I got a Lifan  200 CC Xpect dual sport. It had under700 miles. It's not at800 yet.
The chain had not been serviced and was so sloppy I was feeling the links catch on the sprocket and dismissed the surging as a carburetor issue or the chain. Later I discovered the little thing is injected!!
Coupla days ago it stranded me at the store. It started, but it would only idle around500 rpm and then when I gave it gas it died. After about an hour of poking and trying, it fired up and acted like it had from the first time. Hold the throttle still. Watch the teach wobble JP and down sweeping more than400 rpm.
I've slipped the wires to an LED into the fuel pump connector. It lights when I turn the key on, till the pump kicks off. Start the motor and the pump gets continuous power.
I've disconnected and used electrical connection grease on every connection I can find fuel pump, coil, throttle position sensor, oxygen sensor, injector. Total of nine,,
I pulled the throttle position sensor and tried to get electronic contact cleaner in it.
Next step is to put some jumper wires into the throttle position sensor connection and see if I can see the resistance changing there. If not, the only thing I can see to look at is the crankshaft position sensor.

The symptom is not unheard of at all with these.
I have no idea where the
Computer, if it exists and can be called that, is located.

Anyone who knows about injected stuff and can offer advice
Please call me
Junior Samples
BR549

Don't know this bike at all. But, if I'm understanding the symptoms you're describing, it's the inability to come off idle and a drop in RPM when advancing the throttle. Is that correct?
Appears to be a license built (or knock-off) of the Honda CG125 (you'll still find some of the internal parts stamped CG125). One thing to consider is a faulty CDI. Instead of advancing, they allow an idle, but fail to advance and begin to miss-fire when advancing the throttle, as evidenced in the drop in RPM (these fire every revolution and the dropped firings are causing the tach to drop). If you had a catalytic converter on this bike, you'd see a cherry red cat from it's burning of the hydrocarbons that weren't burned in the combustion chamber due to the faulty firing of the CDI. Not at all uncommon with this engine.
I know it sounds weird, but for a few bucks it's a cheap rule-out in the diagnostic process and saves chasing a bunch of phantom symptoms. It was puzzling to me until the drop in RPM started to make sense when I first encountered them. Likely something like the one below. Your Lifan dealer should have a whole bin of them.

https://www.ebay.com/itm/6-Pin-CDI-Unit-Ignition-CG125-150cc-200cc-250cc-Go-K...

Not trying to be mean, but basically you are saying you bought a cheap POS and it sucks, yes?
And you are surprised?

With so many good used motorcycles available at bargain prices - I am never tempted to buy a Chinese clone copy bike.  I would prefer to go with proven reliable Japanese motorcycles......or maybe even a Buell!
https://cincinnati.craigslist.org/mcy/d/cincinnati-2001-harley-buell-blast/72...

https://cincinnati.craigslist.org/mcd/d/cincinnati-1981-honda-cm400-custom/72...

https://cincinnati.craigslist.org/mcy/d/morrow-suzuki-gs500f/7255431749.html



I wonder if ditching the fuel injection and installing a carb would be a better way to resolve this issue?  The Lifan engines are on a lot of motorcycles with carbs.......maybe you can find one.  OR - maybe a carb for a lawn mower or chain saw would be possible?  Try to get a carb for a similar engine size and HP, and also the same engine type (2 or 4 stroke).  

Those are some pretty good prices you found on those bikes. For a while I was considering a GS500 but never found a decent one at a reasonable price.