Mercury 200 4stroke S/N 2B591535 Plug NGK LKAR7C-9
What’s the proper gap?
Mercury 200 4stroke S/N 2B591535 Plug NGK LKAR7C-9
What’s the proper gap?
I changed mine (300R) around 50 hours and I believe the manual showed .035" for that NGK plug which is used in most of the V6 and V8 outboards.
+1 .035" or 0.9mm
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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Thanks! Has anyone else had issues with plugs in the V6? This is the second time in less than 100 hours that I have had a plug go bad. First time it cost me $150 at the authorized merc dealer to replace one plug. Mercury would not warranty it because they said it was a “maintenance” item. Motor doing the same thing a few months after the repair so I’m replacing all the plugs.
$150 for one plug? That must have included other trouble shooting costs I would think. The plugs are very easy to replace yourself. If this is the 2nd time in 100 hours, is it the same plug going bad again?? If so you might have other issues like a bad / loose plug wire on that cylinder(s), or a bad fuel injector harness on that cylinder? What did the dealer think was the cause?
Yep that's what it cost. The plug was about $25 bucks on the invoice and the rest was diagnosis/labor. I took it back to the dealer because my BRAND NEW motor was running like crap and I was in no mood to start diagnosing the problem that could be something covered by the warranty. It was an intermittent problem and the motor would run right and then lose power then run right again. There were no trouble codes and I had already changed the fuel filter thinking it was fuel related and pulled the plugs and they all looked fine. Now that I know the symptoms that are showing up again and that Mercury considers it "maintenance" item, I will change the plugs myself. Unfortunately they did not tell me or list on the service invoice which plug it was so I won't know if it's the same one.
I guess my question should have been is there a history of the plugs going bad in these new 4 stroke motors? I just find it very strange that in less than 100 hours, I'm having to replace another plug. Maybe there was a bad batch of plugs and others have had the same issue, or maybe it's just the brand of plug is crap and there are better options, or maybe there's another issue with the motor causing the plugs to fail. This just popped up out of nowhere like the last time. I took the boat out a few weeks ago and it ran fine with zero issues. I took it out last Thursday and it started missing as soon as I cranked the motor. Unless sitting in the garage for a couple of weeks makes plugs go bad...
I thought my plugs were bad around 55 hours since it was skipping. So I bought 8 NGK plugs from the local marina here for about $9 each (you can buy them on the internet for less). When I removed them all 8 visually looked fine, but I replaced them anyway, and they worked for about 15 minutes and then started skipping again. I noticed that at least 3 of the plug wires did not snap back down and click. I made an appointment with Don at European Marine and he diagnosed the issue quickly and all under warranty. The issue he found was #7 injector harness wire was defective and causing an over rich condition which caused the misfire. Don repaired the harness injector wire, installed all new plug wires under warranty and it has run flawlessly since. So I thought the plug wires were bad at 55 hours but the root cause was not the plug wires in my case, and I was impressed with Don's service.
Last edited by Pulsare2020; 12-24-2021 at 10:15 PM. Reason: left out a sentence
The gap for your plugs is listed on the engine decal.
New engines need to have the gap check done on the plugs:
http://www.bbcboards.net/showthread....ighlight=Plugs
Plugs.jpg
I pulled the old plugs and replaced with all new plugs. I checked the new plugs and gapped them correctly at .035. With an ohm meter set at 20K, I checked each new plug from tip to electrode and got 6.0 ohm consistently on each new plug. I checked each of the old plugs when I pulled them out cylinder by cylinder and got a variety of readings. Are the resistors breaking down in these old plugs? I would expect to get at least close to the value of the new plugs. I attached a pic of the plugs that I pulled. All look OK, but is it normal to have the brown scorched look where the porcelain meets the metal portion?
#1 - 4.66 ohm
#2 - 4.29 ohm
#3 - 4.39 ohm
#4 - 4.08 ohm
#5 - 4.13 ohm
#6 - 4.11 ohm
I hope to get the boat on the water soon to test. I don't have a way to run it in my driveway.
I tested the resistance of each plug I removed and each one I installed. They were not identical like yours but they were all within 5.2 - 5.5K ohm. I checked the resistance range on an NGK website and all were within spec, and all plugs looked good. Of course, in my case the the missing was not really due to the plugs but a bad harness injector wire on #7 cylinder. This caused a very rich condition according to Don, which caused the missing. The plugs did not look overly rich. After I replaced all 8, the missing disappeared for about 15 minutes, then it returned on the new plugs. After Don repaired the harness, and installed new plug wires the there was no missing on those same 8 plugs.
Ned what were the gap readings on the old plugs?Originally Posted by nwest10
Old plugs gaps were...
#1 - .032
#2 - .033
#3 - .035
#4 - .033
#5 - .040
#6 - .033
I don't know if this was enough to cause the problem I was having or why it would be intermittent, but it definitely wasn't reaching full power even when it seemed to be running OK. This was the exact same symptoms I had several months ago before taking it back to the dealer. It ran great after they replaced the one bad plug for at least 6 months and several trips out, but started up again last week after sitting for about a month.
Last edited by nwest10; 12-26-2021 at 03:21 PM.
Where did you find this info on the NGK web site? I looked but couldn't find the resistance info.
I tried to find it again, and so far have only found several websites that show the resistance as 5K ohm. I have to try and find it but I thought I saw an NGK website that gave a range. I sent Don an e-mail and it said that I checked them and all were between 5.3 - 5.7 K ohm and I told him that was within the range I got from NGK. I will try and find that website and post what it says, but the one below says the same as the first post did, which was 5K ohm. Two different multimeters could easily read different on a range that high.
No problems. The readings I got were well below the new ones. I just don't understand how plug(s) could not last any longer than these did. I'll see how it runs when I can get it in the water.
OEM spark plug literature typically says the resistance of most resistor plugs increases with age. Perhaps my multimeter reads a bit high and yours a bit low? NGK says that they are 5K ohms new, but there must be a tolerance. If it was + or - 250 ohms, both our plugs would have tested close to new values. I doubt that the readings on your plugs or my plugs would cause a misfire if our resistance values were measured on a new spark plug. I agree with you that 50 hours is rather short for plug life on a 4 stroke outboard.
Yep. That's about right. Mercury installs pre-gapped plugs during the build. My 250 CMS V8 repower went on July 2018 and they to were mis gapped. Per my level one Merc monitor readings before/after me gapping them correctly showed a 3% increase in mpg at 4400 rpms/45 mph which is the optimum cruising speed for maximum range on a Z21 Intracoastal. Could you feel an actual difference in performance after gapping them correctly? No.
I haven't found anyone yet that there plugs were at .035 from the factory.
I was surprised to find that the new plugs I installed were all gapped out of the box at .035 and I only had to make a minor adjustment to 1 as it was slightly under. I need to find a good Merc shop that I can trust if I need to take this one back in for a checkup. When I was replacing the plugs I noticed that the shop installed one of the plastic covers on the side of the motor and instead of tucking it back in where it wouldn't interfere with they cowl, they installed it and left the corner hanging out into the area where the cowl sits. It has since worn a big divot in the fiberglass where they let it overlap. Not sure why they had that cover off unless they were checking the wiring under it, but they did a sloppy job putting it back together. I was able to take it back off and reinstall it correctly. I'm certainly disappointed in their work and they were the ones that did my repower.
Great information and reminds me that I need to check the gap on mine. What gap tool does everyone recommend when dealing with these NGK plugs? Thanks.
WIRE type tool, and use the "go-and-no-go" method. NEVER apply ANY force between the electrode and the arm as even the slightest force will cause fractures in the plug.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
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