Mercury 8.1S stumbling after Spring Splash

Jimmy Buoy

Jimmy Buoy

Well-Known Member
Dec 3, 2008
2,468
Sarnia, Ontario, Canada
Boat Info
2003 Cobalt 293
Engines
Twin Volvo-Penta 5.0 270hp & DuoProps
Boat's worked flawlessly when put away into heated storage last fall. Both engines ran up to 4450 RPM and sounded so sweet...

Then I launched yesterday… I thought the STB engine's exhaust sounded a bit "wonky" at idle and sure enough after warming them up to operating temps the ugliness happened. Powering up with the tabs down to get on plane I immediately noticed that sound of mismatched engine RPMs and the sync gauge was pointing to the STB engine as the cause. Tried more throttle, but the tach said "NO WAY" and could not get that engine to go beyond 3200RPM. Even at lower revs I could see the sync gauge bouncing around while the engine started to cut out, then catch, only to start cutting out again.

I had something similar happen 4yrs ago and someone suggested replacing the plugs. I did and like a light switch, the engine was back to full strength. This time, I'm replacing the plugs and wires for about $200. Hopefully it's nothing more, but all the indicators appear good. Oil Pressure 40, oil is still honey gold, transmission fluid is still nice and pink and not burnt smelling. Pulled the plugs and found that two of the eight (one on each side of the engine) were quite black and sooty compared to the other 6. Perhaps the ones not firing?

Something new though… After my hour run from storage to my marina I noticed a definite clunking noise in fwd with the STB engine at idle. I have turned the prop shaft by hand after all was shut down and it seems to turn smoothly and freely. If I stop and change the direction of rotation there is a small "click" noise from the gear play but both transmissions do the same thing. Could the noise heard in forward be the engine missing to the point of "jerking" the prop shaft momentarily? Thoughts??

At least I'm at the dock for our LONG WEEKEND!! (sure would be nice to have some heat though…)
 
Jim,

The clunking noise at idle is indicative of plugs as well. I went through the ACs a lot faster than 200 hrs. If I still had mine, I would have switched brands. Krole Kid posted a Champion number substitute.
 
I searched for the post from Kreole Kid, but couldn't find it. Anyone know the best plugs to replace with on '08 8.1's? I'm having the shudder on the port side. Starboard is okay but I'm well overdue, I think. 180 hours.

Kevin
 
Thanks for the alternate plug info guys! Just for the record, I've put 175hrs on the mains since changing the plugs. Probably 1/2 of that time is running at 1400RPM (we like running at 7kts with the auto helm on when the lake is calm to get to our destination)

I'll update when I complete the replacement on the STB engine and complete a test run! Then I'll complete the Port Engine. That would probably be a good time to complete a compression test for my records - while the plug boots are easy to remove!
 
Jim,

Get a cheap boot puller tool to help pull the boots up from the bottom. They are $5-6 and will save you pulling a wire out of the boot.
 
carterchapman said:
Jim,

Get a cheap boot puller tool to help pull the boots up from the bottom. They are $5-6 and will save you pulling a wire out of the boot.
Click to expand...

Yes - thanks. A fellow boater just told me that he has a boot puller that he uses too. Sounds like a good tool to add to my collection!!
 
Thanks for all the tips, Guys! I'm going to give it a shot next weekend. Is there any consensus on which plug is best? AC Delco or Champion? I think the AC plug is a double platinum, and a bit more money (unless I just haven't found the right sources for both.

Kevin
 
Kevin,

The ACs are about $6.50 each at the auto part stores (41-983s from memory, but you better check) the exact same AC plug is $20 from MerCruiser...

My last set of ACs only last 30 hours on my last boat - when I pulled them, one or two were missing the platinum tip. I had one more set in my spares kit, and I installed them and later sold the boat. The new owner has not had any problems so far, but I won't even put ACs in my Denali or Tahoe anymore.
 
Lanier Tom said:
Looks like Champion Plug for the 8.1L per Kreole Kid is a 13-RS14YC6.
This is the thread I found: http://clubsearay.com/archive/index.php/t-62368.html
Click to expand...

That's the plug, and I'm still running the same champions from way back then. When I do my spring maintenance, I'm gonna pull a couple out and see how they look. I ran a little while last Saturday mostly at idle, but did get up on top a couple of times to knock some slime off and I got up to about 35 mph in about a 2 ft chop.
 
Well, I'm feeling the pain after changing the plugs today. I used the Champions, and so far so good. Runs great at idle and up to 2000 RPMs. Haven't taken the boat out yet though. The stumbling is gone, and both engines run like new. ~$2.00 each at Autozone.

I ended up not needing the boot puller. It was easy enough with my hands. I used the dielectric grease and anti seize. I recommend both. You shouldn't need to remove any hoses, but YOU WILL NEED "2" socket extensions. You should be able to get to all of the plugs using combinations of these.

Kevin


Sent from my iPhone using Tapatalk
 
UPDATE

Completed the replacement of plugs/wires, fired it up, PROBLEMS GONE! No clunking in gear at idle. Back up to 4450RPM on both engines. Flawless!!

That's a good day. (although I did manage to get a nasty gash on my hand while prying off a spark plug boot that slipped - rammed the top of my hand into a hose clamp end - OUCH!!)

Thanks everyone for your input.
 
Jimmy Buoy said:
UPDATE

(although I did manage to get a nasty gash on my hand while prying off a spark plug boot that slipped - rammed the top of my hand into a hose clamp end - OUCH!!)

Thanks everyone for your input.
Click to expand...

That would be reason #2 for the plug boot puller tool...
 
I had the wrong puller also. WORTHLESS! Put it back in the bag after trying the first one. Only one of them was really tough. The one with the worst access. If you have the puller, it's probably worth using. If not, I wouldn't bother buying one. Just be careful. Gloves might help with that. I didn't use them and got a little bloody. But, what else is new...

Kevin
 
Hey guys, I'm chasing an increase in fuel burn on my starboard engine the past month. 8.1s, I did new AC 41-983 plugs in May, but after reading this and several other post going to swap out for a new set, cheap and easy first step. I may try the Champions even though been running the ACs for years with no issue, usually do new every other season. My question is what replacement wires has anyone used as a replacement for the zillion dollar Merc wires? If I find any signs of a plug not firing and new plug doesn't resolve the issue figure wires the next logical step. Any input appreciated.
 
Good 2 Go said:
Hey guys, I'm chasing an increase in fuel burn on my starboard engine the past month. 8.1s, I did new AC 41-983 plugs in May, but after reading this and several other post going to swap out for a new set, cheap and easy first step. I may try the Champions even though been running the ACs for years with no issue, usually do new every other season. My question is what replacement wires has anyone used as a replacement for the zillion dollar Merc wires? If I find any signs of a plug not firing and new plug doesn't resolve the issue figure wires the next logical step. Any input appreciated.
Click to expand...

I know what you mean! Here's where I got mine. Super fast delivery too!!

http://www.michiganmotorz.com/ignition-wire-delco-p-933.html

Good luck with your fix.
 
Quick update on my increase fuel burn on one engine, had a spark plug wire off from some work done earlier in the season. So was indeed as several members have indicated, these 8.1s will run remarkably well on less than 8 cylinders, my telltale signs were fuel burn which got my attention, 25% more, loss of WOT RPM, 200 in my case, and in hindsight remember my idle RPM was off 10 or 20 RPM, now they are back exactly in sync at 650.

I was all set to do plugs and wires as several have recommended as a first step but turned out to be even easier, thanks to those who contribute their advice and experiences and make the forum a great resource.
 
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