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Technical Backfiring through exhaust

Discussion in 'The Hokey Ass Message Board' started by 63401nailhead, Sep 16, 2022.

  1. 63401nailhead
    Joined: Apr 4, 2008
    Posts: 164

    63401nailhead
    Member
    from DE

    Did a little tune up this week and replaced a few things, now she’s idling weird and backfiring out the exhaust.
    1940 chevy truck has a sbc 350 pulled from a 70 Chevelle.
    Replaced plugs (delco r45ts, gapped at 40), plug wires (delco 908w), distro cap (delco d308r), rotor (delco c435), pcv valve (standard v178).
    Also replaced (but surely unrelated) valve covers and valve cover gaskets, master cylinder, pump to carb fuel line and filter, oil and oil filter.

    Any thoughts or ideas on why it’s idling weird and backfiring so much through the exhaust? Doesn’t sputter or hesitate, and actually acceleration is pretty smooth through second and third gear.

    wrong plugs? Wrong gap? Etc? It ran fine before I did all of the above. Thanks!
     
  2. Jethro
    Joined: Mar 5, 2001
    Posts: 1,759

    Jethro
    ALLIANCE MEMBER

    Did you maybe get the firing order wrong?
    Could be that
     
  3. 1-8-4-3-6-5-7-2. 1,3,5,7 on the left hand side and 2,4,6,8 on the right hand side.
     
  4. Wanderlust
    Joined: Oct 27, 2019
    Posts: 648

    Wanderlust

    5&7 very easily mixed up
     
    Tow Truck Tom and 427 sleeper like this.

  5. 427 sleeper
    Joined: Mar 8, 2017
    Posts: 2,033

    427 sleeper
    ALLIANCE MEMBER

    My money's on the wrong firing order or a cracked plug
     
    nunattax and Tow Truck Tom like this.
  6. Tow Truck Tom
    Joined: Jul 3, 2018
    Posts: 938

    Tow Truck Tom
    Member
    from Clayton DE

    This
     
  7. cheepsk8
    Joined: Sep 5, 2011
    Posts: 616

    cheepsk8
    Member
    from west ky

    "Out the "ass" it's in the gas, out the carb it's in the timing"
    Old saying I learned 40 plus years ago from an old school mechanic. I just had a carb bite the dust a couple of weeks ago on my c10, just out of the blue. Pulled one off my 55" put it on,and back to purring agaain. I chased down everything but my issue was surging at higher speeds.
     
  8. indyjps
    Joined: Feb 21, 2007
    Posts: 5,257

    indyjps
    Member

    Sounds timing related with backfiring.

    Check firing order,

    **You didn't mention setting points.

    Pull cap and look for any contacts that aren't "contacting" :D
    .040 spark plug gap may be OK,..... .035 points, .045 HEI is common.
    Plug wires are new - BUT - doesn't take much time to multimeter check for a dud.

    If you rule out ignition, could have picked up junk during fuel filter swap that went into carb.
     
  9. 63401nailhead
    Joined: Apr 4, 2008
    Posts: 164

    63401nailhead
    Member
    from DE

    Thanks guys, gonna double check the firing order, ohm out the plug wires, maybe I’ll set the gap at 35 on the plugs and I’ll report back.
     
  10. Tow Truck Tom
    Joined: Jul 3, 2018
    Posts: 938

    Tow Truck Tom
    Member
    from Clayton DE

    Fifty four years ago, my Chevy was acting out that way. A used condenser from the trash can, saved the day.
     
  11. 1971BB427
    Joined: Mar 6, 2010
    Posts: 7,803

    1971BB427
    Member
    from Oregon

    I always set plug gap at 35 for stock points, and no hot coil, or MSD box.
     
  12. atch
    Joined: Sep 3, 2002
    Posts: 5,229

    atch
    Member

    Backfiring out the exhaust; it's firing while an exhaust valve is open.

    Backfiring out the carburetor; it's firing while an intake valve is open.

    Now why it's doing that is for you to figure out (sounds like lots of good advice above). One thing that I don't see above is an exhaust valve sticking.
     
    NoelC likes this.
  13. greybeard360
    Joined: Feb 28, 2008
    Posts: 2,021

    greybeard360
    Member

    An exhaust leak can make one backfire on decel also.
     
    bchctybob likes this.
  14. NoelC
    Joined: Mar 21, 2018
    Posts: 609

    NoelC
    Member

    Hmm? I'd like to believe that's all you did. Because then I'd say check your coil. But if you did tinker, tweak and twist stuff (cause that's what I'd do), all bets are off. You don't mention the carb? But you do mention fuel line and filter.
    There sure has been a lot of back firing issues it seems... how about bad gas?
     
  15. 63401nailhead
    Joined: Apr 4, 2008
    Posts: 164

    63401nailhead
    Member
    from DE

    Okay so I just triple checked the firing order- definitely all good there. Gapped the plugs to 35 now instead of 40, all the plug wires have continuity and are all between 7k and 9-10k ohms.
    Tomorrow going to through the old cap and rotor on to rule those out. I’m wondering though if these are the best plugs for this motor? I know there are different temp ranges for plugs but I never knew what that meant in practical application. I might also get a new coil for the hell of it.
    The backfiring persists, it’s not loud in fact it’s barely noticeable but it’s kind of a steady every three seconds or so low popping sound.
     
  16. Wanderlust
    Joined: Oct 27, 2019
    Posts: 648

    Wanderlust

    That would be a miss in my experience
     
  17. 63401nailhead
    Joined: Apr 4, 2008
    Posts: 164

    63401nailhead
    Member
    from DE

    Just noticed something that I hadn’t even noticed before…
    The plugs that were in it previously were Champion RJ12YC and have a flat base with a washer. The plugs that I have in there now are tapered seat with no washer. I’m gonna pick up a set of flat seat/washer plugs and throw them back in and see what it does. Hoping I didn’t do any damage if I did in fact have the wrong plugs in there… the tapered plugs look to be longer than the old champions that were in it before.
     
  18. 427 sleeper
    Joined: Mar 8, 2017
    Posts: 2,033

    427 sleeper
    ALLIANCE MEMBER

    If that's the case, you might find a smashed electrode or two, that's been causing the miss.
     
    Wanderlust and Moriarity like this.
  19. nunattax
    Joined: Jan 10, 2011
    Posts: 2,964

    nunattax
    Member
    from IRELAND

    and you could be sucking air in at the plugs.i had an issue with breaking plugs with speedway headers clearance was tight when the plug screwed out with the plug spanner on.when clearance ran out the plug snapped.think clearance issue was with 1,2 ,7 and 8 would have to check that.no issue with corvette cast manifolds
     

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