This is hard...
'jerking as it slowed down'
let's work on that part... Did it spit back through the carburetor?
As in a cough?
That could mean it's starving for fuel.
If the engine 'burbled' to deth.... it was flooding out.
What did the plug look like after it stopped?
OK, I gotta say the engine is probably cleared out quite well now...
That is to say, NOT a good idea to spin it any more without the fuel in the system, as I suggested earlier to clear it out. It NEEDS that fuel/oil mix for lubrication.
At this point, since you are initially getting it to pop and sputter, I think it's safe to assume you have effective spark. Never-the-less, as AL.Fisherman had said earlier, definitely change the plug! That's an EZ improvement.
There's whole threads here on the proper plug selection...
Since these engines are so small, you will need to learn about them.
A problem is frequently caused by a combination of bad actors, so to speak.
There is
something wrong with the carb...
Pls don't get frustrated with the little machine...
Take a break!
Gosh, you are learning a lot!
Can you imagine all the mistgakes I have made over the years?
LoL I ain't
even gonna fix that spelling error!
Welcome to the world of kit bikes!
and more so, to the magnificent china engines we all love so well !
Probably need to take that carb off the bike, dry it off and clean it out.
Or vice-versa...
Buncha little pieces that will be new to you, and you can not lose any of them!
Need to determine if the float and needle valve are adjusted properly and working well... it may be plugged or obstructed.
Then, you need to check if the jets, and pasages within the carb are clear.
Something could be wrong with the slide, from the manufacturer, or the needle in the slide could be bonkers, in some crazy way.
Again, I must say, a china engine is a crap shoot...
No telling what may be amiss on a brand new unit.
LoL I'm really hoping someone will come along with a different way of expressing their opinion so we can get this bike running well for you !
Is their a possibility of intake leaks?
Douse the intake manifold connection to cylinder, and the intake to carb connection with some thick soapy water and see if that makes a difference in a startup... it won't last long, but if it makes a difference, that would be an indication you need to take it apart and inspect gasket surfaces, and perhaps use some sealer on them???
...anyhow, deal with it manana! When you are fresh, and able to consider things one at a time.
Best
rc
Unfortunately, there is really no telling what may be defunct inside one of them. Some of the carb models and versions seem to be hated far and wide.
Oops! I goofed that line up... !
Can you give us the numbers off the carb so we can ID it?