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  1. #1
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    Jul 2005
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    Murfreesboro
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    07 250 pro xs surging

    07 250 pro xs 1B550078
    Kinda long. Up for suggestions. I have a friends 07 250 pro xs that while running it surges. Comes outta the hole fine but if you level off the rpms, itll start surging. Hammer down on it, itll pick up to 53-5500 then surge to 57-5800, then hold back again. Heres everything ive done and checked. No codes shows up on computer.
    New spark plugs, ngk-izfr6j gapped to .042
    All coils were replaced in 2010
    New fuel/water separator
    Comp on all 6 is 120
    Fuel pressure is 108
    Air pressure is 96
    Good spark
    Run 93 ethanol free
    Checked tuner gasket for leaks.
    The burn on the plugs has me puzzled. 2&4 looked normal, good tan color, while the rest looked clean. Plugs have maybe 4-5 hrs on them. Im stumped. Heres some pics of the plugs. Any suggestions?
    Email: bullet_20xrd@yahoo.com
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  2. Member
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    Jul 2009
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    Junction City, Kansas
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    2,589
    #2
    Not related. But absolutely no reason and waste of power and money to be running 93 octane. You are actually robbing the engine of power. You may be also causing alot of carbon issues as that is not burning off as well as 87 octane which is what it is designed to be run on.

  3. Member lpugh's Avatar
    Join Date
    May 2009
    Location
    Sacramento Ca
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    5,164
    #3
    Quote Originally Posted by Bulleteer View Post
    07 250 pro xs 1B550078
    Kinda long. Up for suggestions. I have a friends 07 250 pro xs that while running it surges. Comes outta the hole fine but if you level off the rpms, itll start surging. Hammer down on it, itll pick up to 53-5500 then surge to 57-5800, then hold back again. Heres everything ive done and checked. No codes shows up on computer.
    New spark plugs, ngk-izfr6j gapped to .042
    All coils were replaced in 2010
    New fuel/water separator
    Comp on all 6 is 120
    Fuel pressure is 108
    Air pressure is 96
    Good spark
    Run 93 ethanol free
    Checked tuner gasket for leaks.
    The burn on the plugs has me puzzled. 2&4 looked normal, good tan color, while the rest looked clean. Plugs have maybe 4-5 hrs on them. Im stumped. Heres some pics of the plugs. Any suggestions?
    First things I would do is pull the filter out of the VST and look in the bottom of it for debris, then I would pull the cover off the pulse pump to check the diaphragm and for contaminates. Next check the fuel lines back to the tank for kinks or leaks and with lines disconnected blow compressed thru them in reverse flow direction. Use a container to catch any thing that may blow of them.

    Run the 93 fuel out, Fill it with Top Tier 87, double dose of quick clean and add 8 0z of premium plus to the fuel for every 10 galls of gas and run at least 40 gallon thru it to evaluate results and report back. Also how much carbon inside the prop, I suspect quite a bit

    I do not understand why so many are insistent on running high octane fuel when it detrimental to the motor.
    The only thing different about 87 octane and Premium is the Premium Price, quality is the same it just burns slower and cooler as a result. Run the fuel the manufacture recommends, they know far more about what is right for their product than we do.

    I will offer a little food for thought here to think about:
    My best friend and I both have Optimax motors operated in in the same ways. The inside of his props have a heavy buildup of black carbon deposits, it is ugly. Mine has only very light film of deposits. The difference is he always use 91 and I use 87

    I used to crew chief for my son when he was involved in circle track racing, his class was restricted to 360 cubic inch, 9-1 compression and 500 cfm carburetor. Track rules mandated using only their 110 octane race fuel, I preferred 91 octane pump gas. The reason being it made about another 40 HP and the exhaust system did not run Red Hot. The race fuel burned way to slow for a restricted motor like this. I combated this by running 43 degrees of total timing all in at 6k RPM which helped, with pump gas I could not run more than 36 degrees but it did run stronger.
    The slower burn was still burning all the way out of the exhaust wasting energy. I never could get them to go to a lower octane fuel. Most of them were stuck in this pattern of the higher the octane the better.
    The fuel used in a motor is determined by many factors that all revolve about how fast it burns. Octane is very important to proper operation, many times lower is less harmful than higher

    I see this stressed over and over by many here the know what's best and yet they are ignored.
    Thank You Leon Pugh

