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cb750 'bogging down' under acceleration, carbs? pipe dent? ?

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by philem63, Mar 15, 2008.

  1. hi folks,
    just today i bought a 94 cb 750. it needed the pipes refitted and the battery is totally stuffed. but shes a good buy at $650...
    i refitted the pipes and took it for a ride...the fuels possibly 3-6 months old. the battery was totally dead and one pipe has a really bad dent in it.

    when i rode it she seemed to be bogging down real bad under acceleration...
    it seemed to idle fine and revved great on the spot but no good under load. could thid be the flat battery affecting it or the dent in the pipe?

    if not either of those how about the stale fuel or even blocked up carbs?

    any help greatly appreciated!
  2. all sound feasible except maybe the bent pipe, it would have to be pretty bad for it to slow the bike dramatically.

    most probably stale fuel, low voltage, old plugs/leads, or out of tune
    drain the fuel and try some new stuff

    also not sure of how the carby works but any moving parts like in an SU could be sticky giving bad acceleration, therefore a clean would proabably help
    also air leaks could also do it but not as likely, same with ignition timing.

    to see the condition of plugs and mixtures when the bike is cold see which pipes heat the quickest by putting ur hand near them and feeling the heat coming off, carbys or plugs a bit down will most likely heat slower

    good luck
  3. It'd be looking at giving the carbs a service. To me it sounds like the air/fuel mixture is wrong. A clean and correct adjustment of the floats would be a good start.
  4. is it leaking any fuel (carbs)
  5. Re: cb750 'bogging down' under acceleration, carbs? pipe den

    Most likely both. Definitely be worth cleaning the carbs, I'd also replace the sparkplugs and replace/clean the air filter (probably not the cause but at least you'd eliminate two variables).
  6. Start off with a full fluid and filter change, a new battery and fresh petrol and then see what it feels like before delving deeper.
  7. Re: cb750 'bogging down' under acceleration, carbs? pipe den

    OK, so what did you do to get it to accelerate?

    Would it wind out through the gears if you took it easy, opening the throttle gradually?

    Was it spluttering or misfiring?


    Trevor G
  8. If the battery was dead that suggests the bike's been standing a long time.

    Fuel not only goes "off", it also evaporates in the carbs, leaving a gummy/rubbery residue. It doesn't take much of this stuff to block the jets. If it revs Ok in neutral, there's probably not too much wrong with the engine, though a service/tune up wouldn't hurt.

    It sounds like you've got a blockage in the jets. There are various jets to handle the fueling from idle right up to full throttle, as well as the progression from one jet to another. If any of these are blocked it will affect the running under load.

    Drain/flush the carbies - carb cleaner added to the fuel might work, but I've never had any luck with these products. It may be better to remove the carbs - a fiddly job if you still have original airbox fitted.

    Give the jets a good clean using soft metal wire (brass/copper) to poke through the jets to make sure they're all clear.