.
One point though, it takes more coil output to fire a plug in a compression than in the open. Air is a dielectric, compressed air is a better one.

 

[thunderroad]

Re: Weak spark on Merc 160?

Sounds good. The plugs aren't burnt at all so I'll clean them up, set the gap and leave well enough alone.

thanks