Question about 1997 Suzuki GSF 1200 Bandit S

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Bandit idles HIGH when hot Just bought the bike! My bandit 1200S, 1997, runs smoothly when the engine is cool. But at hot temps when the bike warms up, the idle increases to 3000rpms. It doesnt want to fall back(unless i rev it high and let it the rpms fall from high revs). Also when it gets hot, gas pours out of the fuel supply tube at the male/female connection between the fuel supply valve and the tube that leads to the carbs. The two symptoms seam to be linked to hot engine temps. I would like to hear your opinion on the issue. I think it is a carb or fuel supply problem. I also think it could be caused by excess pressure in the gas tank when the engine is hot. I think this because the high pressure in the gas tank(related to heat) could expand the fluids and force more gas out of the tank: causing the gas leak, and causing the increase in rpms(because there is more gas forced into the carbs) Also the gas leak stops after i take off the gas cap. The gas cap is aftermarket and is just a spin off(no key). Could this cause the pressure increase. I havent checked if taking the gas cap off while running will bring the rpms from 3000 back to normal idle. I will check that next. I have been messing with the idle screw to change it to compensate the idle but it doesnt make much different. Thanks for your help guys... you are the pros

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  • rdlinder23 Nov 13, 2009

    Thanks for the advice on the bandit: high idle.

    Do fuel caps have vents? isnt there another mechanism to equalize tank pressure... like a vaccum tube or a pump.

    Also I dont think the idle problems are related to the throttle because the problem only occurs when the engine is running hot. Also the throttle seems to action fine.

    When the engine gets hot the idle either falls and stalls or it increases and wont fall down. I know that there is a pin sized tube at the top of each carb that draws air. The first two carbs have a rubber seal over the pin sized tube. The third one has a rubber tubbing that goes to the fuel suppy valve. The fourth is uncovered. When i plug the fourth with my finger, the idle revs high. This makes me think that my idle problem is related to the carb.

    Maybe the carbs need to be cleaned, rebuilt, etc. Do you agree. How much will that cost. Can i do it myself: Im good with tools, but unfamilliar with bikes.



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OK.... firstly change the fuel cap, the tank needs to be vented correctly so pressure will equalise. Normally a rich fuel concentration will cause the engine to die not run high. From what your telling me it sounds like the throttle cables are not adjusted properly, if they are too tight the rpm's will not come back to idle. if this is the case you might be able to twist the throttle the opposite direction to lower the revs at idle. For the excess fuel problem, it could be what you say about the fuel cap, another thing will be the Needle and Seat might need changing or the float levels are not adjusted correctly. Whilst your there fit a good inline fuel filter, it will stop problems in the future.

Posted on Nov 13, 2009

  • Noel O'Kell Nov 15, 2009


    If your able to plug that hole and the revs go high, it appears you have an air leak that needs to be plugged. I would try to plug the hole with some sort of cap and the adjust the idle back to normal. My best guess is this has been missing and someone just upped the idle to compensate rather than find the issue. This would definately cause the symptoms you mentioned, you will probably hear a hissing noise as well as it sucks air in.

    Fuel caps have a one way vent, it will allow air in but not out. This is something that was created for antipollution, if you have a non vented cap it will cause your bike to run lean because the fuel is fighting a vacuum.

    Let me know how you go.



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Try buying new diaframs for your carbs, because of the diafram and piston operation, if the diafram becomes weak due to heat from your engine, and by the diafram not beeing rigid enough, it hesitates to lift the needle out of the jet causing the lack of fuel at low tack/rpm when throttling up or accelerating slowly.

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