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Backfiring at idle

Discussion in 'The Hokey Ass Message Board' started by Dave Downs, Aug 20, 2007.

  1. Dave Downs
    Joined: Oct 25, 2005
    Posts: 947

    Dave Downs
    Member
    from S.E. Penna

    I've got 2 very different engines with the same problem. Both are straight 6's. They start fine, but when they're warmed-up they both backfire randomly out the exhaust while at idle.

    #1 is a 225 cu 6 in a 1955 International dumptruck, stock

    #2 is a well-worked 235 Chev; cam, pistons, 3 carbs

    Both seem OK under light load (can't run either one under wot due to roadability and legal status)

    Any ideas?
     
  2. SinisterCustom
    Joined: Feb 18, 2004
    Posts: 8,277

    SinisterCustom
    Member

    Valve adjustment???
     
  3. stealthcruiser
    Joined: Dec 24, 2002
    Posts: 3,750

    stealthcruiser
    Member

    What does a compression check show?
     
  4. spudz
    Joined: Mar 3, 2001
    Posts: 535

    spudz
    Member

    fouled plugs
     
  5. 55chieftain
    Joined: May 29, 2007
    Posts: 2,190

    55chieftain
    Member

  6. GassersGarage
    Joined: Jul 1, 2007
    Posts: 4,726

    GassersGarage
    Member

    How old are the engines? You may have carbon build-up and they are pre-igniting. With engine running at a high idle, pour a small, steady stream of water down carburator. It will blow off any carbon.
     
  7. FIRat
    Joined: Jul 15, 2007
    Posts: 52

    FIRat
    Member
    from MidWest

    Short of a leak down test. It sounds like when the engine is cold the cylinder is sealed off. But when warm and things expand, it sounds like an exhaust(s) valve is tight and stays open. That is a hot valve that cannot transfer heat. Usually that is how a valve can burn. Not enough heat transfer time. It is just a guess, but I will guess with Josh and suggest to at least check the clearances.
     
  8. Badfast
    Joined: Apr 5, 2005
    Posts: 41

    Badfast
    Member

    Had a similar problem, start with changing all the plugs with new ones properly gapped.
     
  9. 32v
    Joined: May 20, 2007
    Posts: 952

    32v
    Member
    from v.i.

    condenser
     
  10. roundvalley
    Joined: Apr 10, 2005
    Posts: 1,776

    roundvalley
    Member

    Try a diagnoses with a vacuum gauge and symptoms chart. Old method but it will tell you alot.
     
  11. FIRING ORDER!
    check the wires too
    recheck and dont have them old plug wires to close to each other even if it is purdier that way
     
  12. Rootie Kazoootie
    Joined: Nov 27, 2006
    Posts: 8,130

    Rootie Kazoootie
    Member
    from Colorado

    After sitting most of the winter, my 327 started doing the same thing-- adjusted valves, cleaned plugs, set timing etc.etc., no change---new gas and all was good again.
     
  13. Dave Downs
    Joined: Oct 25, 2005
    Posts: 947

    Dave Downs
    Member
    from S.E. Penna

    Thanks for all the replies

    1)New gas in the Chev, old gas in the truck

    2) Firing order OK in both

    3) Shouldn't be carbon in the Chev - it's only got 3 hours running time

    4) I'll do a compression check first

    5) I was thinking maybe wire or dist cap, wires on both are old
    (yeah, I should know better on a new engine)
     
  14. Little Wing
    Joined: Nov 25, 2005
    Posts: 7,565

    Little Wing
    Member
    from Northeast

  15. 6inarow
    Joined: Jan 24, 2007
    Posts: 2,468

    6inarow
    Member

  16. Sounds very like the condensor - either a duff unit or a bad earth. Miht even be missing altogether.
     
  17. 302GMC
    Joined: Dec 15, 2005
    Posts: 8,451

    302GMC
    Member
    from Idaho

    Adjust the valves - they both need it anyway ... and you can eliminate "tight valves" from the list.
     

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