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Results 1 to 9 of 9
  1. #1

    Default Hotter spark plugs for 9.9 4-stroke?

    My 2009 Merc 9.9 4-stroke is used only as a trolling motor, mostly at very low rpm, 800-1100 rpm. I motor out onto Lake Michigan using the 100hp, turn off the 100hp, then fire up the 9.9 to troll at 800-1100 rpm for 2-3 hours. Fouling spark plugs is a common occurrence. Would a hotter spark plug help elleviate the problem of fouling plugs? It doesn't burn oil, and all other systems seem to work fine. Sometimes, after 2-3 hours, it starts running on one cylinder. Using NGK standard 14mm plugs. Would like to try a hotter plug to help burn up more of the gas-air mixture at combustion. Suggestions?

  2. #2
    Join Date
    Sep 2010
    Posts
    19,802

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    Are you sure you understand the concept of a hotter spark ?----Explain what you think happens with a hotter plug.------Is there a working thermostat in your motor ?

  3. #3
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    16,836

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    A hotter plug will help, just don't run it wide open with them in there.

    Jeff

  4. #4

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    Fastjeff: Thank you. It never runs wide open. At most, on rare occasion, I'll run it up to 2000 rpm to move to another fishing location. Since I have 3 down riggers down, I cannot go too fast, or damage my trolling equipment. I don't want to pull all line and down riggers, move 1 mile, then reset. It is easier to increase speed from 2.2mph to 3.5-4.0 mph, the slow down once I reach my new spot. That rarely occurs.

    Racerone: Yes, hotter plugs don't dissipate heat to the cylinder head as fast as colder plugs, ergo they stay hotter to help burn up more of the gas-air mixture.

  5. #5
    Join Date
    May 2013
    Location
    TX.
    Posts
    416

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    I have a 115 hp 2+2 Mercury 2 stroke. It is basically 2 alternate firing twins on one crankshaft 90 degrees out of phase. It's basically a 57 HP trolling engine and a 115 get up and go. At lower rpms the top two alternate firing cylinders generate thrust and the bottom two plugs fire, like the top two, and get fuel, but not enough to generate combustion. As a result the plugs sit there cold, getting wet with fuel mix. What this does is tends to foul the bottom two and if not too fouled, they will kick in when the throttle it advanced, but it takes awhile to clean up and be able to run at full rpm.

    In looking at the problem and the plugs, I saw that they weren't burning off contaminants when they were running. Since I do mostly midrange or slower running, like you, I am not worried about getting too hot a plug. The other thing is that I wanted to increase the surface distance that spark leakage had to travel. Finally, having had a career in high voltage and capacitor discharge systems, I know that the more concentrated the area of the spark, the more intense the plasma.....sharp points are easier to light off.

    I went to the NGK online catalog and pulled up my service manual listed NGK plug for my engine. It was heat range 8. I dropped it to heat range 6, lower since NGK runs hotter with lower numbers unlike most popular brands that do just the opposite. Then I went to the Iridium section with my other dimensions and found an iridium plug that matched my OEM plug dimensions.

    They have been in now for two outings and upon removing and having a look, the insulators are nice and tan as they should be and all 4 plugs came out DRY, not top two grey and dryish and bottom two grey and wet. The other thing I did that was a immediately detectable improvement, was to tilt the engine up in the rear, like 10-15* ANY time I am off plane, which keeps fuel from puddling around the spark area.....I have posted several times on this and referred to early Mercury smaller engines deliberately built to run with this tilt for this reason.

    So that is what I have done to correct my engine to suit me and I have no plans to change anything. If I still had a boat full of kids, and if Sunday afternoons were spent water skiing and lots of high speed running and all that then I wouldn't have done what I did. But those days are past, my boating has changed, as have my spark plugs and driving habits.
    Last edited by Texasmark; 06-29-2020 at 08:54 AM.

  6. #6

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    As it turns out, my spark plug (NGK DCPR6E) does not come in a hotter version. And the person at www.sparkplugs.com could not find if there was another brand with a one-step hotter plug. I live in a remote location, so going to auto parts or marine supply stores to check if they have a hotter version (assuming www.sparkplugs.com was incorrect) isn't as convenient as ordering online. Since a hotter version of my plug is not available, I may consider replacing the standard copper plug with iridium, DCPR6EIX. Apparently, iridium plugs are less prone to fouling, and the cost is about $8/plug. Any thoughts?

  7. #7
    Join Date
    May 2013
    Location
    TX.
    Posts
    416

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    I went hotter for the fouling reason due to the engine design. The plugs weren't burning off contaminants. I'm having morning coffee and then going fishing. Later on today, I'll look into the NGK catalog and see if I can find you something that should work for you (I would use in your engine if mine) and possibly a supplier.

  8. #8

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    Texasmark,
    Thank you for trying to help. Based on my research of calling marine stores, spark plug sellers (sparkplugs.com) I don't think a hotter version of any brand is available to go one step hotter than NGK's DCPR6E. What I've concluded is that the Iridium version of the DCPR6E is less prone to fouling. It is available for $8-10/plug, NGK DCPR6Eix. If you come up with another brand of standard plug that is one range hotter than the DCPR6E, I would be interested in hearing what it is. Trolled 3 hours this morning at 800-1200 rpm on plugs I had sand blasted and things worked well. If the problem reoccurs, I may invest in IRIDIUM plugs.

  9. #9
    Join Date
    May 2013
    Location
    TX.
    Posts
    416

    Default Re: Hotter spark plugs for 9.9 4-stroke?

    I have no experience with hotter plugs and just do for my things.....limited background even though I have been at all types of engines for over 60 years. Folks in the know on forums discuss plugs from time to time and I have picked up on comments about plugs being too hot. Obviously there can be problems there if misapplied.

    I was out again today, and had a stellar outing. Fast starts, smooth trolling, smooth hole shots, great acceleration. Besides the plugs and other things I did, adding a check valve in the fuel line between the tank and the squeeze bulb has made for super fast initial and stopping to fish starts and not a lot of bulb pumping between my once every month outing first starts of the day.

    Ebay is where I got my iridiums and they weren't all that expensive. NGK tells you how to tell theirs vs counterfit plugs....had a comment from someone on a forum say that ebay sold counterfits. Well they may and they may not. I got the real Macoy.

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