Incorrect spark plug gap cause loss of RPMs at WOT?

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by impy03, Mar 6, 2021.

  1. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    Would a larger spark plug gap on a Mercruiser 4.3 cause a loss of RPMs at WOT?

    I am only able to reach a max RPM of 3500-3700 RPMs (engine decal says it should run between 4400-4800) and this occurred after I changed the plugs last season. When I changed the plugs, I accidentally gapped them to 0.45, instead of the manufacturer-recommended spec of 0.35. I have twin 4.3s and both are having this same issue. I've done some other troubleshooting such as a tuneup, new fuel/water separator filters, checked for water in the fuel tank, and installed new coils. Should I just go ahead and pull the plugs and re-gap them correctly or would a 0.10 gap difference even be noticable at high RPMs?

    Thanks in advance for any helpful info!
     
  2. bobeast

    bobeast Dance the Tide SILVER Sponsor

    Oct 22, 2017
    Isleton, CA
    2002 310DA
    350 MPI w/V-drives
    Excessive gap can lead to misfiring which could hurt your top end. However, you would typically see some rough idling as well.

    That said, go with what you know. You know the gap is too big, so start by fixing that. Excessive gap can also lead to fouled plugs, so while you are in there, consider replacing them, or at least giving them a good cleaning.

    I believe your era Sea Ray has carbs, so that would be my next area of exploration.
     
  3. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    That is my next plan of action, I guess I was just looking for someone to verify my theory of too large a gap causing a loss of top end. Idling isn't rough, but it does have a bit of a hard start condition. It typically takes 4-6 throttle pumps to get them going but once they warm up, both engines idle smooth.

    You are correct, I do have carbs and I have no clue if they have ever been rebuilt as I am the third owner and have only had it for 2 seasons so far. The strange thing that I also remember is that it had Champion plugs when I first bought it and I remember the RPMs being in the 4000 range the first season. I changed over from Champion to NGKs and ever since I've noticed the loss of top end.
     
    Mauler34Rod likes this.
  4. Chip S

    Chip S Active Member

    316
    Jun 17, 2019
    Bordentown, NJ
    1993 Sea Ray 200 Overnighter
    1993 Mercury 150 hp Black Max Outboard
    If it ran well with the Champions I would replace the NGK spark plugs with correctly gapped Champion spark plugs and see if that solves your problem. If it ain't broke, don't fix it. :)
     
    Chris-380 and Mauler34Rod like this.
  5. Mauler34Rod

    Mauler34Rod Active Member SILVER Sponsor

    105
    Aug 26, 2020
    Lake Michigan IL/IN
    2001 380 Sundancer, Raymarine radar, chart plotter and gps
    496 Mercruiser 8.1 w/ V Drives
    +1 with Chris S , some engines just don’t like NGK plugs. I had an issue with car and NGK.

     
  6. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    Thanks for the suggestions. I think I will switch over to AC Delco MR43T plugs instead of Champions due to the champions having a bad reputation. I'll also say that the reason I switched plugs in the first place is because one of the original Champion plugs on cylinder 3 on my starboard engine detonated and blew the electrode clean off of the plug. Thus I am a little hesitant to go back to the Champion brand.
     
  7. techmitch

    techmitch Well-Known Member GOLD Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    Wise choice to go with the O.E. plug. Champions are for Chryslers and lawnmowers and nothing else.
    As for your detonation on cyl.#3, that maybe a whole other situation in itself.
    (cap, rotor, wires not to mention proper wire orientation)
     
    impy03 and Espos4 like this.
  8. Espos4

    Espos4 Well-Known Member

    Jan 1, 2017
    Long Island NY
    2007 240 Sundeck
    350 MAG Bravo 3 W/DTS
    While the plugs are being replaced, do a proper compression test. You may have a bigger issue than just improperly gapped plugs.
     
    impy03 likes this.
  9. paulswagelock

    paulswagelock Well-Known Member

    Oct 25, 2010
    pa
    2018 SDX 270 OB 300 Verado
    Verado 300
    And verify you did not switch any wires on both motors to the wrong firing order.
     
    dtfeld and impy03 like this.
  10. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    I did a complete tune-up when I installed the new NGK plugs which consisted of new caps, rotors, and wires. I replaced each wire one at a time on each motor, so as to not mess up the firing order.

    "While the plugs are being replaced, do a proper compression test. You may have a bigger issue than just improperly gapped plugs."

    I did already complete a compression check after the Champion plug detonation and all was still normal, although I don't remember the exact PSI results for each cylinder.

    "And verify you did not switch any wires on both motors to the wrong firing order."

    The firing order should still be the same as it was before I replaced the plugs as I replaced those individually, but I will check that again. I will report back once I change over all the plugs. Thanks guys!
     
    Last edited: Mar 8, 2021
  11. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    So I can report that I changed out all the plugs to OEM spec and double-checked the firing order and both engines are running a touch better than before. At WOT, both engines are still only reaching around 3600-3700 RPMs but they both ran smoothly the entire weekend. I also checked my fuel/water separators again and I did not have any water in the fuel.

    At this point, I'm not sure where to go. I was thinking of maybe tinkering with my carbs to adjust them to see if they are possibly running too lean as they still present a bit of a hard starting condition. I also plan to pull both props to check their condition and make sure I don't have any fishing line/braid wrapped up around the splines.

    On another note, I have never cleaned the spark arrestors since owning the boat and have no clue if or when they were last cleaned out. Would 2 dirty 30 something-year-old spark arrestors potentially cause lower RPMs at WOT?
     
