Off throttle backfire

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by 5 speed, Oct 26, 2012.

  1. 5 speed

    5 speed Been here awhile

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    Hi there. So It took my bike in for a valve adjustment. Generally do everything myself but havent adjusted the valves and figured I would have it done professionally. The bike was at the shop for 4 weeks as I had springs ordered for the front end and also sent my seat out to be redone. I figured the gas may have gone bad but I did have the Marine Stabilizer in it. I sprayed seafoam in the carbs and also ran some through the gas and then rode it about 500 miles. My conclusion because, it idles fine and performs well is that there is an airleak. It backfiring and spitting when the throttle is chopped. I remember putting the carbs in myself once and thought they seated but then took the tanks off and could see through the access panels that they weren't quite right.

    So, before I pull the tanks, any ideas of how I might tell where the airleak is coming from (with suspect that it may have to do with the dis/reassembly from the valve job). your thoughts would be appreciated.
    #1
  2. snowhawk jockey

    snowhawk jockey Slack Jaw Gaper

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    Did the shop fiddle with your exhaust at all? Air leak there is a sure backfire.
    #2
  3. TheMuffinMan

    TheMuffinMan Forest Ranger Magnet

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    I'd make sure the carbs are seated in the boots, and then I'd be going over the exhausts for cracks/leaks.
    #3
  4. Spiritwalker2222

    Spiritwalker2222 Been here awhile

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    Check you exhaust header. If the bolt and nut are loose it can cause your bike to bikefire whenever you let off the throttle.
    #4
  5. 5 speed

    5 speed Been here awhile

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    Cool I will try those suggestions before I jump on the carbs and report back. Thanks much
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  6. DirtyADV

    DirtyADV Long timer

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    Had bad backfiring when engine braking from like 4500 rpm and below.

    Carbs were not touched when the bike developed this, found it to be an air leak on one of the hoses on the intake below the carbs that are used when syncing the carbs (smog stuff removed).

    Poorly seated or torn intake rubber could probably cause it also.

    /Johan
    #6
  7. 5 speed

    5 speed Been here awhile

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    I guess I got ahead of myself. I fired it up in the driveway and it ran and idled fine, no backfiring. So I put it together last night (had not gone so far as pulling the tanks) and today it ran terrific, idled perfect, but still backfires when winding down from about 4,000 RPM. It seems to be the right pipe which is the front cylinder. I checked around the head and the pipe joint and did not find a leak. as far as the carbs i did as much inspection from topside I could and the carbs seem seated (although I have been fooled before on that one). I would say its definitely an airleak.
    #7
  8. MrEndo

    MrEndo Been here awhile

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    U can spray a carb cleaner like Berryman B12 on the areas you suspect are leaking. If the idle goes up or down it is sucking the spray in through a leak. Be cautios not to spray so much that it is getting sucked in the carb intake. You might need to hold it at a fast idle for best results.
    #8
  9. geometrician

    geometrician let's keep going...

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    WD40 works just as well & isn't caustic as carb cleaners are

    air leak WAY more likely to be the cause than exhaust
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  10. MrEndo

    MrEndo Been here awhile

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    I was suggesting it for an intake leak....
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  11. 5 speed

    5 speed Been here awhile

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    Yes I will try that and see. I do have a question however, I have noticed that when looking at the slides in the carbs, when pulling the throttle cable one opens slightly sooner than the other, I think the rear carb is opening first. Is that correct?
    #11
  12. StmbtDave

    StmbtDave AKA Invisible Dave

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    No, that is not correct. Both should open exactly at the same time and open the exact same amount. What you're seeing may be caused by one of the throttle cables not being seated correctly or as it was in my case, one of the carb diaphrams being torn. I would first check the cables as they may have been disturbed during the shop work.

    Dave
    #12
  13. Boatman

    Boatman Membership has it's privileges ;-)

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    Slides are vacuum controlled... butterflies are operated by the cables. All should work identical (for the most part) between carbs. If one of the slides are lagging, it could be hanging up or a vacuum leak or a pin hole in a diaphram or some other small issue could cause that.

    Also carb sync could be at the root of the problem.
    #13
  14. geometrician

    geometrician let's keep going...

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    that's what we're talking about... not trying to set the bike on fire!

    on the many bikes that use this CVK carb the air-cut valve diaphragm is the known culprit for backfiring on closed throttle

    it's item #34 on the carb parts diagram...
    #14
  15. K2m

    K2m ....58....

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    When a LC8 is perfectly tuned they do this.

    It may be possible to tune it out by changing the Air Leak jet on the ACV (air cut valve) it's on the pilot circuit.

    If you wish to explore this option I can help further.

    If I understand correctly :-
    It came back from the shop popping on decel. ..... Spot on.
    #15
  16. crofrog

    crofrog Long timer

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    +1 every well jetted lc8 I've been on does it, _and_ it's fun.
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  17. DirtyADV

    DirtyADV Long timer

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    #17
  18. kulvershtukas

    kulvershtukas Adventurer

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    Have the same issue. Seems like the right pipe is backfiring. Also seems like it is going hot a little bit faster than the left one. As soon as I am not a real technician, have asked the guy from the service to do 15kt service and check the issues. So far:
    - no leaks or cracks in pipes
    - no vacuum leaks
    - air filter is clean
    - carbs are synced
    - valves are cleaned, regulated
    - Leo Vince cans are checked from the inside
    All is in order, nothing wrong with the bike, but it still backfires... I guess it just has to do that :D
    #18
  19. kulvershtukas

    kulvershtukas Adventurer

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    Also spark plugs are replaced, but have not seen the bike yet
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  20. Sumi

    Sumi Long timer

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    If it pulls as it should and idle is fine, I'd say it's normal. They really all do that!

    If your slides are not in sync (if they are way out of sync I mean), it might be a carb sync issue (way out of sync carbs will also cause backfiring - but probably not just off throttle).

    But IMO if it has the power it should have, and the idle isn't hunting too badly neither hanging, then it's normal - the off throttle backfiring. Also it's a better practice to ride these bikes instead of worrying about every hickup it throws at us / repairing (what isn't really broken) it constantly:)
    #20