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My Yamaha '77 XS 400, mitsumi carbs, starts very hard, regardless of choke setting. When it finally starts, it roars at 5,000 to 8,000 rpm. I've lubricated the throttle cable to make sure it isn't sticking. The throttle link arm seems to be almost resting on the remote throttle stop screw when that screw is all the way backed off. I've disassembled and cleaned the carburetors, but that didn't help. Also removed and cleaned the gas tap and gas filter. The vacuum tube from the tank is a little loose where it goes into the inlet stub, but it was like that when the bike was running okay. This problem started when I fired up the bike for the first time this Spring. Had been running okay last year. I'm all out of ideas to try to fix this problem. Any suggestions would be hugely appreciated! Thanks.
 

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hi guys check your carb holders for cracks thats the rubber mounts for the carbs if they are cracked and old they can leak air thus the hi rev they can be fixed with silicone or replaced also make sure the carb slides are dropping back down all the way and smoothly
 

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How did you clean the carbs? I first tried to clean my carbs with some carb cleaner and a nylon brush but this was no good, my carbs were so gummed that I had to do a carb dip.

Ingredients:
One part water
One part yamaha carb dip
one or more carbs

Bring contents to a boil in some container and rotate carb for like 30 minutes or so, the sludge literally rinsed off with the water hose. (Some scrubbing may be required)

If you did the dip then great, but if you didn't then, trust me, it's the only way.
 

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it depends on which carbs you have some have adjusting screws but if your unlucky some are fixedlook for the screw on the front of the carb up top good luck
 

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I had the same problem. I adjusted my floats and the revving got better. It doesn't leak gas out the air boots or rev high on startup, but after a long ride, it will stay revved up to 4-5 grand, instead of idling. I was told it was too lean, when this happens. Gotta be an air mix adjustment.
 
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