Hi everybody on the forum.
I am sending you this message concerning my VRF400 NC24, which starts easily, that runs great up to 5000 RPM and then it starts to run less regularly (up to 8000 RPM), in fact if I need to overtake, I need to leave 2 gears (from 4 to 2 for example) and then the engine runs perfectly as soon as the frontier of 8000 rmp is reached.
The sparks are new, oil and filter new, tank has been cleaned and has some cream (RESTOM kit for old tanks) , the fuel is perfectly reaching the carbs, the carbs have been cleaned with an ultrasonic engine (perhaps I could do better on this side). the battery is new.
I have made a test without the air filter (no improvement).
If somebody has had the same problem and has some clues, I do thank you

Please, forgive me because my english is not perfect, I spent some years in England but that was ages ago...
I hope that everyone is going through this covid crisis without any damage.
stay safe with you familes.

Manu from the south of France

Hi Manu,sounds like a carb or fuel supply problem...is the vacuum pipe on correctly and diaphragm in good condition on the fuel tap??.float heights correct??.they need to be correct to work properly,but check vacuum pipe is not collapsing...quite common on these..good luck...