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'80 Xs400 Idle RPM randomly skyrockets


SirBieber
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So I put my bike away for the entire winter and finally pulled it out today. I never owned a vehicle where I had to store it for an entire season so I never knew to put in special liquid if the fuel to prevent varnishing, or to disconnect the battery.

When I started the bike I found out the battery was completely shot. I bought a new one, and they guy suggest and empty the tank, and start with fresh fuel which I did. Then I cleaned the carburetor once again very thoroughly, and I made sure the rubber connections to it are very very tight as to not leak any air.

The first few kicks did absolutely nothing. (My electric starter doesnt work... I just hear something spinning freely and not catching but I'll save that for another day). Finally on about the 10th kick it started up while the choke was on and primed, gave it a couple minutes to warm up, and set the idle rpm below 2k.

I rode around for a few minutes and was at a dead stop again, then my rpms just randomly shot way up. I adjusted the idle needle so that it brought them back down again. but then the rpms drop again and wont stay idle so i gotta raise them up a bit. I did some research and found that bad gas and air leaks can be 2 factors causing this, but I'm pretty sure I eliminated these possibilites. If any one has any ideas please let me know. I can post a video of it if I figure out how.

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Recheck the intake boots, as even small cracks in an older, dried out one can change the mixture....spray WD-40 around the intakes while it is idling...if the RPM cganges, you still have air leaks. The boots are available on ebay, or from Yamaha with more money, and change the intake gaskets as well.

Good luck.

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I had a bad gasket under a nylon washer, under a circlip for the butterfly valve rod. Found it by dropping oil onto each of the exposed sections against the carb body, and when the RPMs dropped every time I hit that spot with oil, I knew I found my leak.

I strongly agree with Runningdog, WD40 is a good choice for when you're searching all over hells half acre.

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