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Help, bike revs up then stalls, carby? Mixture screws?

Discussion in 'Technical and Troubleshooting Torque' started by brent88, Dec 10, 2012.

  1. Hi, this is actually a question for my dads motorbike which is a 1992 kawasaki gpx 250cc.

    When started it revs to about 4,000 then slowly comes down and depending where the throttle stop is stalls, as soon as its revved again it hovers around 4,000 and slowly drops back down to 1,000 or stalls.

    We have set the mixture screw to 1 and a half turns out but have tried 1, 2 and 2 and a quarter turns out. the carby's have been pulled apart and all the jets cleaned. The diaphrams are good and slide properly forgot to ask him about the air cleaner but if it didnt have 1 or have a good one could this be the problem? the spark plugs and spark has been tested by having the spark plug earthed against the cylinder head and the engine turned over and its a consistent blue spark. as far as I know the vacum lines are hooked up correctly but as the carby's have been off this is a potential contributer to the problem.

    once running it runs fine but could be lacking a bit of torque IMO though this could be the mixture screws not set right.

    Please offer suggestions of things to check/ do

  2. When the revs hang like that it usually means a lean mixture.
    It could either be an air leak, like from a vacuum hose that's been incorrectly fitted, or loose inlet manifold clamp, or it could be a carbie issue.
    You should try as an experiment to screw the miture screws out four or five turns to see what this does. Yes the one & half to two & half turns is the normal range of adjustment, and if you have to go outside of this it usually means you perhapps need to change pilot jet size or needle position etc.
    Those carbs have plastic slides which tend to wear & cause lean mixtures. One trick I used to compensate for worn carbs is to fit tiny washers under the slide needles which richen the mixture. It will never make it run perfect, but it does improve it.
    Another thing could be the balance screw between the two carbs could be way out. You need a set of vacuum gauges to check this out.
    Also, when you cleaned the jets, did you actually remove the pilot jets & check that you could see daylight through them? Sometimes jets can get partially restricted by fuel gum. That is you can still see daylight through them, and they seem fine but the hole is reduced in size equivalent to a few jet sizes. One trick I use is to gently ream the jet hole by passing an appropriately sized strand of copper wire back & forth through the jet with a little solvent to help soften & dissolve any gummy deposits. You can salvage the strand of wire from a short piece of insulated electrical wire.
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  3. Is ok but lacks a bit of torque?
    Definaltey something to do with mixture screws and I would also check float heights again? maybe the float is sticking and not moving? float not measured correctly? Had a few of the similar things happen on my 400 that sounds like it was running similar to yours
  4. As well as the other suggestions re base settings etc, if the carbs have been removed, disassembled, cleaned and re assembled and refitted, they will need balancing ( ie vacuum gauges )
  5. Thanks every1 this has given me something to think about. I know he should get the carby properly balanced but we just want the problem to go away first and think that trial and error with the mixture screws should be able to be got right if thats the problem. will try having the mixture screws out 5 turns.

    Where can i get vacum gauges? i cannot find them on ebay (these are the things with the ball bearings that you hook to the vacum lines correct?

    I will have to check the air filter as im not sure if he has yet could a stuffed or no air filter cause the symptoms?

    Jets were cleaned really well but there was some rust in the tank when he brought it but got thouroughly cleaned and has 2 fuel filters just incase( we have also had the jets out after a while of running with the once rusted tank and no rust is getting into the carby so though some may not agree im quite confident that is not the problem. we have also ran it with the fuel supply coming from another tank to eliminate this theory.

    Will check floats again if the other things do not work. what is the base setting for the balance screw inbetween the carbies and could this be the problem?

  6. The base settingd for my 400 are 2 and 1/4 turns out, 5 sounds like a lot
  7. I'd get a balance done first, with the idle screws set equally to around the stock setting, then go back to the mixture fiddling.

    Some CV carbs can also be quite prone to the slides misbehaving if there are airbox mods or even an aftermarket air cleaner. Hopefully nobody has snipped coils off the slide springs and/or drilled the bleed holes to try to liven things up.

    Come to think of it, for it's age, the springs might have just weakened and are letting the slides lift too much when it starts (or with a touch of throttle), but better to sort first things first so you can either solve the issue or rule out the easy to check stuff.
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  8. #8 Tinkerer, Dec 11, 2012
    Last edited: Dec 11, 2012
    Ebay doesn't have everything. Try a google search. Suzuki used the floating ball bearing type gauge, but they can bleed a little air down the vacuum line & can make some bikes run poor when hooked up to them making tuning more difficult. You can sometimes pick up a mercury set cheap, but there are safety issues with using mercury & you have to keep them upright otherwise the mercury can leak out. The modern digital ones are the most acurate, but very expensive especially for home use. I prefer the old school dial gauges, reasonably priced, rugged, and acurate enough to get most bikes running smooth.

    Yes it could be a contributing factor, but not likely to be the main cause.

    There is no base setting, unless you want to remove the carbies, hold them up to the light & adjust the screw until you see the same amount of daylight around each carb butterfly. This method will only get you in the ball park, the best way is to hook up the vacuum gauges & then adjust the screw until both gauges read the same. If the balance is way out, then it could definately cause your problem. If it's only a little out, then there are probably other causes.
  9. So why do manuals say base setting is........?
  10. You should be making sure everything is as it should be before mucking around with mixture settings.

    This includes making sure the tap and the tank vent are good, as is the fuel filters.

    Once you get to more than 4 turns out, that is suggesting you need a bigger idle jet. Unless some fairies come in and swapped out your jets overnight, I'd say your problems lay elsewhere.
  11. This happened to my GPX, the problem was a crack in one of the inlet manifolds. Duct taped her up until it could be replaced. make sure you check properly for air leaks, this one was not visible until taking the inlet manifold off and squeezing it slightly so the crack opened up.
  12. You might be confusing "balance screw" with "mixture screw" settings. Yes mixture screws do have a base setting, but I was talking about balance screw settings, which are not set by a number of turns, but set by using a vacuum gauge and adjusting the balance screw so that each carb has equal vacuum.
  13. Fair odds of cracking on something of this age, especially the rubber bits. You can also check by spraying on brake cleaner or something similar that's combustible around the each inlet. If the revs pick up, you know it's sucking air.