    Last edited: Mar 22, 2021
  12. bobeast

    bobeast Dance the Tide SILVER Sponsor

    Oct 22, 2017
    Isleton, CA
    2002 310DA
    350 MPI w/V-drives
    You keep asking whether you should correct problems you already know about. If you know the carbs need attention, then I would be having a nice conversation with them. If your spark arresters have never been cleaned, then time to break out the cleaning kit. Only after you have resolved all known issues can you begin troubleshooting in earnest.

    Inspecting the props is a good general maintenance habit, but if they were both damaged, I would expect you to have other issues, such as vibrations. One thing you should check, while you are there however, is that they are the factory original props. If a P.O. changed them out to a different pitch, that could definitely affect max rpm.
     
    Last edited: Mar 22, 2021
  13. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    I am not a mechanic, thus is why I am here asking questions. I don't really truly believe the carbs are the issue, but that is one more thing I could try adjusting if a lean condition would negatively affect RPMs at WOT, same with cleaning the arrestors. I will plan to clean the arrestors, but I was more or less wondering if both of them being dirty would cause such a dramatic drop in RPMs.

    The props look to be original and do not show any visible cracks or chips, but they do show rough surface areas and could for sure use a good polishing. I will definitely be checking the size to see if they are correct OEM spec for pitch and diameter when I pull them next weekend. Thanks for the suggestions!
     
  14. bobeast

    bobeast Dance the Tide SILVER Sponsor

    Oct 22, 2017
    Isleton, CA
    2002 310DA
    350 MPI w/V-drives
    I think a fouled carb could definitely affect WOT and you are already experiencing hard starts. It's easy enough to check the spark arrestors as a potential cause. As long as you are not experiencing back-fires, you could simply remove them and go for a test run. That said, but them back on, as they are a safety feature. You couldn't get away with this on a MPI, but a carb'ed engine is not as closely monitored by the computer.
     
  15. hughespat57

    hughespat57 Active Member

    406
    Sep 25, 2016
    Rock Hill, SC
    300 Sundancer 1994, trailered tri-axle LoadRite roller
    Mercruiser 5.7 260HP Alpha One Gen II, twin
    That could have been just one poor quality plug.

    However, true "detonation" in a cylinder is due to
    very high compression (not a 4.3 characteristic)
    low octane fuel, (also not likely, you don't need high octane)
    lean mixture = carburetor issues

    You can only externally adjust the idle mixture on a carb
    The internal jets are fixed, or variable with a vacuum controlled piston. If either of these get clogged or gummed up you could have a lean mixture at WOT. Leading to poor performance and possible detonation. The only way to deal with that internal clogging is to disassemble and clean them thoroughly.

    As @bobeast pointed out if you already suspect carb issues, work on that

    p.s.
    On an unmuffled marine engine it's nearly impossible to hear mild detonation. But detonation will do more than just ruin the plugs it can ruin the pistons.
     
  16. Sea-Ya

    Sea-Ya New Member

    25
    Sep 29, 2016
    Long Island
    2004 Sea Ray 215 Express Cruiser
    5.0 Mercruiser
    Just a thought, but how old is the gas? Was it sitting all winter? From what I understand gas that sits can loose some octane while sitting. I know you said there was not water in the fuel but possibly old fuel from being stored over the winter could be a problem
     
  17. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    I agree, but both engines are running smooth and at the same RPM, so I find it hard to believe both carbs are fouled, but I guess bad fuel or trash could have done so to both at the same time. I will definitely try removing the arrestors and going for a run to see if there is any difference as I am not experiencing any backfires or issues once the motors are cranked and warmed up.

    I too thought it could be a fuel issue, but I only had about 10-15 gallons left over from last season in the tank and I was sure to treat it with plenty of sea foam at the end of the season. I have also run that existing fuel out and added some fresh since the beginning of March and checked my fuel/water separators 3 times so far this season and replaced the internal filters and seen no change.

    The next things I will be troubleshooting in this order are: remove/clean flame arrestor, check existing prop for the correct size/pitch, check engine timing, check fuel pump pressure, and finally rebuild the carbs if all else fails.

    Thanks for all the suggestions fellas!
     
  18. Lazy Daze

    Lazy Daze Well-Known Member TECHNICAL Contributor GOLD Sponsor

    Apr 21, 2009
    PA
    Various
    Various
    One thing I don't see mentioned is when exactly the issue happened. Did it happen RIGHT after you replaced the plugs, or was there some time between when you replaced them and then ran the boat again? Has this boat sat in the water? If the bottom is dirty, that can greatly affect top end speed/RPM.
     
  19. impy03

    impy03 New Member

    22
    Mar 26, 2019
    1989 Sea Ray Sundancer 280
    4.3l Mercruiser
    Yes, I noticed the loss of RPMs at WOT after the first season I owned the boat and after I had replaced the original spark plugs. This boat stays in freshwater year round, so I couldn’t tell you how dirty the bottom is. I do know I have already put it through it’s paces some this year to try and remove as much of that gunk as possible.
     
  20. Lazy Daze

    Lazy Daze Well-Known Member TECHNICAL Contributor GOLD Sponsor

    Apr 21, 2009
    PA
    Various
    Various
    If I'm reading this correctly, it sounds like you had this issue before you changed the plugs, too?

    You won't get barnacles in fresh water, but you can get weeds growing and this won't always come off just by running the boat. I think the first course of action here is to pull it out and do a proper power wash on it. Otherwise, you might be doing all this other stuff for no reason. It doesn't take much growth - even just a thin layer - to cause the problems you're having.
     
    techmitch likes this.

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