As a an "old enough to know better, but still young enough to do it" (and done it) aged person here is the simplest way to do it.
Take a second ignition coil and spark plug assy. Mount the plug about 2 inches in from the end of the tail pipe. Take the set of primary wires going to the actual engine coil and run them to a double pole double throw switch. or DPDT relay triggered by a momentary push button switch. Put the engine coil on the normaly closed side. when the switch or relay is triggered it moves the primary power to the ignitor coil shutting off the engine coil.
Wind that engine up, push the button. All of that mixed gas and air go right through the engine into the exaust and get ignited at the end of the pipe!
Being ignited at the end of the pipe the flame front has to travel back against the outgoing exaust pressure it does not cause back firing. It just gives a good multicolored blow torch effect. Very Very Cool at night!
Washes down the cylinder walls though, shorter engine life. Unless its a two stroke then it dont care!
P.S. If you think your lil engine is cool do it with a cammed out big block V8 with an 800 cfm carb, on propane! A good 5000 RPM burp will do a 10 footer!
And thats also what the cops call an exibition ticket too! (They cost way more than the ignitor setup)
I miss my old pickup. Darn little old lady drivers!