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  1. #1
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    2.4 Mercury Formula Blown Powerhead

    I recently blew up my 1985 Mercury 2.4 EFI Formula with oval port exhaust and down draft analog EFI system and was looking for some help with the rebuild. The motor is on a 19 ft bullet bass boat and I'm the third owner since last May. The guy I bought the boat and motor from had owned it since sometime in 1985. Last summer, on about my third time out, the motor started cutting out while on plane at high RPMs. This felt like a sudden misfire or like bumping the kill switch on a tiller engine for those that can relate. The motor would only cut out for a split second and would recover and never died and only did this at higher RPMs. This motor had the factory external fuel pump after the water/fuel separator and the fuel pump was extremely loud and had a lot of static noise while cycling which I assumed to be as cavitation from a worn pump. For preventative measures, I replaced the water/fuel separator just to make sure that it was not the problem. The fuel pump still had the frequent static noise and the engine would still cut out at high fuel demands and higher RPMs. Over the winter, I purchased a new Bosh fuel pump online, part number 0580 254 911, that was sold as a cross reference to the Mercury part number 8M8026000. I installed the new pump and it has the same loud noise but only has a brief second or two of cavitation noise until the fuel system is pressurized and circulating. On my first to water test the motor I had instant startup and normal warm up prior to plane off. The boat came out of the hole with ease and ran smooth at midrange. I ran it for a few miles and then turned to come back and got in it pretty good on my return to see if the motor would cut out like before. I assume I was turning about 5k RPM when I noticed a slight loss of power and then a clicking noise which led to a strong knocking noise. I immediately killed the motor at the key switch and trolled my way back to the boat launch. I was running what was sold at the pump as 91 octane non-ethanol gas at a 50:1 ratio with Amsoil HP Marine synthetic oil. A few days later I did a cold compression test with the following results: Piston 1, 120 lbs. Piston 3, 120 lbs. Piston 5, 90 lbs. Piston 2, 120 lbs. Piston 4, 60 lbs. Piston 6, zero lbs. I have since removed the heads and have attached pics of the damage which looks like piston 4 and 6 failed on the exhaust port side. Now that I have a blown motor what are my options for the rebuild and what else should I check as the potential contributing factor? Can I just replace the pistons that were lost or does the whole powerhead need to be rebuilt? The cylinder walls seem to be pretty clean but there are some fine scratches on number 4 and 6 from what appears to be broken ring particles. Does the block look like it can be cleaned up or honed or will these cylinders require a sleeve? Can the damaged head be cleaned up? Are there good shops in the southeast that can do the block work? Does anyone have any information on a repair manual for this motor? Any help or suggestions are greatly appreciated!
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  2. #2
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    FYI that motor is not a Yamaha

    i think you will prolly need some sleeves and heads but I'm not the expert on how much you can "clean up" a chrome bore motor

  3. #3
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    Thank you sir for that discovery. The "Yamaha tech" prefix was an error made from a dropdown while drafting the post. I tried to edit the post and remove the prefix with no luck and I'm not an outboard tech of any sort for that matter.

  4. #4
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    Quote Originally Posted by 2.4Formula View Post
    Thank you sir for that discovery. The "Yamaha tech" prefix was an error made from a dropdown while drafting the post. I tried to edit the post and remove the prefix with no luck and I'm not an outboard tech of any sort for that matter.
    I figured it was an error when trying to get Merc tech on the drop down and was teasing you.

  5. #5
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    Definately will need sleeves,that will not clean up...give a call to talk about it if you like,305-394-3354,Chris

  6. #6
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    Go ahead and call Chris. Great rep.
    I carry a gun because a cop won't fit in my pocket.

    1987 Glasstream 15V
    1986 Mariner "Tower of Power" 115

  7. #7
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    hey what setup are you running on the bullet? i have a 1989 19 v and about to buy a 2.4 to put on it

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