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.

7000rpm is right on the RC Valve transition. Check the valves are operating correctly, and properly adjusted. Even for 88/89, the RC Valve and PGM air correction is quite sophisticated, so you want it set up right! If the RC Valves are set incorrectly, and effectively opening a little early or late, then the mismatch with the air correction could be delivering a lean/rich mixture at just the wrong time, under certain riding conditions. It's unlikely, but it is possible.

While pod filters will require re-jetting, I wouldn't necessarily expect to see "much larger" main jets, however please refrain from an specific jetting discussion in the public area. Jetting discussion is reserved for the members area.

Bubbles coming from breathers are usually a bad thing. It may sound obvious, but just in case, make sure you have installed the one-way valve the correct way round! The little screw-on breather cap at the front of the fuel tank is the only one on MC18s.
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Andy.
NSR-WORLD.COM

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Thank you Andy for your advice and recommendations.

The RC valves appear to be functioning correctly but I will look into the air correction circuit. However, I am not sure how the air correction circuit functions. I have read some of the sources on the website where people have put jets inside the fittings and hoses but am not sure how this affects the desired carburation effect.

As per the board rules I will not talk about jetting but I just went with the recommended MJ size that Tyga had as a baseline.

It is really strange because it is intermittent and seems to be more apparent when the bike is traveling uphill. This is a headscratcher for me and I've pulled off the carbs more times than I can count.

Check the float levels. We've had really fussy MC18s before, that were very sensitive to different float heights.
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Andy.
NSR-WORLD.COM

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Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

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