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Hi,
I got a bit confused.
I took the two plugs for inspection. On the right he looks dry and "beautiful". On the left you can see signs of black soot, which means that the mixture is too rich.
According to the motorcycle book I should close the screw mixture.
I saw an explanation for other motorcycles which required to open the screw. Since this screw reduces the amount of fuel, it should be closed. Is that so?
Thanks
 

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The screws adjust the mixture at idle. They have diminishing effect at the throttle is opened. With the engine warm, bike on centerstand, idle at 1200=/-100rpm, adjust one mixture screw to achieve highest rpm. Re-adjust idle speed. Adjust opposite mixture screw to achieve highest rpm. Unless the rich side was a long ways off, this is not going to fix your problem.

Possibly your float valve is not sealing, causing a high fuel level in the bowl. Possibly you have a cracked float which allows fuel into the float so it "floats" lower. I'd start by removing the bowl, float, and valve. Inspect the float and valve, flush out the line. While you have the bowl off ensure the jets are in place/tight and the rubber plug over the slow jet is in place and in good condition.
 

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Which side carb has the accelerator pump on a CM400C, I thought I remember it being on the right. Could the linkage be sticking or missdjusted?
 

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How is your compression?
How is the coil and spark plug end caps resistance?
When were the carbs last sync'd?
These are all things that can effect the mixture and leave a dry soot deposit on the plugs
 
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