Kotz is a very mediocre oil so I wouldn't run it at less than 17% -- especially the Techniplate (which is straight synthetic).
We run AeroSave/Aerosynth here at 12% but that's a much better oil.
There are two reasons why many of the new generation Chinese engines run so poorly in the mid-range.
1. the carbies are crap. Replace the Chinese carby with an OS one and the engines often run much better. Unfortunately, an OS carby is the same price as a Chinese engine so it's not really economical to do that.
2. there is a tendancy to overbore engines so as to get maximum displacement from smaller packages. That's why engines that used to be .40-sized are now sporting capacities of .56 or more and engines that were .60-sized are regularly overbored to .90 size. When you overbore an engine like this you end up making the transfer ports much shallower and that produces choking during the transfer process. In effect, all the tiny fuel droplets coming from the carby are squashed through such a narrow path that they bang together and become larger droplets. Large droplets don't burn nearly as well as small ones (due to reduced surface-area to volume ratios) and so the engines appear to run rich. You can adjust for this at the top end and at the bottom end with the high and low needles -- but in the mid-range there is no adjustment so the engine appears to run rich -- mainly because the big droplets cool the plug more than the little ones.
It's a complex bit of gasflow really but the general result is that most overbored engines will have an inferior transition and mid-range. The JBA 120 may have deep enough transfer ports to avoid this (since it's probably not an overbored crankcase) so then you only have the issue of the carby.
The reason that some Chinese engines run better than others that are the same make/size is down to quality control and manufacturing processes.
In a good factory that produces quality product, the tooling is regularly changed -- when you're making an engine to a price, the tooling is often only changed if it breaks or the parts will no longer go together without using a sledgehammer. That's why, with only a few exceptions, you usually get what you pay for with RC engines and cheapest usually means you're playing lotto.
RCModelReviews.com, just the facts.