Yeah, you've got the Linhai 257cc engine, so check out the intake valve cover on top, two bolts hold it on, if your drill a couple holes in the battery box you can access the valve without taking any plastic off. Just use a swivel 8mm socket on an extension, and a magnet to keep the bolts captive so you can lift out the cover.
Then the exhaust cover has 3 bolts, kinda tricky to get to, one requires an 8mm box end wrench, but again, just loosen the right bottom side panel and drop the belly pan on the right side, prop it down with a board, then reach yer little monkey hand up in there and git her done.
Tighten the connections to wires on the back of the coil, and make sure the grounds are properly connected. Some little gremlins appear after running for a while because the vibrations of the egine loosen nuts and bolts.
No telling where the problem may be, but if it's minor, and you check and secure everything, chances are you'll find it.
Once you are familiar, especially if you're not successful, you can move on to valve adjustment and carb tuning.