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Discussion Starter #1
Hey guys, I’m new here but wanted some advice. I just got my first bike a couple days ago and don’t know much about them but I wanted to learn. When I test drove it it idled perfectly and rode perfectly and has for a couple days. Now I’m experiencing a problem where when I start it from cold it idles OK but it will warm up and start revving without me touching anything. Ive adjusted the idle screw but it’s not changing much, the bike ends up just dying when I mess around with it. I’ve heard things about vacuum leaks and such but really don’t know if it’s that or not. When I’m riding it I’ll get into first gear and it’ll rev up to about 5000 RPMs and stay there as I shift and then when I’m at a stop light it will eventually just die out. Also the electric start just started to not work, it’s just clicking. Not exactly sure what the problem is, I just hope I didn’t make a mistake in buying this old bike. Any advice is very appreciated!

thanks!
 

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You could have an air leak at the intake manifold. But does your throttle snap back OK? Your starting issue may just be a weak battery.
 

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Discussion Starter #3
The throttle does snap back ok. And I got the starter figured out, it was the battery. I just tried starting it up again and sounded fine but as soon as I put it in gear it just made a very slow “lugugugug”...hopefully that’s understandable haha. And then as soon as I was shifting to second and third it got stuck at a high RPM again but then would die at stop lights. Had to take an Uber home cause it started pouring. :(
 

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Jap bikes of that vintage are known to develop cracks in the rubber tubes between the carbs and the engine. My buddy had the same symptom on his yamaha. Lean running on one or more cylinders increases rpm. Put on the choke, if equipped to get things under control, remove the carbs and check, and most likely replace the rubber tubes.
 

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When I fire up my XS400 from cold. I apply full choke, which is an enrichener device. After a few moments running, it will increase revs without me touching anything. This is normal. The engine used to be cold, and it required more fuel. It is now warming up and requires less fuel. If you do not back off the choke thing, it will rev higher. Push the lever in a bit.
To check for air leaks. which usually expose themselves when the engine is hot, spray the area with WD40. If there is an air leak the engine will increase in revs. The rubber boots can be cracked on the outside, but not leaking any air.

UK
 

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Discussion Starter #9
Thanks for help guys! I’m definitely in over my head...thinking I maybe should have waited and gotten a 350 or something... but oh well. I’m sure once this thing is up and running well it’ll be great.
 

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I had an '06 CB600F. A lot newer, but still breathed thru carburetors. I had to choke it and warm it up, even if it wasn't cold weather. When it warmed up a certain amount it would start revving up just like you describe. So you just back off the choke to about half, and let it finish warming up and then take the choke completely off. It might start revving up again at half choke, so adjust as needed. You'll get used to how much to choke and when.
Mine had a temperature gauge so I knew when it got to 150 F it was ready to roll. The other way you can tell is, with the choke off, blip the throttle pretty hard. If it's not ready, it'll stall/sag a little. If it's warmed up it'll rev without any hesitation.
This is assuming there's nothing wrong and it's just the normal warmup behavior mine had.
 
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