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Thread: K75rt backfire

  1. #1
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    K75rt backfire

    my k75 sometime backfires on deceleration. change of plugs seemed to help some but after 7000 miles it is getting worse. There was talk about this here sometime back but I can not yremember what was said Seems to run fine otherwise

  2. #2
    Mars needs women! 35634's Avatar
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    they all do that! A little backfiring is normal, or you TPS could need adjusting, check out this ... http://www.ibmwr.org/ktech/backfire.shtml
    1987 K75S
    Original litter
    Original owner
    2012 Ural Gear Up

  3. #3
    It's a way of life! oldnslow's Avatar
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    Before you get drawn and quartered by the mob, I'll let you know its really and 'afterfire' and not a 'backfire', that is unless the noise is from an explosion of fuel being blown back through your air cleaner!

    My K100 did it too, and the first time I thought I blew the muffler off! It would happen if I was going down hill, using engine braking to slow down, but I would be lazy and relax my right hand and hang it loosely on the throttle, , causing the throttle to be just barely open, which caused the fuel injector to keep pumping fuel into the cylinders and then, POW!! So, before you do any adjustments, be sure that you are completely closing the throttle on deceleration.
    Mike Davis
    "Old n Slow" It's a way of life!
    1985 K100RT

    1998 R1100RT

  4. #4
    3 Red Bricks
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    There is a difference between the normal "burrbling" and light popping on deceleration and a loud KAPOW like a gun shot. If it is the latter, then you are either slightly holding the throttle open, as Mike said(thereby not activating the throttle position switch) OR the throttle position switch is out of adjustment (not activating at fully closed throttle).


    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  5. #5
    Nickname: Droid
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    Thanks for the replies defining what "backfiring" is not. One of my pet peeves on cycle forums are riders claiming their bikes backfire. Heck, even techs at dealerships refer to it as backfires which is really wrong. Decel "popping" or afterfire are better terms to define the characteristic common to lean-burn engines.

  6. #6
    Registered User jwetering's Avatar
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    Thanks for the clarification as well....but the real story here is that "they all do that".

    I was talking to a guy a while back who was reminiscing about his K75. I asked if he ever had any problems with it and he said the only thing he remembers was that he bought a set of new injectors to resolve a persistent backfiring issue - but that it didn't help. Guess what I told him?

    My riding buddies are always telling me that my bike backfires and once in a while I start worrying that it's getting excessive as well. There are no loud bangs though - and I've checked and adjusted my TPS a half dozen times so I know it's right.

    I guess that this persistent popping and layshaft gear whine (my bike's PO sold the bike partially because it "sounded like a food processor) are just two of the things that make bricks special.

    jasper
    north vancouver

    1990 K75S

  7. #7
    A bozo on the bus deilenberger's Avatar
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    The K75 is particularly prone to this, I suspect for a few reasons:

    1 - it uses the same air-flow meter as the K100 bike, and I suspect it responds a bit more slowly to the smaller airflow, so it thinks the engine is calling for more air/fuel then it is on deceleration.

    2 - the single connection between the header pipe assembly and the muffler is more prone to air leakage, especially if the very expensive and rare mica gasket that came on them has disappeared (and bet by now most have.) It allows an air leak giving the rich exhaust mixture the air it needs to burn - and you end up with the "backfire" (after-fire.)

    It's one of those "character" sort of things, especially if you're using engine braking on a long downhill. My riding companions knew to back off a bit when following me down a hill.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  8. #8
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    Quote Originally Posted by deilenberger View Post
    especially if the very expensive and rare mica gasket that came on them has disappeared (and bet by now most have.)
    Not all that expensive or rare (but very well could be the culprit):

    18 12 1 457 142 GASKET $13.73


    Just part of normal maintenance on a 27 year old bike.

    It could very well be your problem if the TPS is adjusted properly.


    And they DO NOT "all do that" (if properly maintained). My bike has 131,000 miles and my wife's has 83,000 miles. Yes, the gaskets have been replaced a couple of times and my bike gets tweeked alot for various reasons. They run great and only give a slight burbling with a very rare (like every couple of months) light pop (more than likely from not fully closing the throttle all the way as Mike has mentioned). I predominantly use engine braking and high RPM, so if it was going to happen, it would happen A LOT with me.




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  9. #9
    A bozo on the bus deilenberger's Avatar
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    Lee - interesting. At one time that gasket was > $35. I thought Brian Curry was going to keel over when he heard the price of it. He tried using high-temperature silicone sealant in place of it. Not only did it not work, it made a lovely smell of burning rubber and a small smoke cloud as it burned away..
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

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