Re: strange bogging at 1/2-wot
I would start by suggesting you do research on the carb tuning threads. The NT carb is very common and a popular choice for conversions and some even consider it an upgrade compared to recent factory "performance" carbs.
You might simply need to adjust the slide needle, or that along with a fuel jet re-size.
It also could be a fuel flow issue. Due to poor quality control at the factories, many of the stock fuel valves struggle to flow enough fuel at higher engine speeds causing a lean condition.
It could be a slow flowing fuel filter (you are using one, right?).
It could be an issue with the float setting.
It could be a poorly flowing muffler.
It could be a bad spark plug.
It could be a leaking head gasket.
It could be something I haven't listed here.
There is no one thing to look at for a solution.
It could be one of the above suggestions or a combination of them.
Check them out individually and write down any thing you do.
A written log of changes makes troubleshooting in the future easier if things go wrong.
Do not make multiple changes all at once because if one of them is wrong you only have multiplied the possible problems.
Which change actually helped?
Which change is now causing the new problem and masking the good result that other changes have done?
Do each change and test, then move on to the next change if the last one either helped or did nothing. If it did nothing at least you ruled it out....for now.
"It's a chinagirl. It's a sometimes fun little engine that often will break your heart. It's disposable.
Does not come with a fortune cookie."