I've finally reverted the OEM carburetor back to stock settings (just the choke valve (flap) removed), and put one washer under the jet needle, and the bike is running great.
It does have noticeable acceleration gain across the board compared to the stock settings.
Still a little lean on the top end, but kind of starts working better as the engine warms up. No bogging.
It does idle pretty high at 2k RPM idle from cold with the idle screw to the lowest setting, and indications of a lean running bike (when revving the bike, it lowers to 2k RPM quite slowly), despite the exhaust smelling still a bit rich.
Once the bike is warmed up, that problem disappears, and idle is closer to 1500RPM.
I would try a second washer under the jet needle clip maybe another time, as with the stock airbox removed (and using a pod filter) I only need to remove the 2x 10mm nuts, to lower the carburetor enough to access the jet needle. I can do this without removing any of the fairings now.
So changing out the jet needle settings should take less than 30 minutes.
I did install the performance exhaust with 2 copper gaskets:
https://www.ebay.com/itm/385603922733
Which apparently, I could order from Alibaba for half the price...
The pipe doesn't quite fit the bike, the bottom is touching the frame, causing the header to be in a slight angle, hence the 2 copper gaskets. I do hear the air coming out the header section when riding slow, but there's nothing I could do about that.
My recommendation, if you are going to use a POD filter, is not to cap off the crankcase vent hose.
I did cap it off, but drilled a small pilot hole in the tube, so gasses or pressure can exit the case, but hopefully oil will drip back into the crankcase.
I did cap off the fuel tank vent hose.
The ~10in aluminum pod filter pipe didn't fit the bike, so I screwed the pod straight on the carburetor.
If I found another pipe with vacuum line connection, I could add the fuel tank vent hose to that.
Top speed (with a little tailwind) in 5th gear was 68MPH indicated (this was with 1 washer under jet needle clip, tucked in, and the fastest recorded speed), though generally it'll top out around 63MPH in 4th gear (60MPH GPS verified) at 8-8.2k rpm without tail wind, or 55MPH GPS verified in 5th gear.
I use it more as a cruising gear than a gear to go fast with.
Around 8k RPM, the engine also is nearly vibrationless (balanced).
I also supplied the bike with 5W30 oil, after the top speed test.
This should mean the bike could run even faster with this type of oil.
The ~200 mile oil change had some minor metallic flakes, but nothing serious. The oil screen was clean.
My recommendation for new riders on the shipping oil, is to use it to start the bike, set the idle, let it reach up to temperature idling, lightly revving the engine (to like 4k RPM), to feel throttle response, and go for a few hundred feet of a ride and back at low RPM, to see if everything is working normally.
Then dump it, never use it for actual riding, and fill up with break-in oil (Lucas oil with zinc) added to standard 10W40 oil, for the first 50 miles.
Then use any regular 10W40 oil (you can add some of the zinc oil as well), for the next 150 miles.
After this, the engine is pretty much broken in, and you should use 10W40 motorcycle oil for the first 1000-3000 miles.
Don't follow my example on using 5W30. I only do it for testing purposes.
Other things I disliked:
The passenger handles are weak. I almost bent one from picking up the bike's tail by one handle. Always pick up the rear end using both handles; and even then, I feel the construction is too weak, and prone to breaking.
The Lifan 150 engine used is not a very efficient engine.
It barely ekes out at 60MPH, meanwhile Honda Grom 125cc engines, and the older Chinese 125cc pushrod engines top out at 65-70MPH; and the pushrod engines can actually get 120MPG easily (with the right gearing, and 100MPG when driven hard).
The lifan 150 makes most power at a high 7.5k RPM, which costs MPG and efficiency. It doesn't pull a 19T with the stock 34T rear sprocket at speeds over 6-6.5k RPM in 5th gear, but I can get the bike up to 8.5-9k RPM in 4th gear.
I will swap out my 34T rear sprocket with a 30T sprocket, which should give me higher top speed in 4th, and better cruising speed between 30-50MPH in 5th.