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Intermittant backfiring....

Discussion in 'Technical and Troubleshooting Torque' started by Bogus69, Aug 22, 2006.

  1. My VTR250 has started back firing.
    A couple of services ago it started happening when I got the bike back, but only when I forgot to turn the choke off.

    Gradually it has increased to the point where it happens with no choke on at all.
    It doesn't happen all the time or anything, but it is increasing in frequency - slightly.

    It happens when coasting with the throttle off. (coming of the freeway, approaching lights etc)

    Is there some easy adjustment I can make on my own to prevent this? The engine runs fine otherwise.

    Not sure if relevant or not....
    It's done 33,000 KM
    Recently ran out of petrol

  2. Backfiring on a closed throttle means one of two things:

    1. it's running rich, or
    2. you have an exhaust leak.

    The running rich thing could be because you have a stuck float or the float hieght is wrong, or just the mixture screws are set incorrectly.
  3. A "backfire" sounds like a shotgun going off. Is the sound like that?

    The other thing that will happen on closed throttle deceleration is "lean popping". My bike does this a bit and started doing it after I put the new free flowing exhaust on it.

    So is the noise a loud "bang!" (backfire), or like a popping burble (lean pop)?

    If it's a popping burble, it's running a little lean and can be fixed by turning out the carburettor pilot screws by perhaps a quarter turn, and maybe up to a half turn.

    If it's a loud "BANG!", then turn in the pilot screws by a quarter to half-turn.
  4. When was the air filter last checked?, and were the correct heat range plugs fitted at the service you mentioned?
  5. I wonder if the jets are wearing out.
  6. Could be that too. Jets can wear out in as little as 10000kms if they're low quality, and if they've not been changed in over 30000kms (since new) then it's worth looking into. As they wear out, they make the mixture richer.
  7. Yo bogus, as mentioned above a "backfire" is loud as hell and it is mixture buring back through the inlet tract (the path air and later fuel takes to get into the engine) hence back-fire. Can be caused by myriad problems from gunked up valves not closing all the way to a crossfire caused by crap plug leads etc.

    or is the sound more like a bit of pop and crackle when you close the throttle sometimes? Different probs diff causes diff solutions :grin:
  8. Hmmm.

    I wear ear plugs always so I'm not 100% sure of the sound.
    I would say it is quieter then a firearm but I figured that would be something to do with being on a small bike...

    I guess a few times it could be accurately descibed more as a pop. but other times it is a bit louder. Yesterday it gave me a fright (which was unusual).

    It is definately a single noise, not a series of noises if that helps?

    As for servicing, I take it to Redwing so would hope they are doing all they are supposed to. I'm not really sure. They haven't had to replace any parts.
    (except oil filter etc)
    The previous owner took it to PS. Since I've had the bike it has had 1 major and 1 minor service.

    I mentioned the petrol running out cos I was thinking maybe there was some crud in the tank that got through the system?

    I will take a look for Carb pilot screws.
    How many should there be?
  9. If you ride with someone else, ask them to follow you and to let you know if they see any smoke or smell more petrol than usual(rich fuel/air mixture).
  10. A leaking exhaust is the most likely culprit, either where the headers meet the engine, or at a join in the pipework. Leave those carbies alone!
  11. Any way I can test that Heinz?
  12. I'd agree it's running lean for sure from where it's back fireing.

    That was one of the symptons of my problems recently solved here, I knew it was a lean back fire from doing it while coasting.

    Thats the problem, listen to others how to fix your particular model.

    Your plugs should not be running that nice shade of grey for healthy plugs, they will by much lighter to confirm running lean.

    Hope you get it sorted and thats your prob. Fault diagnosis is always suk.
  13. Put you boot over the exhasut outlet, before it gets too hot. Look around the joints.
  14. Thanks, I'll give it a go.