Register now to get rid of these ads!

Hot Rods Flathead knocking

Discussion in 'The Hokey Ass Message Board' started by Boden, Mar 9, 2020.

  1. Boden
    Joined: Oct 10, 2018
    Posts: 707

    Boden

    So I was timing my 8ba today. And when I was finished. I revved the motor up. No more than 3500 Rpm and when jt was de accelerating it started knocking. I shut if off instantly. So I tried retarding the timing and it still knocked. It was running rich so that made me think it couldn’t be pre ignition. I then pulled one plug wire at a time and the knock continued. So I pulled the pan and all of the rods are tight and only have play side to side like they should. I’m totally stumped. I’ve heard piston slap but I’m hoping it’s not that. Wanted to get some other ideas that it could be from you guys.


    Sent from my iPhone using H.A.M.B.
     
  2. Boden
    Joined: Oct 10, 2018
    Posts: 707

    Boden

    Pre ignition?


    Sent from my iPhone using H.A.M.B.
     
  3. 51504bat
    Joined: May 22, 2010
    Posts: 1,816

    51504bat
    ALLIANCE MEMBER

    Need more info. Brand new rebuild/first time start up? Running without the knock before you worked on the timing?
     
  4. Boden
    Joined: Oct 10, 2018
    Posts: 707

    Boden

    So it’s a merc motor. High compression heads,bored .040 over, cam, adjustables, it ran fine before. I even drove it up the block. It had 60psi of oil pressure idling which leads me to believe that it’s not a rod bearing. And the rods have no play on them which means it’s not a wrist pin. So the only other thing I can think is pre ifniton. It’s regular fuel with a tiny bit of lead additive.


    Sent from my iPhone using H.A.M.B.
     
    Register now to get rid of these ads!

  5. dan31
    Joined: Jul 3, 2011
    Posts: 1,045

    dan31
    Member

    If it was sitting awhile could it be a valve stuck in a guide? Guide retainer come off?.
     
  6. Boden
    Joined: Oct 10, 2018
    Posts: 707

    Boden

    It is freshly rebuilt. No more than 10 miles on it. I was also thinking something on top of piston. But I don’t want to pull a head if I don’t have to


    Sent from my iPhone using H.A.M.B.
     
  7. Back up to the last thing you did.
    You set the timing,,,,,look in that direction first .

    Tommy
     
  8. oldiron 440
    Joined: Dec 12, 2018
    Posts: 1,507

    oldiron 440
    Member

    Get a bore scope and have a look.
     
    41 GMC K-18 likes this.
  9. Pete Eastwood
    Joined: Jul 27, 2011
    Posts: 912

    Pete Eastwood
    Member
    from california

    check the mains .
     
    kidcampbell71 likes this.
  10. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 2,874

    Flathead Dave
    ALLIANCE MEMBER
    from So. Cal.

    Video with good sound. There is a difference of sound for pre ignition and other problems.
     
  11. Budget36
    Joined: Nov 29, 2014
    Posts: 3,450

    Budget36
    Member

    You did check clearances for the head on all cylinders?
     
  12. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 2,874

    Flathead Dave
    ALLIANCE MEMBER
    from So. Cal.

    There is a song about knocking... Just saying...
     
    Boden likes this.
  13. alchemy
    Joined: Sep 27, 2002
    Posts: 15,477

    alchemy
    ALLIANCE MEMBER

    Make sure your plugs aren’t smashed flat.
     
    Boden and 34 GAZ like this.
  14. jaracer
    Joined: Oct 4, 2008
    Posts: 323

    jaracer
    Member

    In my experience, knocks that only occur on deceleration have been rod bearings. The only real way to check is to remove the rod caps. However, it could be something like a loose flywheel or pulley. It would have to be something that is free to move when unloaded.
     
    Bandit Billy likes this.
  15. Boden
    Joined: Oct 10, 2018
    Posts: 707

    Boden

    I checked on a few and I have .060 on them. And I’ve run the motor for a long amount of time and gotten it hot. So why would it happen when it’s not fully warmed up?


    Sent from my iPhone using H.A.M.B.
     
  16. chicken
    Joined: Aug 15, 2004
    Posts: 161

    chicken
    Member
    from Kansas

    It wouldn't unless something changed to let the piston rise higher,like excess rod clearance. So...maybe pull the plugs and turn the engine over by hand feeling for resistance on any cylinder in case there's something stuck in a piston top. If you find a suspicious spot where resistance is higher,see which pair of cyls is at tdc and try to inspect the pistons...but the heads may need to come off if a scope isn't available.
    I'd try to do anything I can to troubleshoot this before taking it apart. Like turning the crank back and forth every few degrees of rotation to see if you hear anything that sounds/feels like excess clearance. It's a knock just on one cyl, correct?

    Terry
     
  17. 42merc
    Joined: Dec 19, 2010
    Posts: 701

    42merc
    Member

    Sounds like you checked piston to head clearance.
    Did you check valve to head clearance ?
    Don't recall what cam you are using, but, a 400jr will contact most heads if not massaged for clearance.
     
    Boden likes this.
  18. Boden
    Joined: Oct 10, 2018
    Posts: 707

    Boden

    It turns out it was pre detonation. I went to retard timing and actually advanced it. So I finally got the timing right and it ran good. It’s very retarted. And it somewhat diesels when I turn the key off. So I bet if I put some hightest in it, it will run better and will let me advance the timing more. It also has 170psi max on a few cylinder


    Sent from my iPhone using H.A.M.B.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.