I have been chasing this problem with my MC18, but it still eludes me to what could be the issue.

For the most part the bike runs very well, but intermittently on part throttle under load (at around 6k-7k rpm), the bike bogs and will not accelerate no matter the throttle opening. It certainly feels like a lean condition (alike running out of gas). If I close the throttle for a short while and roll it back on, everything is fine and it accelerates as it should.

Now, the one issue I am suspecting is the tank vent system. The stock tank breather has been dripping fuel out of the line and I installed a one-way check valve to prevent this. However, I notice that the tank was bubbling fuel out of the keyhole of the gas cap today. I have the stock gas cap, and I do not see a breather hole in the tank, just a drain tube for the overflow. Is there one?

I have disassembled and cleaned the carb and replaced all gaskets, and checked float levels, adjusted air screw etc. The bike is stock with the exception of pod air filters, and I am running much larger main jets to accommodate this. As I said, it starts and runs beautifully and doesn't exhibit any poor running characteristics otherwise.

Any advice would be welcome. Cheers.