Mercruiser 4.3 no spark issue...Help!

Despite all the warnings of boats being money pits, we, like so many others this year, became first time boat owners. Earlier this summer we picked up an '88 Invader V177 with a 4.3l Mercruiser. It has been a great experience that the whole family has enjoyed. We've probably put about 60 trouble free hours on it so far, until yesterday.  

About half way through a 7-8 hour day on the water I noticed at maybe 3/4-full throttle it would occasionally bog a little and then surge again a few seconds later. When it would bog down I would throttle back to about 3000-3200 rpm (from about 3800rpm) and it would run fine but wouldn't get back up to full song again until about 30-90 seconds later. It got worse by the end of the day but we were able to drive it back to the ramp under it's own power.

Full disclosure...a few days before I made some adjustments to the shift linkage to improve the shifting from forward/reverse back into neutral as it had a tendency to stay in gear despite the :shifter being in the neutral position. The adjustment improved things significantly. Since then I used it for about 3 hours one day and again 3-4 hours yesterday before experiencing any issue.

Step 1 was to check the plugs. They were in decent shape however the top half of one actually crumbled and split in half lengthwise when I was removing it.  It broke under minimal pressure so I assumed it may have been cracked already and was perhaps the cause of my misfire. I put new plugs in tonight and now it won't fire at all, not even with quick start. No spark can be seen at the plugs. I swapped in a known good coil and no change. Firing order has been checked as well and the wires are where they should be.  Any suggestions on what i'm missing or what to try next?

Thanks for your help!

Since you have been working with the shift linkage lately...i would look at a neutral safety switch causing the no-spark.  The other issue with the surge/bog sounds like a fuel issue to me.  I had family member with 4.3liter powered boat that had similar issue caused by weak fuel pump.  

yep, check the neutral micro switch on the left exhaust riser.     also,  try pulling the throttle lever towards the seat (neutral setting).

I'm also assuming your safety kill switch is in place and not pulled out.

In terms of bogging,  have you been running regular unleaded or Rec Fuel?        Ethanal will berkeley up your fuel filters and carburetor if it sits at all (short as a few days)

I’d check fuel filters too, and make sure you’re running ethanol free

Thanks for the input so far!

I'm going to check the neutral safety switch tonight. Fuel pressure seems good so I suspect that's not the issue but will keep it in mind.

I'm running regular gas which from what i've read could be up to 10% ethanol. I hadn't thought about that but will look for ethanol free next time.  The tank is almost empty right now so it's a good time to switch.

I'll check the fuel filters as well.

I just put a meter on my coil and the resistance of the primary windings was 1.6 and the secondary was just over 9. From what I can tell 9 may be on the low side. I know I switched out the coil to try another one that I was told was good but maybe it isn't. I'll have to meter that one as well to be sure. Anyone know if 9 is in fact low?

Don't worry a single second about ethanol fuel. Certainly not the cause of your issue, and the only thing it might do in your boat is shorten the life of fuel lines.

If you changed the shift cables at the back to alter the shift position at the throttle lever, you may have adjusted it away from where the neutral safety switch is in contact.

And yes, also check the neutral return switch.

The boat itself isn't a money pit. You're basically working on an S10 Blazer.

In reply to Riley_88 :

Take a good look at the distributor cap, rotor, wires as well.  If one of the plugs was that deteriorated what else has aged out?  

X3 on the neutral lock out switch plus one other somewhat related possibility: If the outdrive is an “Alpha” series (should be with the 4.3) they use a momentary contact switch to briefly stall the engine when shifting into and out of reverse.   The brief interruption drops the torque enough to let the dogs that engage the gears in the lower unit to disengage and allow the gears to shift. If that switch is being held in contact after your adjustment for whatever reason you would not have spark. 

Thanks for all the feedback, I have spark, the boat starts and idles smoothly.  I ran 12v directly to the coil and it started right up.  I started tracing the 3 wires that connect to the positive terminal on the coil in hopes of finding a short.  The wire running to the oil pressure switch seemed to be the culprit. After messing with that a little and a good luck tap of the switch it fired right up without the needing to wire power directly to it.  Hopefully tomorrow I can get out and do a water test to see if teh new plugs addressed the original issue. Fingers crossed!

Thanks, Mark