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Bike backfiring and surging above 5000 rpm

Discussion in 'Technical and Troubleshooting Torque' started by V4bloke, Dec 13, 2007.

  1. Hi,

    not sure if this has been answered before.

    My bike has been sitting for some months, and while riding it for the first time it ran fine up to 5000rpm, but over that it began to miss, backfire and lose power very badly.

    I'd drained and refilled the tank with fresh fuel prior to the ride, but didn't drain the fuel bowls. I thought it might clear its throat after a few goes but it seems to be getting worse.

    Any ideas on what I can try or whats causing it? I have next to no mechanical experience!
  2. From someone lacking in mechanic knowledge and ability;

    I'd guess its the carbs are just full of crap. I'd put some of the nulon fuel injection/carb cleaner stuff in, and possibly a bit of metho (half a cup in a full tank - gets rid of water), then go for a rather spirited ride. Warm it up properly then thrash its tits off. Should make a difference. If not, go for the bowls and whilst you're there you might as well clean the rest of the carbs.
  3. +1 Phizog sounds very much like a fuel starvation problem to me , time to clean carbies !!
  4. It's leaning out.

    How did you drain the fuel? Check your fuel lines and vacuum lines for good condition and fit.

    also make sure your tank vent is clear.

    It may also be gum in the carbies, so try a tank of e10 if the above doesn't work.

    After that I'd say you need to clean the carbies.
  5. Thanks guys :) I'll give those ideas a go and hopefully it will do the trick without having to have the carbies removed.
  6. So i'm guessing by your username you have a vfr/rvf? Yeah i can understand not wanting to remove the carbies! :cool:
  7. Thats right davesta - I have an old '86 VFR 750 which I bought midway through the year, and it was sitting for quite a long time (about 8mths) before I bought it.

    The carb setup is not as simple as some, and I'm not game to attempt removal and fiddling myself!
  8. Actually, the symptoms suggest something other than blocked jets. It's more like a lack of fuel (which blocked jets cause, of course).

    Is the fuel tap vacuum operated? If so (and most Hondas have been for years) it will have a vacuum line feed from one of the inlet manifolds. This opens the tap once the engine is running, but when the diaphragm fails - they only need a microscopic hole - it won't open the tap correctly.

    To test, either try running the bike in the prime position if it has one (this bypasses the vacuum operation) or remove the vacuum line from the manifold and suck on it.

    Danger! If you get a mouthful of fuel, that is your responsibility.

    Check the line for fuel leakage (there should be none) before putting it anywhere near your mouth. As soon as you produce vacuum, seal it against your tongue. If the vacuum won't maintain but dies away within a second or two you have a faulty diaphragm.

    Another possibility is carb diaphragm failure in one or more of the carbs. This also lets unwanted air into the system to make it run lean.

    All the best

    Trevor G

    PS This (fuel tap diaphragm) is not an uncommon problem and is easier to check than pulling the carbs...

    The carb diaphragms should be easily checkable with a vacuum gauge set, I think...
  9. Thanks Trevor :) I'll give that a try as well. Hopefully one of them will be an easy fix, or its off the mechanic! (which I can't afford :cry: )