farting.....

You dont need to modify anything to make a car backfire on the overrun.
I have had a quite a few old classic cars and with things set up properly never had problems with missfiring ect until unleaded petrol was introduced.
For the last nine years I have owned a 1969 MGB roadster which I only drive in the summer.
I leave it in a garage during winter with only occasionally starting and when I bring it out for the summer it will missfire pop and bang like a boy racer until I fill the tank with fresh petrol then everything is ok again.
I have had the same problem with a 1949 Ferguson tractor that I have recently acquired. It kept missfiring, popping and banging. I checked the carb, ignition and timing and could find nothing wrong. Then I filled the tank with fresh petrol and problem solved.
Must say that it is quite fun to drive the MG for its first outing before filling up with fresh fuel.
Fun to hear the pops and bangs from the exhaust. 
Is there such a thing as an old boy racer?

landyman wrote:You dont need to modify anything to make a car backfire on the overrun.

On older cars, that may be true, but on current-model cars the engine conditions are so tightly controlled - assuming there are no faults - that you certainly shouldn't get any of the popping and banging that you hear currently.

I'd be interested - purely out of curiosity - to understand how they modify the engine map to make it happen. I don't - of course - need any further explanation of why - young males and showing off in cars is nothing new.

Thankfully the fashion for adding blow-off valves to turbocharged cars has finally ended, though. It's been a while since we last had young lads hooning up and down the local high street on and off full throttle, just so everyone could hear their show-off valve in operation.

A friend of mine, when we was young (mid '60's) when coming down Church Street in Gt. Malvern, used to disable the ignition on his very old Rover and then almost immediately, back on. This would result in the un-burnt gasses in the exhaust igniting with a very loud BANG!
This practice was repeated, especially after drink having been consumed, until one evening he was a little slow in turning the ignition back on and the resulting LARGE accumulation of combustible gasses in his exhaust ignited with a spectacular BANG that blew his silencer apart! 
 frederic

Remember when having a bit of an exhaust leak made your car sound sporty (well in your head maybe)
Back in the day (70's) me and my mates had many camping hols in many vehicles.
We thought it was fun when alighting from the car to slam each door in turn bang bang bang bang!
Well one time in a rover 90 we pulled up at the pub slammed the doors bang bang bang bang, and headed to the pub.
after a couple of steps a fifth 'bang' we all turned to see who had slammed the fifth door? and realised it was a backfire
the engine used to run-on a bit!!
aah! lummy days