constant backfire in intake runner

Discussion in 'Engine, Fuel and Exhaust' started by mttdad, Jan 1, 2009.

  1. mttdad

    mttdad New Member

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    I have a 2000 ultra classic with efi, I bought the bike this past Sept. This problem existed when I bought it but the price was so good!
    #1 It starts hard, it cranks way to many times for being injected.
    #2 When it does start there is a constant small backfire going on in the intake manifold. I have run it with the air cleaner off and can see small explosions in the manifold.
    #3 It runs like this going down the road also, the only time it seems to run close to correct is when it is under a heavy load, ie: pulling up a hill or rolling on the throttle.
    #4 There are no codes the engine light is not on.
    #5 I have pulled the intake apart to change the intake gasket thinking maybe it had a leak. I noticed that the rear cylinder intake track is carboned up some, the front one is clean.
    #6 I don't know the history of the bike, it has 35k miles on it. I beleive I am the third owner. Someone has changed the exaust to a two in one. I dont know if any mapping changes were made.

    I just found this sight the other day, Super good! I appreciate the tight control on language and content.
    Please help, I know someone can, Thank you,
    mttdad.
     
  2. glider

    glider Veteran Member

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    Thanks for your comment on the site.

    As far as your problem, I would check the compression in the cylinders first. What you are explaining here sounds like a bad valve in the rear cylinder possibly. You should see about the same compression in both cylinders +- 10% otherwise you found the problem and the head has to come off.

    Have you tried to clean out the idle bleeds in the throttle body on your engine? I believe you have the MM system on that bike.

    You'll find two small holes in the intake of the throttle body at the 10 and 2 o'clock positions. Use some spray carb cleaner and clean them out

    [​IMG]
     
  3. mttdad

    mttdad New Member

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    Before I started to pull things apart I did a compression test, both cylinders were the same, around 145lbs, if I remember right.
     
  4. glider

    glider Veteran Member

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    That's a reasonable compression for that motor.

    Something else to consider after reading your post again is the fuel pressure and fuel filter also both of which would cause hard starting and some other symptoms as well if it was low.

    Food for thought.

    Also have a look at the injector wires near the injectors too. They had a habit of the copper breaking inside the insulation and causing problems from being strapped to tightly when they were made. Do a wiggle test on the wire with the bike running and see if the engine falters.
     
  5. mttdad

    mttdad New Member

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    I will take a look at the fuel filter, the bike did not get much use this past year. The first time I filled the tank I was amazed to see that the gas cap was missing!! So I ran sea foam through it, knowing what the ethanol does. Two tanks of gas and no changes.