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Discussion Starter #1
Hello,
New here and just picked up a "new" 78 xs400. Anyway, the bike starts runs and idles fine once warm. However starting it is the biggest pain ever when cold, and once its started I cant get it to idle well. It is very slow when cold, Id say around 600-700 rpm at best, and you need to crank the thing forever. Using even half choke seems to only flood the engine. As I unerstand if the bike is hot, using the choke should increase engine speed, on my bike even half choke just stalls the motor. I have the haynes manual for the bike but it says that the carbs arent to be tampered with, therefore is rather useless. I know nothing about these carbs, and therefore any help would be appreciated. I am not familiar where all the adjustment screws are and dont want to further make problems for myself. They are mikuni bs34 units.
Thanks.
 

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Check the float needles and seats, sounds like a carb problem. Also check the fuel tap if it's a vacuum operated one. Does it do the same thing if you turn it to "prime" (does it have a prime setting?)

Anyway putting on the choke when its hot will usually flood a motor out after a bit.
 

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Seems like a Yamaha (or an old bike in general) problem.
My 83 XJ650 has the same problem... Same carbs (4 vs. 2)?
I'll be rebuilding the carbs this winter - for now she's stripped down for paint.
Post here again when you figure out the problem.
 

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Discussion Starter #6
Unfortunately I dont have time untill next weekend, but ill be sure to post the progress.
 
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