  4. Member Macsimus's Avatar
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    Jan 2005
    Location
    Savage, MN
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    6,571
    #4
    Leon, the reason, at least around here, that some run 91 is that it is the only gas available without ethanol. The availability of non-oxy 87 seems to be on a state-by-state basis and it is frustratingly not available in MN. I noticed during a recent trip that IA has non-oxy 87 octane but MO does not. How much real difference is there in the burn rate between 87 and 91? I've run my 250 ProXS for four years on 87 with ethanol. As an experiment, I ran non-oxy 91 for two months during a winter escape trip to southern MO. At the end of the two months, I pulled a plug from each bank and they looked as clean, if not cleaner, as they do when I used the 87. Note that I was on a large reservoir and did a minimum of idling. Maybe the way the motor is run is more important than the octane difference between 87 and 91? I also will mention that my service manual says to run a minimum of 87 octane.
    There was a time that I didn't fish but I cannot remember it.


  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    Apr 2008
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    Greenville, SC (US)
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    #5
    No need to debate fuel octane- it's stated in the owner's manual, on Mercury's site, and quite clearly at: Oil, Battery, Spark Plugs, Fuel and Fuel Additive Recommendations

    Bulleteer: Given your description, I would recommend starting with EVERYTHING listed in the below link.

    Do not skip any items. Use ONLY the materials recommended.

    FUEL LINES AND PERFORMANCE PROBLEMS


    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

  6. Member
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    Nov 2016
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    Danvillle, CA
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    836
    #6
    Quote Originally Posted by Macsimus View Post
    Leon, the reason, at least around here, that some run 91 is that it is the only gas available without ethanol. The availability of non-oxy 87 seems to be on a state-by-state basis and it is frustratingly not available in MN. I noticed during a recent trip that IA has non-oxy 87 octane but MO does not. How much real difference is there in the burn rate between 87 and 91? I've run my 250 ProXS for four years on 87 with ethanol. As an experiment, I ran non-oxy 91 for two months during a winter escape trip to southern MO. At the end of the two months, I pulled a plug from each bank and they looked as clean, if not cleaner, as they do when I used the 87. Note that I was on a large reservoir and did a minimum of idling. Maybe the way the motor is run is more important than the octane difference between 87 and 91? I also will mention that my service manual says to run a minimum of 87 octane.
    If you have super high compression engine like my M5 BMW, the fuel/air mixture gets compressed so high that it heats up to temperatures where if you use 87 octane the gas will ignite before top dead center (prior to the spark from the spark plug) causing knocking which is really bad for an engine. The mercs must apparently run cool and lower compression ratios so the fuel/air mixture is not hot enough to ignite on its own at 87 octane or higher. Which is good.

    My tow truck is the same way. Low compression ratio - 87 octane is fine. No ping at all. But I could most certainly use 91 octane ( if I want to waste the money ) and it would not cause any noticeable carbon build. The fuel still ignites powerful with the spark. The only issue would be that my wallet will be less full.

    Personally the only reason I would use 91 would be right before a long time of storage. As I believe octane is lost with time ( slight loss in 6 months ). But then again you could use stabil in the fuel and be just fine for 12 months. So high octane is a waste of money.

    Here is a nice article... https://www.amsoil.com/newsstand/art...t-performance/

    and another...

    https://www.bp.com/content/dam/bp-co...icle-tanks.pdf

    https://www.livescience.com/58117-do...ne-go-bad.html

    https://auto.howstuffworks.com/engine-knock1.htm

    and here is a good one...

    http://www.boatpokers.com/data/piston.pdf

  7. Member
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    May 2011
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    Spangle, WA
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    6,180
    #7
    From the FAQ topic Don provided the link to:

    Fuel Selection:

    Make your fuel selections based on the following criteria, IN THE FOLLOWING ORDER:

    -High Traffic, Top-Tier (Name Brand) Fuel
    -Appropriate Octane Rating (see above info)
    -Lowest possible ethanol content (NEVER to exceed 10%).
    -While it may be preferable to utilize a fuel that contains lower (or no) ethanol, use of fuels that are SUBSTANTIALLY higher octane can result in decreased performance, decreased fuel economy, and potential for carbon buildup. Higher octane fuels burn SLOWER and COOLER.
    _______

    Phil
    '09 Hewescraft ProV
    '09 150 Optimax