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Kombi Backfiring Problem

Discussion in 'Bay Tech Clinic' started by Chris Williams, Nov 5, 2018.

  1. Chris Williams

    Chris Williams New Member

    Messages:
    15
    Location:
    Melbourne
    Hello, I have just recently rebuilt my 1976 2.0 Litre, dual carby engine, it also incorporates a new Pertronix ignition system. The carbie I bought are Roger Penrose Racing similar to the Weber IDF-XE.
    Engine works fine, except a back firing problem, which I can isolate down to cylinder #1 only.
    The reason I think it is only cylinder #1, is that when I remove the spark plug lead from #1, back firing problem goes away and I run on 3 cylinders no problems.
    Other symptoms are as follows:
    - When I put my timing gun on #1 spark plug lead, the light does not consistently go off, indicating that the spark is not being sent every time per stroke. When I move the timing gun to all other spark plug leads, I get a very consistent light which is what I would expect to see.
    - When I disconnect the spark plug lead from #1 spark plug and hold it close to ground, I now get a consistent spark with no noticeable interruptions and also the timing gun light is also firing, missing no sparks.
    - I have changed spark plugs from other cylinders, no difference.
    - I have changed spark plug leads, no difference.
    - I have re-adjusted the tappets, no difference.
    - I have checked compression and all cylinders are around 125 psi.
    - I have not made any adjustments to the carbies, they have been installed "out of the box".

    Can anyone please help me with some more troubleshooting ideas, or solutions?

    Thanks in advance for your help.
     
  2. Wayne murray

    Wayne murray Well-Known Member

    Messages:
    2,073
    Location:
    Seven hills
    Have you check that you haven't got any leaks under the manifold or exhaust system. Does the coil meant for the pertronix ignition system?
     
  3. Chris Williams

    Chris Williams New Member

    Messages:
    15
    Location:
    Melbourne
    How do I check for leaks under the manifold etc? The coil is a Flamethrower 3.0 Ohm, which I believe is the correct coil. Seems to be working alright on the other 3 cylinders.
     
  4. Wayne murray

    Wayne murray Well-Known Member

    Messages:
    2,073
    Location:
    Seven hills
    Spray water on the manifold
    Should change the revs if there is a leak.
     
  5. Barry

    Barry Well-Known Member

    Messages:
    2,750
    Location:
    Abbotsford NSW
    Is it a backfire or sneeze?

    A sneeze is a out the top of the carb and backfire is out the exhaust

    sneeze - lean
    back fire - rich

    Maybe dumb to ask - but have you set the mixture screws?
    Did they respond accordingly?
    Same same on each cylinder?
     
  6. Barry

    Barry Well-Known Member

    Messages:
    2,750
    Location:
    Abbotsford NSW
    Ooops - I missed that

    You need to "dial in" the carbs

    NOTHING works "out of the box"
     
  7. Barry

    Barry Well-Known Member

    Messages:
    2,750
    Location:
    Abbotsford NSW
  8. Mordred

    Mordred Super Moderator Staff Member

    Messages:
    2,705
    Location:
    Penna, Hobart
    Barry likes this.
  9. Barry

    Barry Well-Known Member

    Messages:
    2,750
    Location:
    Abbotsford NSW
  10. Mordred

    Mordred Super Moderator Staff Member

    Messages:
    2,705
    Location:
    Penna, Hobart
    Thanks Barry, I flicked too quickly over the original post this morning and missed the fact he had changed them out.

    I dont know if they are more complex but often misunderstood and worn, or with mismatched parts which make them difficult to get right, but matched to the correct standard engine in good condition they are pretty good.

    I hope he gets the problems sorted with out too much effort.

    Cheers

    Adrian
     
    oldman likes this.
  11. Chris Williams

    Chris Williams New Member

    Messages:
    15
    Location:
    Melbourne
    Thanks very much everyone for your help and links.
    Will report back next weekend when I can get under the engine again.
     
  12. Chris Williams

    Chris Williams New Member

    Messages:
    15
    Location:
    Melbourne
    Just reporting back on the outcome of my problem for others future reference.
    Given the advice above, I could hear a hissing sound from under the engine. When I removed my RHS heater box and associated exhaust, I found that on number #1 cylinder, there was black soot all around the mating surfaces between the exhaust and the head exhaust valve outlet. Therefore, as predicted above, the source of the backfiring was found.
    The reason why however, was because the new exhaust did not mate closely enough, so that even though I had torqued the exhaust down, there was still a small gap between the exhaust valve outlet and the exhaust (heater box). I ground down the heater box / exhaust surface (highlighted in purple) in the attached picture, which gave me the ability to pull the two closer together, hence providing a great seal between the two. Now, no leaky exhaust and no back fire.
    This could be the case where the brand new parts are slighly of different dimensions. Not sure.

    But, I still can't explain why the ignition fires every time now, without fail ????

    Thanks everyone for your help.

    Chris
     

    Attached Files:

  13. Alpal

    Alpal Moderator Staff Member

    Messages:
    8,338
    Location:
    Melbourne Bend of Islands
    Happy spark?
     

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