Ignition timing check.
Most multi ( 4 ) cylinder engines use a
"wasted spark". That is; they spark on the
exhaust stroke as well as the power stroke. Thus, cylinders
1 and 3 use a common coil, as do cylinders 2 and 4.
If, when you hit the start button, you get loud backfiring and "blowbacks" through the air filter, then it's a good bet that some joker has swapped your plug leads over.
To check the timing
If your bike has got timing marks, they should line up just as the points begin to open.
If you have no timing marks, remove a plug and CAREFULLY put something down the plug hole. Don't use anything that could be dropped into the cylinder, or that would damage the piston. A pencil is ideal, or a drinking straw. Turn the engine over (NOT with the starter motor) slowly until the piston is almost ( about 3mm from ) at top dead centre. At least one set of points should just be about to open. If they are "miles" out, then the timing has slipped, reset them as best as possible using this method but take it steady, it's all too easy to hole a piston.
If you set the timing too advanced on a big single you risk a broken leg if it kicks back.
Trust me on this one