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I made a long title to try and get some interest. I am a new rider for about a year and this is my second motorcycle. Here is my situation: I start my gs450l and it takes a little bit to warm up surprising in an Illinois summer. After a few minutes you can feel the idle level off and it sits at 1100 rpm which seems great. No sputter sounds good.

When I take off it takes a couple minutes of trying to accelerate and it kind of sputters and then will kick in. After that it runs nice. Shifting up and down is fine. After about 10 minutes of riding a new problem occurs when I am at a stop and downshift into 1 the rpms will actually go up to 3000. As I down shift the rpms will go down just when I stop the rpms go up until I accelerate again.

I thought it might be a clutch issue but even if I put it in Neutral the rpms will increase to 3000. It doesn't rev or anything. Its a level idle, just a high one lol.

The bike has 900 miles and had 2 owners. It had new carbs and tires installed before I bought it. Thats about it.
 

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It sounds like you have a vacuum leak. Being that it is a 30 year old bike, unless all the vacuum hoses, airbox boots and intake boots have hardened. The rubber orings between the intake boots and cylinder head may have hardened and are loosing their seal once it is heating up. Check out #25 and 26. https://www.babbittsonline.com/oemparts/a/suz/50d3f281f8700230d8b4be71/cylinder-head
While it is running you can use some carb cleaning spray to locate if there are leaks. The rpm will raise if the spray gets sucked in.

Also since you said it has new carbs, make sure the air box boots are on the carbs properly all the way around.
 
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