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  1. #1
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    I think I have a blown head gasket on my M8

    I'm right at about 4300 miles on my 2018 SGS and it appears that I might have a blown head gasket. What drew my attention to a possible problem was the way the bike acted when I went to start it up after it sat for about a month and a half. It idled high (3500 rpm) for about 6 seconds and then dropped down to a normal idle. I took it out for a short ride and it sounded like it has an exhaust leak. The bike sat for a couple of weeks and when I started it up, it idled high again for about 3 seconds and died. I began looking around the front and rear heads with an LED light and noticed that there is carbon on the edge of the head gasket in the front center. Damn! I'm aware that some M8s have encountered blown head gaskets, but I was hoping I wouldn't become one of them. The pic is sideways for some crazy reason, but you can see the carbon that I am speaking of. The rest of the gasket edges (front and rear cylinders) are rust colored.
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    Last edited by NGOT8R; 11-29-2019 at 02:43 PM.


    2015 Vivid Black SGS w/107 motor: V&H Pro Pipe, Thundermax tuner, SE Ventilator Elite AC with K&N filter, Andrews 30T pulley mod, AIM variable pressure clutch, Evolution Industries billet clutch basket, Twin Power compensator eliminator sprocket, HD highway pegs, HD passenger floorboards, RB Racing modified stock swingarm (2" over adjustable), HD 7" Wind Splitter windshield, HD black windshield trim, JM Corp 250W 2 channel amp and Focal 165 KRC speakers in fairing

  2. #2
    Administrator hulkss's Avatar
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    Maybe an intake manifold leak? That can cause erratic idle and poor running. The stock plastic manifolds have had issues.
    2019 FLHCS

  3. #3
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    Thanks Hulk. I'll take a look at the intake as well. What are your thoughts on the black soot that appears to be carbon on the edge of the front head gasket? This problem is so weird, in that it just popped up all of a sudden. I'm going to investigate things more thoroughly once I finish up with my Sportster build.


    2015 Vivid Black SGS w/107 motor: V&H Pro Pipe, Thundermax tuner, SE Ventilator Elite AC with K&N filter, Andrews 30T pulley mod, AIM variable pressure clutch, Evolution Industries billet clutch basket, Twin Power compensator eliminator sprocket, HD highway pegs, HD passenger floorboards, RB Racing modified stock swingarm (2" over adjustable), HD 7" Wind Splitter windshield, HD black windshield trim, JM Corp 250W 2 channel amp and Focal 165 KRC speakers in fairing

  4. #4
    Administrator hulkss's Avatar
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    I guess you could do a cranking compression check or a cylinder leakdown test.
    2019 FLHCS

  5. #5
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    Quote Originally Posted by hulkss View Post
    I guess you could do a cranking compression check or a cylinder leakdown test.
    I do plan to perform a leakdown test, just as soon as I can pick up a tester.


    2015 Vivid Black SGS w/107 motor: V&H Pro Pipe, Thundermax tuner, SE Ventilator Elite AC with K&N filter, Andrews 30T pulley mod, AIM variable pressure clutch, Evolution Industries billet clutch basket, Twin Power compensator eliminator sprocket, HD highway pegs, HD passenger floorboards, RB Racing modified stock swingarm (2" over adjustable), HD 7" Wind Splitter windshield, HD black windshield trim, JM Corp 250W 2 channel amp and Focal 165 KRC speakers in fairing

  6. #6
    Administrator Mojo's Avatar
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    I have a friend that just had rear head gasket blown/ 2017 FLHXS - 24,000 miles. Out of warranty, Dealer fixed it no charge.
    2018 FLHXANV
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  7. #7
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    Quote Originally Posted by Mojo View Post
    I have a friend that just had rear head gasket blown/ 2017 FLHXS - 24,000 miles. Out of warranty, Dealer fixed it no charge.
    I'm probably on my own if the head gasket is blown Mojo. I installed an S&S cam and tuner a year ago.


    2015 Vivid Black SGS w/107 motor: V&H Pro Pipe, Thundermax tuner, SE Ventilator Elite AC with K&N filter, Andrews 30T pulley mod, AIM variable pressure clutch, Evolution Industries billet clutch basket, Twin Power compensator eliminator sprocket, HD highway pegs, HD passenger floorboards, RB Racing modified stock swingarm (2" over adjustable), HD 7" Wind Splitter windshield, HD black windshield trim, JM Corp 250W 2 channel amp and Focal 165 KRC speakers in fairing

  8. #8
    Prospect
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    I had an Evo motor on a chopper that used to blow head gaskets every year or so. You should be able to hear it and feel it - you'll hear the "pfft pfft pfft" of compression leaking and feel the blow by at the leak.
    2015 Street Glide Special - Vivid Black

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  9. #9
    Founding Member TomA's Avatar
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    Quote Originally Posted by NGOT8R View Post
    I'm probably on my own if the head gasket is blown Mojo. I installed an S&S cam and tuner a year ago.
    Sound's like your up for another build of course if your taking the head off why not have them worked on just a little bit. You have t replace the second one just for peace of mind right. That Zipper's kit Michael had installed looked like fun.


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