Pull both plugs, hold to cylinder with wires attached & crank. Should be Blue Hot Snappy Spark going to both. If not fix Ur Ignition, points, condenser, coil, wires etc. whatever. Good Spark, could be bad Compression. U didn't say 2 or 4 Stroke, so lets try 4 first. Pull the "Rocker Arm Covers & check Valve Clearance. Make Adjustments per Manual. Work? Yes, have Fun! No? It may need a Valve Job where they Re-Finish the Seats & Face the Valves. If it is a 2 Stroke it "Kinda" has Valves, either a "Reed, Rotary or Piston Skirt" will be used. The "Reed Block" is in between the Carb & the Cylinder & is easy to replace. Often when bad this will "Backfire" out the Carb. If it is "Piston Seal" look into the Cylinder from both Carb & Exhaust Ports & check for "Wear" (Slop/Scarring excessive Ring Grove Play etc.) If this is the problem, it needs new "Top End" (Piston & Rings over sized) & a "Boring" of the Cylinder. U Must do both Cylinders at a time as well. Can get a bit much but should make it "Brand New"!!! John