I got this question this last week: I am doing my ladies bike- which is a '94 600 dual carb, it has a rejet and stacks, the idle is high (goes up and down) and not consistent at the RPM it idles it. It will be fine then jump to 2500 to 3000, then might take it for a spin and might act fine or stay the same. How can I make my bikes idle stay consistent?
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So in this situation what I would recommend is to pull the carbs out and clean them. Especially if you don't know the condition, and even if a previous owner said they cleaned them. Many times people will tell you that they have rejetted a carb and unfortunately that is not always the case. The second thing is, take a look at the intake boots. If the intake boots are old, hard and you cannot squeeze them, they are CRAP. You need to throw them away and get them replacements. I am not just trying to sell you something here, but seriously most likely fix here, because they can create serious vacuum leaks and when the bike is vibrating and running those boots are shaking as well. If they are hard and don't seal well, this allows air into your combution chamber affecting your RPM. If you haven't done those two things, those are the first things I would try.
If you have hunting idle, you most likely have a vacuum leak, you may have a torn diaphragm. Those are two other things you might want to look at, that is why the first thing I said to do is pull the carbs out and inspect everything and rebuild it. We do have rebuild kits that are SUPER helpful. Starting from ground zero is usually your best bet and making sure it all squeaky clean!
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