Won't accelerate

a pop usually means a crank seal or head gasket is leaking - look for oil around head, behind rotor & behind small bevel gear

Yup... what crassius said.... you can also run a compression check if you got a compression tester.
If it's a new engine it's most likely the head nuts came loose, this is a common problem. They only need to be torqued to about 10 ft/lb so don't over tighten or it could snap a stud or pull the stud out of the aluminum in the cases below.
You can usually see the oily residue around the head if they came loose. Another tip would be to get some regular nuts from the hardware store since the acorn nuts have been known to bottom out on themselves giving a fake torque reading when torqued, leaving the head loose after its torqued. Just take one of your acorn nuts with you and match it up. You can also get extra tall nuts from Fred at crmachine.com or from mzmiami on ebay if looks are important...

A pop can also mean a CDI or coil went kaput. One last, improperly timed, spark causes a backfire when the CDI goes out.

Also very likely to be compression. Just don't rule out spark issues

CDI failure is common, but depends on what OP means by 'won't accelerate' - bad CDI won't run at all

head & seal leaks will run really slowly